xnu-7195.81.3.tar.gz

[apple/xnu.git] / bsd / pthread / pthread_workqueue.c

/*
 * Copyright (c) 2000-2020 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/* Copyright (c) 1995-2018 Apple, Inc. All Rights Reserved */

#include <sys/cdefs.h>

#include <kern/assert.h>
#include <kern/ast.h>
#include <kern/clock.h>
#include <kern/cpu_data.h>
#include <kern/kern_types.h>
#include <kern/policy_internal.h>
#include <kern/processor.h>
#include <kern/sched_prim.h>    /* for thread_exception_return */
#include <kern/task.h>
#include <kern/thread.h>
#include <kern/zalloc.h>
#include <mach/kern_return.h>
#include <mach/mach_param.h>
#include <mach/mach_port.h>
#include <mach/mach_types.h>
#include <mach/mach_vm.h>
#include <mach/sync_policy.h>
#include <mach/task.h>
#include <mach/thread_act.h> /* for thread_resume */
#include <mach/thread_policy.h>
#include <mach/thread_status.h>
#include <mach/vm_prot.h>
#include <mach/vm_statistics.h>
#include <machine/atomic.h>
#include <machine/machine_routines.h>
#include <vm/vm_map.h>
#include <vm/vm_protos.h>

#include <sys/eventvar.h>
#include <sys/kdebug.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/param.h>
#include <sys/proc_info.h>      /* for fill_procworkqueue */
#include <sys/proc_internal.h>
#include <sys/pthread_shims.h>
#include <sys/resourcevar.h>
#include <sys/signalvar.h>
#include <sys/sysctl.h>
#include <sys/sysproto.h>
#include <sys/systm.h>
#include <sys/ulock.h> /* for ulock_owner_value_to_port_name */

#include <pthread/bsdthread_private.h>
#include <pthread/workqueue_syscalls.h>
#include <pthread/workqueue_internal.h>
#include <pthread/workqueue_trace.h>

#include <os/log.h>

static void workq_unpark_continue(void *uth, wait_result_t wr) __dead2;
static void workq_schedule_creator(proc_t p, struct workqueue *wq,
    workq_kern_threadreq_flags_t flags);

static bool workq_threadreq_admissible(struct workqueue *wq, struct uthread *uth,
    workq_threadreq_t req);

static uint32_t workq_constrained_allowance(struct workqueue *wq,
    thread_qos_t at_qos, struct uthread *uth, bool may_start_timer);

static bool workq_thread_is_busy(uint64_t cur_ts,
    _Atomic uint64_t *lastblocked_tsp);

static int workq_sysctl_handle_usecs SYSCTL_HANDLER_ARGS;

#pragma mark globals

struct workq_usec_var {
        uint32_t usecs;
        uint64_t abstime;
};

#define WORKQ_SYSCTL_USECS(var, init) \
                static struct workq_usec_var var = { .usecs = init }; \
                SYSCTL_OID(_kern, OID_AUTO, var##_usecs, \
                                CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_LOCKED, &var, 0, \
                                workq_sysctl_handle_usecs, "I", "")

static LCK_GRP_DECLARE(workq_lck_grp, "workq");
os_refgrp_decl(static, workq_refgrp, "workq", NULL);

static ZONE_DECLARE(workq_zone_workqueue, "workq.wq",
    sizeof(struct workqueue), ZC_NONE);
static ZONE_DECLARE(workq_zone_threadreq, "workq.threadreq",
    sizeof(struct workq_threadreq_s), ZC_CACHING);

static struct mpsc_daemon_queue workq_deallocate_queue;

WORKQ_SYSCTL_USECS(wq_stalled_window, WQ_STALLED_WINDOW_USECS);
WORKQ_SYSCTL_USECS(wq_reduce_pool_window, WQ_REDUCE_POOL_WINDOW_USECS);
WORKQ_SYSCTL_USECS(wq_max_timer_interval, WQ_MAX_TIMER_INTERVAL_USECS);
static uint32_t wq_max_threads              = WORKQUEUE_MAXTHREADS;
static uint32_t wq_max_constrained_threads  = WORKQUEUE_MAXTHREADS / 8;
static uint32_t wq_init_constrained_limit   = 1;
static uint16_t wq_death_max_load;
static uint32_t wq_max_parallelism[WORKQ_NUM_QOS_BUCKETS];

#pragma mark sysctls

static int
workq_sysctl_handle_usecs SYSCTL_HANDLER_ARGS
{
#pragma unused(arg2)
        struct workq_usec_var *v = arg1;
        int error = sysctl_handle_int(oidp, &v->usecs, 0, req);
        if (error || !req->newptr) {
                return error;
        }
        clock_interval_to_absolutetime_interval(v->usecs, NSEC_PER_USEC,
            &v->abstime);
        return 0;
}

SYSCTL_INT(_kern, OID_AUTO, wq_max_threads, CTLFLAG_RW | CTLFLAG_LOCKED,
    &wq_max_threads, 0, "");

SYSCTL_INT(_kern, OID_AUTO, wq_max_constrained_threads, CTLFLAG_RW | CTLFLAG_LOCKED,
    &wq_max_constrained_threads, 0, "");

#pragma mark p_wqptr

#define WQPTR_IS_INITING_VALUE ((struct workqueue *)~(uintptr_t)0)

static struct workqueue *
proc_get_wqptr_fast(struct proc *p)
{
        return os_atomic_load(&p->p_wqptr, relaxed);
}

static struct workqueue *
proc_get_wqptr(struct proc *p)
{
        struct workqueue *wq = proc_get_wqptr_fast(p);
        return wq == WQPTR_IS_INITING_VALUE ? NULL : wq;
}

static void
proc_set_wqptr(struct proc *p, struct workqueue *wq)
{
        wq = os_atomic_xchg(&p->p_wqptr, wq, release);
        if (wq == WQPTR_IS_INITING_VALUE) {
                proc_lock(p);
                thread_wakeup(&p->p_wqptr);
                proc_unlock(p);
        }
}

static bool
proc_init_wqptr_or_wait(struct proc *p)
{
        struct workqueue *wq;

        proc_lock(p);
        wq = os_atomic_load(&p->p_wqptr, relaxed);

        if (wq == NULL) {
                os_atomic_store(&p->p_wqptr, WQPTR_IS_INITING_VALUE, relaxed);
                proc_unlock(p);
                return true;
        }

        if (wq == WQPTR_IS_INITING_VALUE) {
                assert_wait(&p->p_wqptr, THREAD_UNINT);
                proc_unlock(p);
                thread_block(THREAD_CONTINUE_NULL);
        } else {
                proc_unlock(p);
        }
        return false;
}

static inline event_t
workq_parked_wait_event(struct uthread *uth)
{
        return (event_t)&uth->uu_workq_stackaddr;
}

static inline void
workq_thread_wakeup(struct uthread *uth)
{
        thread_wakeup_thread(workq_parked_wait_event(uth), uth->uu_thread);
}

#pragma mark wq_thactive

#if defined(__LP64__)
// Layout is:
//   127 - 115 : 13 bits of zeroes
//   114 - 112 : best QoS among all pending constrained requests
//   111 -   0 : MGR, AUI, UI, IN, DF, UT, BG+MT buckets every 16 bits
#define WQ_THACTIVE_BUCKET_WIDTH 16
#define WQ_THACTIVE_QOS_SHIFT    (7 * WQ_THACTIVE_BUCKET_WIDTH)
#else
// Layout is:
//   63 - 61 : best QoS among all pending constrained requests
//   60      : Manager bucket (0 or 1)
//   59 -  0 : AUI, UI, IN, DF, UT, BG+MT buckets every 10 bits
#define WQ_THACTIVE_BUCKET_WIDTH 10
#define WQ_THACTIVE_QOS_SHIFT    (6 * WQ_THACTIVE_BUCKET_WIDTH + 1)
#endif
#define WQ_THACTIVE_BUCKET_MASK  ((1U << WQ_THACTIVE_BUCKET_WIDTH) - 1)
#define WQ_THACTIVE_BUCKET_HALF  (1U << (WQ_THACTIVE_BUCKET_WIDTH - 1))

static_assert(sizeof(wq_thactive_t) * CHAR_BIT - WQ_THACTIVE_QOS_SHIFT >= 3,
    "Make sure we have space to encode a QoS");

static inline wq_thactive_t
_wq_thactive(struct workqueue *wq)
{
        return os_atomic_load_wide(&wq->wq_thactive, relaxed);
}

static inline int
_wq_bucket(thread_qos_t qos)
{
        // Map both BG and MT to the same bucket by over-shifting down and
        // clamping MT and BG together.
        switch (qos) {
        case THREAD_QOS_MAINTENANCE:
                return 0;
        default:
                return qos - 2;
        }
}

#define WQ_THACTIVE_BEST_CONSTRAINED_REQ_QOS(tha) \
                ((thread_qos_t)((tha) >> WQ_THACTIVE_QOS_SHIFT))

static inline thread_qos_t
_wq_thactive_best_constrained_req_qos(struct workqueue *wq)
{
        // Avoid expensive atomic operations: the three bits we're loading are in
        // a single byte, and always updated under the workqueue lock
        wq_thactive_t v = *(wq_thactive_t *)&wq->wq_thactive;
        return WQ_THACTIVE_BEST_CONSTRAINED_REQ_QOS(v);
}

static void
_wq_thactive_refresh_best_constrained_req_qos(struct workqueue *wq)
{
        thread_qos_t old_qos, new_qos;
        workq_threadreq_t req;

        req = priority_queue_max(&wq->wq_constrained_queue,
            struct workq_threadreq_s, tr_entry);
        new_qos = req ? req->tr_qos : THREAD_QOS_UNSPECIFIED;
        old_qos = _wq_thactive_best_constrained_req_qos(wq);
        if (old_qos != new_qos) {
                long delta = (long)new_qos - (long)old_qos;
                wq_thactive_t v = (wq_thactive_t)delta << WQ_THACTIVE_QOS_SHIFT;
                /*
                 * We can do an atomic add relative to the initial load because updates
                 * to this qos are always serialized under the workqueue lock.
                 */
                v = os_atomic_add(&wq->wq_thactive, v, relaxed);
#ifdef __LP64__
                WQ_TRACE_WQ(TRACE_wq_thactive_update, wq, (uint64_t)v,
                    (uint64_t)(v >> 64), 0, 0);
#else
                WQ_TRACE_WQ(TRACE_wq_thactive_update, wq, v, 0, 0, 0);
#endif
        }
}

static inline wq_thactive_t
_wq_thactive_offset_for_qos(thread_qos_t qos)
{
        return (wq_thactive_t)1 << (_wq_bucket(qos) * WQ_THACTIVE_BUCKET_WIDTH);
}

static inline wq_thactive_t
_wq_thactive_inc(struct workqueue *wq, thread_qos_t qos)
{
        wq_thactive_t v = _wq_thactive_offset_for_qos(qos);
        return os_atomic_add_orig(&wq->wq_thactive, v, relaxed);
}

static inline wq_thactive_t
_wq_thactive_dec(struct workqueue *wq, thread_qos_t qos)
{
        wq_thactive_t v = _wq_thactive_offset_for_qos(qos);
        return os_atomic_sub_orig(&wq->wq_thactive, v, relaxed);
}

static inline void
_wq_thactive_move(struct workqueue *wq,
    thread_qos_t old_qos, thread_qos_t new_qos)
{
        wq_thactive_t v = _wq_thactive_offset_for_qos(new_qos) -
            _wq_thactive_offset_for_qos(old_qos);
        os_atomic_add(&wq->wq_thactive, v, relaxed);
        wq->wq_thscheduled_count[_wq_bucket(old_qos)]--;
        wq->wq_thscheduled_count[_wq_bucket(new_qos)]++;
}

static inline uint32_t
_wq_thactive_aggregate_downto_qos(struct workqueue *wq, wq_thactive_t v,
    thread_qos_t qos, uint32_t *busycount, uint32_t *max_busycount)
{
        uint32_t count = 0, active;
        uint64_t curtime;

        assert(WORKQ_THREAD_QOS_MIN <= qos && qos <= WORKQ_THREAD_QOS_MAX);

        if (busycount) {
                curtime = mach_absolute_time();
                *busycount = 0;
        }
        if (max_busycount) {
                *max_busycount = THREAD_QOS_LAST - qos;
        }

        int i = _wq_bucket(qos);
        v >>= i * WQ_THACTIVE_BUCKET_WIDTH;
        for (; i < WORKQ_NUM_QOS_BUCKETS; i++, v >>= WQ_THACTIVE_BUCKET_WIDTH) {
                active = v & WQ_THACTIVE_BUCKET_MASK;
                count += active;

                if (busycount && wq->wq_thscheduled_count[i] > active) {
                        if (workq_thread_is_busy(curtime, &wq->wq_lastblocked_ts[i])) {
                                /*
                                 * We only consider the last blocked thread for a given bucket
                                 * as busy because we don't want to take the list lock in each
                                 * sched callback. However this is an approximation that could
                                 * contribute to thread creation storms.
                                 */
                                (*busycount)++;
                        }
                }
        }

        return count;
}

#pragma mark wq_flags

static inline uint32_t
_wq_flags(struct workqueue *wq)
{
        return os_atomic_load(&wq->wq_flags, relaxed);
}

static inline bool
_wq_exiting(struct workqueue *wq)
{
        return _wq_flags(wq) & WQ_EXITING;
}

bool
workq_is_exiting(struct proc *p)
{
        struct workqueue *wq = proc_get_wqptr(p);
        return !wq || _wq_exiting(wq);
}

#pragma mark workqueue lock

static bool
workq_lock_spin_is_acquired_kdp(struct workqueue *wq)
{
        return kdp_lck_spin_is_acquired(&wq->wq_lock);
}

static inline void
workq_lock_spin(struct workqueue *wq)
{
        lck_spin_lock_grp(&wq->wq_lock, &workq_lck_grp);
}

static inline void
workq_lock_held(__assert_only struct workqueue *wq)
{
        LCK_SPIN_ASSERT(&wq->wq_lock, LCK_ASSERT_OWNED);
}

static inline bool
workq_lock_try(struct workqueue *wq)
{
        return lck_spin_try_lock_grp(&wq->wq_lock, &workq_lck_grp);
}

static inline void
workq_unlock(struct workqueue *wq)
{
        lck_spin_unlock(&wq->wq_lock);
}

#pragma mark idle thread lists

#define WORKQ_POLICY_INIT(qos) \
                (struct uu_workq_policy){ .qos_req = qos, .qos_bucket = qos }

static inline thread_qos_t
workq_pri_bucket(struct uu_workq_policy req)
{
        return MAX(MAX(req.qos_req, req.qos_max), req.qos_override);
}

static inline thread_qos_t
workq_pri_override(struct uu_workq_policy req)
{
        return MAX(workq_pri_bucket(req), req.qos_bucket);
}

static inline bool
workq_thread_needs_params_change(workq_threadreq_t req, struct uthread *uth)
{
        workq_threadreq_param_t cur_trp, req_trp = { };

        cur_trp.trp_value = uth->uu_save.uus_workq_park_data.workloop_params;
        if (req->tr_flags & WORKQ_TR_FLAG_WL_PARAMS) {
                req_trp = kqueue_threadreq_workloop_param(req);
        }

        /*
         * CPU percent flags are handled separately to policy changes, so ignore
         * them for all of these checks.
         */
        uint16_t cur_flags = (cur_trp.trp_flags & ~TRP_CPUPERCENT);
        uint16_t req_flags = (req_trp.trp_flags & ~TRP_CPUPERCENT);

        if (!req_flags && !cur_flags) {
                return false;
        }

        if (req_flags != cur_flags) {
                return true;
        }

        if ((req_flags & TRP_PRIORITY) && req_trp.trp_pri != cur_trp.trp_pri) {
                return true;
        }

        if ((req_flags & TRP_POLICY) && req_trp.trp_pol != cur_trp.trp_pol) {
                return true;
        }

        return false;
}

static inline bool
workq_thread_needs_priority_change(workq_threadreq_t req, struct uthread *uth)
{
        if (workq_thread_needs_params_change(req, uth)) {
                return true;
        }

        return req->tr_qos != workq_pri_override(uth->uu_workq_pri);
}

static void
workq_thread_update_bucket(proc_t p, struct workqueue *wq, struct uthread *uth,
    struct uu_workq_policy old_pri, struct uu_workq_policy new_pri,
    bool force_run)
{
        thread_qos_t old_bucket = old_pri.qos_bucket;
        thread_qos_t new_bucket = workq_pri_bucket(new_pri);

        if (old_bucket != new_bucket) {
                _wq_thactive_move(wq, old_bucket, new_bucket);
        }

        new_pri.qos_bucket = new_bucket;
        uth->uu_workq_pri = new_pri;

        if (workq_pri_override(old_pri) != new_bucket) {
                thread_set_workq_override(uth->uu_thread, new_bucket);
        }

        if (wq->wq_reqcount && (old_bucket > new_bucket || force_run)) {
                int flags = WORKQ_THREADREQ_CAN_CREATE_THREADS;
                if (old_bucket > new_bucket) {
                        /*
                         * When lowering our bucket, we may unblock a thread request,
                         * but we can't drop our priority before we have evaluated
                         * whether this is the case, and if we ever drop the workqueue lock
                         * that would cause a priority inversion.
                         *
                         * We hence have to disallow thread creation in that case.
                         */
                        flags = 0;
                }
                workq_schedule_creator(p, wq, flags);
        }
}

/*
 * Sets/resets the cpu percent limits on the current thread. We can't set
 * these limits from outside of the current thread, so this function needs
 * to be called when we're executing on the intended
 */
static void
workq_thread_reset_cpupercent(workq_threadreq_t req, struct uthread *uth)
{
        assert(uth == current_uthread());
        workq_threadreq_param_t trp = { };

        if (req && (req->tr_flags & WORKQ_TR_FLAG_WL_PARAMS)) {
                trp = kqueue_threadreq_workloop_param(req);
        }

        if (uth->uu_workq_flags & UT_WORKQ_CPUPERCENT) {
                /*
                 * Going through disable when we have an existing CPU percent limit
                 * set will force the ledger to refill the token bucket of the current
                 * thread. Removing any penalty applied by previous thread use.
                 */
                thread_set_cpulimit(THREAD_CPULIMIT_DISABLE, 0, 0);
                uth->uu_workq_flags &= ~UT_WORKQ_CPUPERCENT;
        }

        if (trp.trp_flags & TRP_CPUPERCENT) {
                thread_set_cpulimit(THREAD_CPULIMIT_BLOCK, trp.trp_cpupercent,
                    (uint64_t)trp.trp_refillms * NSEC_PER_SEC);
                uth->uu_workq_flags |= UT_WORKQ_CPUPERCENT;
        }
}

static void
workq_thread_reset_pri(struct workqueue *wq, struct uthread *uth,
    workq_threadreq_t req, bool unpark)
{
        thread_t th = uth->uu_thread;
        thread_qos_t qos = req ? req->tr_qos : WORKQ_THREAD_QOS_CLEANUP;
        workq_threadreq_param_t trp = { };
        int priority = 31;
        int policy = POLICY_TIMESHARE;

        if (req && (req->tr_flags & WORKQ_TR_FLAG_WL_PARAMS)) {
                trp = kqueue_threadreq_workloop_param(req);
        }

        uth->uu_workq_pri = WORKQ_POLICY_INIT(qos);
        uth->uu_workq_flags &= ~UT_WORKQ_OUTSIDE_QOS;

        if (unpark) {
                uth->uu_save.uus_workq_park_data.workloop_params = trp.trp_value;
                // qos sent out to userspace (may differ from uu_workq_pri on param threads)
                uth->uu_save.uus_workq_park_data.qos = qos;
        }

        if (qos == WORKQ_THREAD_QOS_MANAGER) {
                uint32_t mgr_pri = wq->wq_event_manager_priority;
                assert(trp.trp_value == 0); // manager qos and thread policy don't mix

                if (mgr_pri & _PTHREAD_PRIORITY_SCHED_PRI_FLAG) {
                        mgr_pri &= _PTHREAD_PRIORITY_SCHED_PRI_MASK;
                        thread_set_workq_pri(th, THREAD_QOS_UNSPECIFIED, mgr_pri,
                            POLICY_TIMESHARE);
                        return;
                }

                qos = _pthread_priority_thread_qos(mgr_pri);
        } else {
                if (trp.trp_flags & TRP_PRIORITY) {
                        qos = THREAD_QOS_UNSPECIFIED;
                        priority = trp.trp_pri;
                        uth->uu_workq_flags |= UT_WORKQ_OUTSIDE_QOS;
                }

                if (trp.trp_flags & TRP_POLICY) {
                        policy = trp.trp_pol;
                }
        }

        thread_set_workq_pri(th, qos, priority, policy);
}

/*
 * Called by kevent with the NOTE_WL_THREAD_REQUEST knote lock held,
 * every time a servicer is being told about a new max QoS.
 */
void
workq_thread_set_max_qos(struct proc *p, workq_threadreq_t kqr)
{
        struct uu_workq_policy old_pri, new_pri;
        struct uthread *uth = current_uthread();
        struct workqueue *wq = proc_get_wqptr_fast(p);
        thread_qos_t qos = kqr->tr_kq_qos_index;

        if (uth->uu_workq_pri.qos_max == qos) {
                return;
        }

        workq_lock_spin(wq);
        old_pri = new_pri = uth->uu_workq_pri;
        new_pri.qos_max = qos;
        workq_thread_update_bucket(p, wq, uth, old_pri, new_pri, false);
        workq_unlock(wq);
}

#pragma mark idle threads accounting and handling

static inline struct uthread *
workq_oldest_killable_idle_thread(struct workqueue *wq)
{
        struct uthread *uth = TAILQ_LAST(&wq->wq_thidlelist, workq_uthread_head);

        if (uth && !uth->uu_save.uus_workq_park_data.has_stack) {
                uth = TAILQ_PREV(uth, workq_uthread_head, uu_workq_entry);
                if (uth) {
                        assert(uth->uu_save.uus_workq_park_data.has_stack);
                }
        }
        return uth;
}

static inline uint64_t
workq_kill_delay_for_idle_thread(struct workqueue *wq)
{
        uint64_t delay = wq_reduce_pool_window.abstime;
        uint16_t idle = wq->wq_thidlecount;

        /*
         * If we have less than wq_death_max_load threads, have a 5s timer.
         *
         * For the next wq_max_constrained_threads ones, decay linearly from
         * from 5s to 50ms.
         */
        if (idle <= wq_death_max_load) {
                return delay;
        }

        if (wq_max_constrained_threads > idle - wq_death_max_load) {
                delay *= (wq_max_constrained_threads - (idle - wq_death_max_load));
        }
        return delay / wq_max_constrained_threads;
}

static inline bool
workq_should_kill_idle_thread(struct workqueue *wq, struct uthread *uth,
    uint64_t now)
{
        uint64_t delay = workq_kill_delay_for_idle_thread(wq);
        return now - uth->uu_save.uus_workq_park_data.idle_stamp > delay;
}

static void
workq_death_call_schedule(struct workqueue *wq, uint64_t deadline)
{
        uint32_t wq_flags = os_atomic_load(&wq->wq_flags, relaxed);

        if (wq_flags & (WQ_EXITING | WQ_DEATH_CALL_SCHEDULED)) {
                return;
        }
        os_atomic_or(&wq->wq_flags, WQ_DEATH_CALL_SCHEDULED, relaxed);

        WQ_TRACE_WQ(TRACE_wq_death_call | DBG_FUNC_NONE, wq, 1, 0, 0, 0);

        /*
         * <rdar://problem/13139182> Due to how long term timers work, the leeway
         * can't be too short, so use 500ms which is long enough that we will not
         * wake up the CPU for killing threads, but short enough that it doesn't
         * fall into long-term timer list shenanigans.
         */
        thread_call_enter_delayed_with_leeway(wq->wq_death_call, NULL, deadline,
            wq_reduce_pool_window.abstime / 10,
            THREAD_CALL_DELAY_LEEWAY | THREAD_CALL_DELAY_USER_BACKGROUND);
}

/*
 * `decrement` is set to the number of threads that are no longer dying:
 * - because they have been resuscitated just in time (workq_pop_idle_thread)
 * - or have been killed (workq_thread_terminate).
 */
static void
workq_death_policy_evaluate(struct workqueue *wq, uint16_t decrement)
{
        struct uthread *uth;

        assert(wq->wq_thdying_count >= decrement);
        if ((wq->wq_thdying_count -= decrement) > 0) {
                return;
        }

        if (wq->wq_thidlecount <= 1) {
                return;
        }

        if ((uth = workq_oldest_killable_idle_thread(wq)) == NULL) {
                return;
        }

        uint64_t now = mach_absolute_time();
        uint64_t delay = workq_kill_delay_for_idle_thread(wq);

        if (now - uth->uu_save.uus_workq_park_data.idle_stamp > delay) {
                WQ_TRACE_WQ(TRACE_wq_thread_terminate | DBG_FUNC_START,
                    wq, wq->wq_thidlecount, 0, 0, 0);
                wq->wq_thdying_count++;
                uth->uu_workq_flags |= UT_WORKQ_DYING;
                if ((uth->uu_workq_flags & UT_WORKQ_IDLE_CLEANUP) == 0) {
                        workq_thread_wakeup(uth);
                }
                return;
        }

        workq_death_call_schedule(wq,
            uth->uu_save.uus_workq_park_data.idle_stamp + delay);
}

void
workq_thread_terminate(struct proc *p, struct uthread *uth)
{
        struct workqueue *wq = proc_get_wqptr_fast(p);

        workq_lock_spin(wq);
        TAILQ_REMOVE(&wq->wq_thrunlist, uth, uu_workq_entry);
        if (uth->uu_workq_flags & UT_WORKQ_DYING) {
                WQ_TRACE_WQ(TRACE_wq_thread_terminate | DBG_FUNC_END,
                    wq, wq->wq_thidlecount, 0, 0, 0);
                workq_death_policy_evaluate(wq, 1);
        }
        if (wq->wq_nthreads-- == wq_max_threads) {
                /*
                 * We got under the thread limit again, which may have prevented
                 * thread creation from happening, redrive if there are pending requests
                 */
                if (wq->wq_reqcount) {
                        workq_schedule_creator(p, wq, WORKQ_THREADREQ_CAN_CREATE_THREADS);
                }
        }
        workq_unlock(wq);

        thread_deallocate(uth->uu_thread);
}

static void
workq_kill_old_threads_call(void *param0, void *param1 __unused)
{
        struct workqueue *wq = param0;

        workq_lock_spin(wq);
        WQ_TRACE_WQ(TRACE_wq_death_call | DBG_FUNC_START, wq, 0, 0, 0, 0);
        os_atomic_andnot(&wq->wq_flags, WQ_DEATH_CALL_SCHEDULED, relaxed);
        workq_death_policy_evaluate(wq, 0);
        WQ_TRACE_WQ(TRACE_wq_death_call | DBG_FUNC_END, wq, 0, 0, 0, 0);
        workq_unlock(wq);
}

static struct uthread *
workq_pop_idle_thread(struct workqueue *wq, uint8_t uu_flags,
    bool *needs_wakeup)
{
        struct uthread *uth;

        if ((uth = TAILQ_FIRST(&wq->wq_thidlelist))) {
                TAILQ_REMOVE(&wq->wq_thidlelist, uth, uu_workq_entry);
        } else {
                uth = TAILQ_FIRST(&wq->wq_thnewlist);
                TAILQ_REMOVE(&wq->wq_thnewlist, uth, uu_workq_entry);
        }
        TAILQ_INSERT_TAIL(&wq->wq_thrunlist, uth, uu_workq_entry);

        assert((uth->uu_workq_flags & UT_WORKQ_RUNNING) == 0);
        uth->uu_workq_flags |= UT_WORKQ_RUNNING | uu_flags;
        if ((uu_flags & UT_WORKQ_OVERCOMMIT) == 0) {
                wq->wq_constrained_threads_scheduled++;
        }
        wq->wq_threads_scheduled++;
        wq->wq_thidlecount--;

        if (__improbable(uth->uu_workq_flags & UT_WORKQ_DYING)) {
                uth->uu_workq_flags ^= UT_WORKQ_DYING;
                workq_death_policy_evaluate(wq, 1);
                *needs_wakeup = false;
        } else if (uth->uu_workq_flags & UT_WORKQ_IDLE_CLEANUP) {
                *needs_wakeup = false;
        } else {
                *needs_wakeup = true;
        }
        return uth;
}

/*
 * Called by thread_create_workq_waiting() during thread initialization, before
 * assert_wait, before the thread has been started.
 */
event_t
workq_thread_init_and_wq_lock(task_t task, thread_t th)
{
        struct uthread *uth = get_bsdthread_info(th);

        uth->uu_workq_flags = UT_WORKQ_NEW;
        uth->uu_workq_pri = WORKQ_POLICY_INIT(THREAD_QOS_LEGACY);
        uth->uu_workq_thport = MACH_PORT_NULL;
        uth->uu_workq_stackaddr = 0;
        uth->uu_workq_pthread_kill_allowed = 0;

        thread_set_tag(th, THREAD_TAG_PTHREAD | THREAD_TAG_WORKQUEUE);
        thread_reset_workq_qos(th, THREAD_QOS_LEGACY);

        workq_lock_spin(proc_get_wqptr_fast(get_bsdtask_info(task)));
        return workq_parked_wait_event(uth);
}

/**
 * Try to add a new workqueue thread.
 *
 * - called with workq lock held
 * - dropped and retaken around thread creation
 * - return with workq lock held
 */
static bool
workq_add_new_idle_thread(proc_t p, struct workqueue *wq)
{
        mach_vm_offset_t th_stackaddr;
        kern_return_t kret;
        thread_t th;

        wq->wq_nthreads++;

        workq_unlock(wq);

        vm_map_t vmap = get_task_map(p->task);

        kret = pthread_functions->workq_create_threadstack(p, vmap, &th_stackaddr);
        if (kret != KERN_SUCCESS) {
                WQ_TRACE_WQ(TRACE_wq_thread_create_failed | DBG_FUNC_NONE, wq,
                    kret, 1, 0, 0);
                goto out;
        }

        kret = thread_create_workq_waiting(p->task, workq_unpark_continue, &th);
        if (kret != KERN_SUCCESS) {
                WQ_TRACE_WQ(TRACE_wq_thread_create_failed | DBG_FUNC_NONE, wq,
                    kret, 0, 0, 0);
                pthread_functions->workq_destroy_threadstack(p, vmap, th_stackaddr);
                goto out;
        }

        // thread_create_workq_waiting() will return with the wq lock held
        // on success, because it calls workq_thread_init_and_wq_lock() above

        struct uthread *uth = get_bsdthread_info(th);

        wq->wq_creations++;
        wq->wq_thidlecount++;
        uth->uu_workq_stackaddr = (user_addr_t)th_stackaddr;
        TAILQ_INSERT_TAIL(&wq->wq_thnewlist, uth, uu_workq_entry);

        WQ_TRACE_WQ(TRACE_wq_thread_create | DBG_FUNC_NONE, wq, 0, 0, 0, 0);
        return true;

out:
        workq_lock_spin(wq);
        /*
         * Do not redrive here if we went under wq_max_threads again,
         * it is the responsibility of the callers of this function
         * to do so when it fails.
         */
        wq->wq_nthreads--;
        return false;
}

#define WORKQ_UNPARK_FOR_DEATH_WAS_IDLE 0x1

__attribute__((noreturn, noinline))
static void
workq_unpark_for_death_and_unlock(proc_t p, struct workqueue *wq,
    struct uthread *uth, uint32_t death_flags, uint32_t setup_flags)
{
        thread_qos_t qos = workq_pri_override(uth->uu_workq_pri);
        bool first_use = uth->uu_workq_flags & UT_WORKQ_NEW;

        if (qos > WORKQ_THREAD_QOS_CLEANUP) {
                workq_thread_reset_pri(wq, uth, NULL, /*unpark*/ true);
                qos = WORKQ_THREAD_QOS_CLEANUP;
        }

        workq_thread_reset_cpupercent(NULL, uth);

        if (death_flags & WORKQ_UNPARK_FOR_DEATH_WAS_IDLE) {
                wq->wq_thidlecount--;
                if (first_use) {
                        TAILQ_REMOVE(&wq->wq_thnewlist, uth, uu_workq_entry);
                } else {
                        TAILQ_REMOVE(&wq->wq_thidlelist, uth, uu_workq_entry);
                }
        }
        TAILQ_INSERT_TAIL(&wq->wq_thrunlist, uth, uu_workq_entry);

        workq_unlock(wq);

        if (setup_flags & WQ_SETUP_CLEAR_VOUCHER) {
                __assert_only kern_return_t kr;
                kr = thread_set_voucher_name(MACH_PORT_NULL);
                assert(kr == KERN_SUCCESS);
        }

        uint32_t flags = WQ_FLAG_THREAD_NEWSPI | qos | WQ_FLAG_THREAD_PRIO_QOS;
        thread_t th = uth->uu_thread;
        vm_map_t vmap = get_task_map(p->task);

        if (!first_use) {
                flags |= WQ_FLAG_THREAD_REUSE;
        }

        pthread_functions->workq_setup_thread(p, th, vmap, uth->uu_workq_stackaddr,
            uth->uu_workq_thport, 0, WQ_SETUP_EXIT_THREAD, flags);
        __builtin_unreachable();
}

bool
workq_is_current_thread_updating_turnstile(struct workqueue *wq)
{
        return wq->wq_turnstile_updater == current_thread();
}

__attribute__((always_inline))
static inline void
workq_perform_turnstile_operation_locked(struct workqueue *wq,
    void (^operation)(void))
{
        workq_lock_held(wq);
        wq->wq_turnstile_updater = current_thread();
        operation();
        wq->wq_turnstile_updater = THREAD_NULL;
}

static void
workq_turnstile_update_inheritor(struct workqueue *wq,
    turnstile_inheritor_t inheritor,
    turnstile_update_flags_t flags)
{
        if (wq->wq_inheritor == inheritor) {
                return;
        }
        wq->wq_inheritor = inheritor;
        workq_perform_turnstile_operation_locked(wq, ^{
                turnstile_update_inheritor(wq->wq_turnstile, inheritor,
                flags | TURNSTILE_IMMEDIATE_UPDATE);
                turnstile_update_inheritor_complete(wq->wq_turnstile,
                TURNSTILE_INTERLOCK_HELD);
        });
}

static void
workq_push_idle_thread(proc_t p, struct workqueue *wq, struct uthread *uth,
    uint32_t setup_flags)
{
        uint64_t now = mach_absolute_time();
        bool is_creator = (uth == wq->wq_creator);

        if ((uth->uu_workq_flags & UT_WORKQ_OVERCOMMIT) == 0) {
                wq->wq_constrained_threads_scheduled--;
        }
        uth->uu_workq_flags &= ~(UT_WORKQ_RUNNING | UT_WORKQ_OVERCOMMIT);
        TAILQ_REMOVE(&wq->wq_thrunlist, uth, uu_workq_entry);
        wq->wq_threads_scheduled--;

        if (is_creator) {
                wq->wq_creator = NULL;
                WQ_TRACE_WQ(TRACE_wq_creator_select, wq, 3, 0,
                    uth->uu_save.uus_workq_park_data.yields, 0);
        }

        if (wq->wq_inheritor == uth->uu_thread) {
                assert(wq->wq_creator == NULL);
                if (wq->wq_reqcount) {
                        workq_turnstile_update_inheritor(wq, wq, TURNSTILE_INHERITOR_WORKQ);
                } else {
                        workq_turnstile_update_inheritor(wq, TURNSTILE_INHERITOR_NULL, 0);
                }
        }

        if (uth->uu_workq_flags & UT_WORKQ_NEW) {
                assert(is_creator || (_wq_flags(wq) & WQ_EXITING));
                TAILQ_INSERT_TAIL(&wq->wq_thnewlist, uth, uu_workq_entry);
                wq->wq_thidlecount++;
                return;
        }

        if (!is_creator) {
                _wq_thactive_dec(wq, uth->uu_workq_pri.qos_bucket);
                wq->wq_thscheduled_count[_wq_bucket(uth->uu_workq_pri.qos_bucket)]--;
                uth->uu_workq_flags |= UT_WORKQ_IDLE_CLEANUP;
        }

        uth->uu_save.uus_workq_park_data.idle_stamp = now;

        struct uthread *oldest = workq_oldest_killable_idle_thread(wq);
        uint16_t cur_idle = wq->wq_thidlecount;

        if (cur_idle >= wq_max_constrained_threads ||
            (wq->wq_thdying_count == 0 && oldest &&
            workq_should_kill_idle_thread(wq, oldest, now))) {
                /*
                 * Immediately kill threads if we have too may of them.
                 *
                 * And swap "place" with the oldest one we'd have woken up.
                 * This is a relatively desperate situation where we really
                 * need to kill threads quickly and it's best to kill
                 * the one that's currently on core than context switching.
                 */
                if (oldest) {
                        oldest->uu_save.uus_workq_park_data.idle_stamp = now;
                        TAILQ_REMOVE(&wq->wq_thidlelist, oldest, uu_workq_entry);
                        TAILQ_INSERT_HEAD(&wq->wq_thidlelist, oldest, uu_workq_entry);
                }

                WQ_TRACE_WQ(TRACE_wq_thread_terminate | DBG_FUNC_START,
                    wq, cur_idle, 0, 0, 0);
                wq->wq_thdying_count++;
                uth->uu_workq_flags |= UT_WORKQ_DYING;
                uth->uu_workq_flags &= ~UT_WORKQ_IDLE_CLEANUP;
                workq_unpark_for_death_and_unlock(p, wq, uth, 0, setup_flags);
                __builtin_unreachable();
        }

        struct uthread *tail = TAILQ_LAST(&wq->wq_thidlelist, workq_uthread_head);

        cur_idle += 1;
        wq->wq_thidlecount = cur_idle;

        if (cur_idle >= wq_death_max_load && tail &&
            tail->uu_save.uus_workq_park_data.has_stack) {
                uth->uu_save.uus_workq_park_data.has_stack = false;
                TAILQ_INSERT_TAIL(&wq->wq_thidlelist, uth, uu_workq_entry);
        } else {
                uth->uu_save.uus_workq_park_data.has_stack = true;
                TAILQ_INSERT_HEAD(&wq->wq_thidlelist, uth, uu_workq_entry);
        }

        if (!tail) {
                uint64_t delay = workq_kill_delay_for_idle_thread(wq);
                workq_death_call_schedule(wq, now + delay);
        }
}

#pragma mark thread requests

static inline int
workq_priority_for_req(workq_threadreq_t req)
{
        thread_qos_t qos = req->tr_qos;

        if (req->tr_flags & WORKQ_TR_FLAG_WL_OUTSIDE_QOS) {
                workq_threadreq_param_t trp = kqueue_threadreq_workloop_param(req);
                assert(trp.trp_flags & TRP_PRIORITY);
                return trp.trp_pri;
        }
        return thread_workq_pri_for_qos(qos);
}

static inline struct priority_queue_sched_max *
workq_priority_queue_for_req(struct workqueue *wq, workq_threadreq_t req)
{
        if (req->tr_flags & WORKQ_TR_FLAG_WL_OUTSIDE_QOS) {
                return &wq->wq_special_queue;
        } else if (req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) {
                return &wq->wq_overcommit_queue;
        } else {
                return &wq->wq_constrained_queue;
        }
}

/*
 * returns true if the the enqueued request is the highest priority item
 * in its priority queue.
 */
static bool
workq_threadreq_enqueue(struct workqueue *wq, workq_threadreq_t req)
{
        assert(req->tr_state == WORKQ_TR_STATE_NEW);

        req->tr_state = WORKQ_TR_STATE_QUEUED;
        wq->wq_reqcount += req->tr_count;

        if (req->tr_qos == WORKQ_THREAD_QOS_MANAGER) {
                assert(wq->wq_event_manager_threadreq == NULL);
                assert(req->tr_flags & WORKQ_TR_FLAG_KEVENT);
                assert(req->tr_count == 1);
                wq->wq_event_manager_threadreq = req;
                return true;
        }

        struct priority_queue_sched_max *q = workq_priority_queue_for_req(wq, req);
        priority_queue_entry_set_sched_pri(q, &req->tr_entry,
            workq_priority_for_req(req), false);

        if (priority_queue_insert(q, &req->tr_entry)) {
                if ((req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) == 0) {
                        _wq_thactive_refresh_best_constrained_req_qos(wq);
                }
                return true;
        }
        return false;
}

/*
 * returns true if the the dequeued request was the highest priority item
 * in its priority queue.
 */
static bool
workq_threadreq_dequeue(struct workqueue *wq, workq_threadreq_t req)
{
        wq->wq_reqcount--;

        if (--req->tr_count == 0) {
                if (req->tr_qos == WORKQ_THREAD_QOS_MANAGER) {
                        assert(wq->wq_event_manager_threadreq == req);
                        assert(req->tr_count == 0);
                        wq->wq_event_manager_threadreq = NULL;
                        return true;
                }
                if (priority_queue_remove(workq_priority_queue_for_req(wq, req),
                    &req->tr_entry)) {
                        if ((req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) == 0) {
                                _wq_thactive_refresh_best_constrained_req_qos(wq);
                        }
                        return true;
                }
        }
        return false;
}

static void
workq_threadreq_destroy(proc_t p, workq_threadreq_t req)
{
        req->tr_state = WORKQ_TR_STATE_CANCELED;
        if (req->tr_flags & (WORKQ_TR_FLAG_WORKLOOP | WORKQ_TR_FLAG_KEVENT)) {
                kqueue_threadreq_cancel(p, req);
        } else {
                zfree(workq_zone_threadreq, req);
        }
}

#pragma mark workqueue thread creation thread calls

static inline bool
workq_thread_call_prepost(struct workqueue *wq, uint32_t sched, uint32_t pend,
    uint32_t fail_mask)
{
        uint32_t old_flags, new_flags;

        os_atomic_rmw_loop(&wq->wq_flags, old_flags, new_flags, acquire, {
                if (__improbable(old_flags & (WQ_EXITING | sched | pend | fail_mask))) {
                        os_atomic_rmw_loop_give_up(return false);
                }
                if (__improbable(old_flags & WQ_PROC_SUSPENDED)) {
                        new_flags = old_flags | pend;
                } else {
                        new_flags = old_flags | sched;
                }
        });

        return (old_flags & WQ_PROC_SUSPENDED) == 0;
}

#define WORKQ_SCHEDULE_DELAYED_THREAD_CREATION_RESTART 0x1

static bool
workq_schedule_delayed_thread_creation(struct workqueue *wq, int flags)
{
        assert(!preemption_enabled());

        if (!workq_thread_call_prepost(wq, WQ_DELAYED_CALL_SCHEDULED,
            WQ_DELAYED_CALL_PENDED, WQ_IMMEDIATE_CALL_PENDED |
            WQ_IMMEDIATE_CALL_SCHEDULED)) {
                return false;
        }

        uint64_t now = mach_absolute_time();

        if (flags & WORKQ_SCHEDULE_DELAYED_THREAD_CREATION_RESTART) {
                /* do not change the window */
        } else if (now - wq->wq_thread_call_last_run <= wq->wq_timer_interval) {
                wq->wq_timer_interval *= 2;
                if (wq->wq_timer_interval > wq_max_timer_interval.abstime) {
                        wq->wq_timer_interval = (uint32_t)wq_max_timer_interval.abstime;
                }
        } else if (now - wq->wq_thread_call_last_run > 2 * wq->wq_timer_interval) {
                wq->wq_timer_interval /= 2;
                if (wq->wq_timer_interval < wq_stalled_window.abstime) {
                        wq->wq_timer_interval = (uint32_t)wq_stalled_window.abstime;
                }
        }

        WQ_TRACE_WQ(TRACE_wq_start_add_timer, wq, wq->wq_reqcount,
            _wq_flags(wq), wq->wq_timer_interval, 0);

        thread_call_t call = wq->wq_delayed_call;
        uintptr_t arg = WQ_DELAYED_CALL_SCHEDULED;
        uint64_t deadline = now + wq->wq_timer_interval;
        if (thread_call_enter1_delayed(call, (void *)arg, deadline)) {
                panic("delayed_call was already enqueued");
        }
        return true;
}

static void
workq_schedule_immediate_thread_creation(struct workqueue *wq)
{
        assert(!preemption_enabled());

        if (workq_thread_call_prepost(wq, WQ_IMMEDIATE_CALL_SCHEDULED,
            WQ_IMMEDIATE_CALL_PENDED, 0)) {
                WQ_TRACE_WQ(TRACE_wq_start_add_timer, wq, wq->wq_reqcount,
                    _wq_flags(wq), 0, 0);

                uintptr_t arg = WQ_IMMEDIATE_CALL_SCHEDULED;
                if (thread_call_enter1(wq->wq_immediate_call, (void *)arg)) {
                        panic("immediate_call was already enqueued");
                }
        }
}

void
workq_proc_suspended(struct proc *p)
{
        struct workqueue *wq = proc_get_wqptr(p);

        if (wq) {
                os_atomic_or(&wq->wq_flags, WQ_PROC_SUSPENDED, relaxed);
        }
}

void
workq_proc_resumed(struct proc *p)
{
        struct workqueue *wq = proc_get_wqptr(p);
        uint32_t wq_flags;

        if (!wq) {
                return;
        }

        wq_flags = os_atomic_andnot_orig(&wq->wq_flags, WQ_PROC_SUSPENDED |
            WQ_DELAYED_CALL_PENDED | WQ_IMMEDIATE_CALL_PENDED, relaxed);
        if ((wq_flags & WQ_EXITING) == 0) {
                disable_preemption();
                if (wq_flags & WQ_IMMEDIATE_CALL_PENDED) {
                        workq_schedule_immediate_thread_creation(wq);
                } else if (wq_flags & WQ_DELAYED_CALL_PENDED) {
                        workq_schedule_delayed_thread_creation(wq,
                            WORKQ_SCHEDULE_DELAYED_THREAD_CREATION_RESTART);
                }
                enable_preemption();
        }
}

/**
 * returns whether lastblocked_tsp is within wq_stalled_window usecs of now
 */
static bool
workq_thread_is_busy(uint64_t now, _Atomic uint64_t *lastblocked_tsp)
{
        uint64_t lastblocked_ts = os_atomic_load_wide(lastblocked_tsp, relaxed);
        if (now <= lastblocked_ts) {
                /*
                 * Because the update of the timestamp when a thread blocks
                 * isn't serialized against us looking at it (i.e. we don't hold
                 * the workq lock), it's possible to have a timestamp that matches
                 * the current time or that even looks to be in the future relative
                 * to when we grabbed the current time...
                 *
                 * Just treat this as a busy thread since it must have just blocked.
                 */
                return true;
        }
        return (now - lastblocked_ts) < wq_stalled_window.abstime;
}

static void
workq_add_new_threads_call(void *_p, void *flags)
{
        proc_t p = _p;
        struct workqueue *wq = proc_get_wqptr(p);
        uint32_t my_flag = (uint32_t)(uintptr_t)flags;

        /*
         * workq_exit() will set the workqueue to NULL before
         * it cancels thread calls.
         */
        if (!wq) {
                return;
        }

        assert((my_flag == WQ_DELAYED_CALL_SCHEDULED) ||
            (my_flag == WQ_IMMEDIATE_CALL_SCHEDULED));

        WQ_TRACE_WQ(TRACE_wq_add_timer | DBG_FUNC_START, wq, _wq_flags(wq),
            wq->wq_nthreads, wq->wq_thidlecount, 0);

        workq_lock_spin(wq);

        wq->wq_thread_call_last_run = mach_absolute_time();
        os_atomic_andnot(&wq->wq_flags, my_flag, release);

        /* This can drop the workqueue lock, and take it again */
        workq_schedule_creator(p, wq, WORKQ_THREADREQ_CAN_CREATE_THREADS);

        workq_unlock(wq);

        WQ_TRACE_WQ(TRACE_wq_add_timer | DBG_FUNC_END, wq, 0,
            wq->wq_nthreads, wq->wq_thidlecount, 0);
}

#pragma mark thread state tracking

static void
workq_sched_callback(int type, thread_t thread)
{
        struct uthread *uth = get_bsdthread_info(thread);
        proc_t proc = get_bsdtask_info(get_threadtask(thread));
        struct workqueue *wq = proc_get_wqptr(proc);
        thread_qos_t req_qos, qos = uth->uu_workq_pri.qos_bucket;
        wq_thactive_t old_thactive;
        bool start_timer = false;

        if (qos == WORKQ_THREAD_QOS_MANAGER) {
                return;
        }

        switch (type) {
        case SCHED_CALL_BLOCK:
                old_thactive = _wq_thactive_dec(wq, qos);
                req_qos = WQ_THACTIVE_BEST_CONSTRAINED_REQ_QOS(old_thactive);

                /*
                 * Remember the timestamp of the last thread that blocked in this
                 * bucket, it used used by admission checks to ignore one thread
                 * being inactive if this timestamp is recent enough.
                 *
                 * If we collide with another thread trying to update the
                 * last_blocked (really unlikely since another thread would have to
                 * get scheduled and then block after we start down this path), it's
                 * not a problem.  Either timestamp is adequate, so no need to retry
                 */
                os_atomic_store_wide(&wq->wq_lastblocked_ts[_wq_bucket(qos)],
                    thread_last_run_time(thread), relaxed);

                if (req_qos == THREAD_QOS_UNSPECIFIED) {
                        /*
                         * No pending request at the moment we could unblock, move on.
                         */
                } else if (qos < req_qos) {
                        /*
                         * The blocking thread is at a lower QoS than the highest currently
                         * pending constrained request, nothing has to be redriven
                         */
                } else {
                        uint32_t max_busycount, old_req_count;
                        old_req_count = _wq_thactive_aggregate_downto_qos(wq, old_thactive,
                            req_qos, NULL, &max_busycount);
                        /*
                         * If it is possible that may_start_constrained_thread had refused
                         * admission due to being over the max concurrency, we may need to
                         * spin up a new thread.
                         *
                         * We take into account the maximum number of busy threads
                         * that can affect may_start_constrained_thread as looking at the
                         * actual number may_start_constrained_thread will see is racy.
                         *
                         * IOW at NCPU = 4, for IN (req_qos = 1), if the old req count is
                         * between NCPU (4) and NCPU - 2 (2) we need to redrive.
                         */
                        uint32_t conc = wq_max_parallelism[_wq_bucket(qos)];
                        if (old_req_count <= conc && conc <= old_req_count + max_busycount) {
                                start_timer = workq_schedule_delayed_thread_creation(wq, 0);
                        }
                }
                if (__improbable(kdebug_enable)) {
                        __unused uint32_t old = _wq_thactive_aggregate_downto_qos(wq,
                            old_thactive, qos, NULL, NULL);
                        WQ_TRACE_WQ(TRACE_wq_thread_block | DBG_FUNC_START, wq,
                            old - 1, qos | (req_qos << 8),
                            wq->wq_reqcount << 1 | start_timer, 0);
                }
                break;

        case SCHED_CALL_UNBLOCK:
                /*
                 * we cannot take the workqueue_lock here...
                 * an UNBLOCK can occur from a timer event which
                 * is run from an interrupt context... if the workqueue_lock
                 * is already held by this processor, we'll deadlock...
                 * the thread lock for the thread being UNBLOCKED
                 * is also held
                 */
                old_thactive = _wq_thactive_inc(wq, qos);
                if (__improbable(kdebug_enable)) {
                        __unused uint32_t old = _wq_thactive_aggregate_downto_qos(wq,
                            old_thactive, qos, NULL, NULL);
                        req_qos = WQ_THACTIVE_BEST_CONSTRAINED_REQ_QOS(old_thactive);
                        WQ_TRACE_WQ(TRACE_wq_thread_block | DBG_FUNC_END, wq,
                            old + 1, qos | (req_qos << 8),
                            wq->wq_threads_scheduled, 0);
                }
                break;
        }
}

#pragma mark workq lifecycle

void
workq_reference(struct workqueue *wq)
{
        os_ref_retain(&wq->wq_refcnt);
}

static void
workq_deallocate_queue_invoke(mpsc_queue_chain_t e,
    __assert_only mpsc_daemon_queue_t dq)
{
        struct workqueue *wq;
        struct turnstile *ts;

        wq = mpsc_queue_element(e, struct workqueue, wq_destroy_link);
        assert(dq == &workq_deallocate_queue);

        turnstile_complete((uintptr_t)wq, &wq->wq_turnstile, &ts, TURNSTILE_WORKQS);
        assert(ts);
        turnstile_cleanup();
        turnstile_deallocate(ts);

        lck_spin_destroy(&wq->wq_lock, &workq_lck_grp);
        zfree(workq_zone_workqueue, wq);
}

static void
workq_deallocate(struct workqueue *wq)
{
        if (os_ref_release_relaxed(&wq->wq_refcnt) == 0) {
                workq_deallocate_queue_invoke(&wq->wq_destroy_link,
                    &workq_deallocate_queue);
        }
}

void
workq_deallocate_safe(struct workqueue *wq)
{
        if (__improbable(os_ref_release_relaxed(&wq->wq_refcnt) == 0)) {
                mpsc_daemon_enqueue(&workq_deallocate_queue, &wq->wq_destroy_link,
                    MPSC_QUEUE_DISABLE_PREEMPTION);
        }
}

/**
 * Setup per-process state for the workqueue.
 */
int
workq_open(struct proc *p, __unused struct workq_open_args *uap,
    __unused int32_t *retval)
{
        struct workqueue *wq;
        int error = 0;

        if ((p->p_lflag & P_LREGISTER) == 0) {
                return EINVAL;
        }

        if (wq_init_constrained_limit) {
                uint32_t limit, num_cpus = ml_wait_max_cpus();

                /*
                 * set up the limit for the constrained pool
                 * this is a virtual pool in that we don't
                 * maintain it on a separate idle and run list
                 */
                limit = num_cpus * WORKQUEUE_CONSTRAINED_FACTOR;

                if (limit > wq_max_constrained_threads) {
                        wq_max_constrained_threads = limit;
                }

                if (wq_max_threads > WQ_THACTIVE_BUCKET_HALF) {
                        wq_max_threads = WQ_THACTIVE_BUCKET_HALF;
                }
                if (wq_max_threads > CONFIG_THREAD_MAX - 20) {
                        wq_max_threads = CONFIG_THREAD_MAX - 20;
                }

                wq_death_max_load = (uint16_t)fls(num_cpus) + 1;

                for (thread_qos_t qos = WORKQ_THREAD_QOS_MIN; qos <= WORKQ_THREAD_QOS_MAX; qos++) {
                        wq_max_parallelism[_wq_bucket(qos)] =
                            qos_max_parallelism(qos, QOS_PARALLELISM_COUNT_LOGICAL);
                }

                wq_init_constrained_limit = 0;
        }

        if (proc_get_wqptr(p) == NULL) {
                if (proc_init_wqptr_or_wait(p) == FALSE) {
                        assert(proc_get_wqptr(p) != NULL);
                        goto out;
                }

                wq = (struct workqueue *)zalloc(workq_zone_workqueue);
                bzero(wq, sizeof(struct workqueue));

                os_ref_init_count(&wq->wq_refcnt, &workq_refgrp, 1);

                // Start the event manager at the priority hinted at by the policy engine
                thread_qos_t mgr_priority_hint = task_get_default_manager_qos(current_task());
                pthread_priority_t pp = _pthread_priority_make_from_thread_qos(mgr_priority_hint, 0, 0);
                wq->wq_event_manager_priority = (uint32_t)pp;
                wq->wq_timer_interval = (uint32_t)wq_stalled_window.abstime;
                wq->wq_proc = p;
                turnstile_prepare((uintptr_t)wq, &wq->wq_turnstile, turnstile_alloc(),
                    TURNSTILE_WORKQS);

                TAILQ_INIT(&wq->wq_thrunlist);
                TAILQ_INIT(&wq->wq_thnewlist);
                TAILQ_INIT(&wq->wq_thidlelist);
                priority_queue_init(&wq->wq_overcommit_queue);
                priority_queue_init(&wq->wq_constrained_queue);
                priority_queue_init(&wq->wq_special_queue);

                wq->wq_delayed_call = thread_call_allocate_with_options(
                        workq_add_new_threads_call, p, THREAD_CALL_PRIORITY_KERNEL,
                        THREAD_CALL_OPTIONS_ONCE);
                wq->wq_immediate_call = thread_call_allocate_with_options(
                        workq_add_new_threads_call, p, THREAD_CALL_PRIORITY_KERNEL,
                        THREAD_CALL_OPTIONS_ONCE);
                wq->wq_death_call = thread_call_allocate_with_options(
                        workq_kill_old_threads_call, wq,
                        THREAD_CALL_PRIORITY_USER, THREAD_CALL_OPTIONS_ONCE);

                lck_spin_init(&wq->wq_lock, &workq_lck_grp, LCK_ATTR_NULL);

                WQ_TRACE_WQ(TRACE_wq_create | DBG_FUNC_NONE, wq,
                    VM_KERNEL_ADDRHIDE(wq), 0, 0, 0);
                proc_set_wqptr(p, wq);
        }
out:

        return error;
}

/*
 * Routine:     workq_mark_exiting
 *
 * Function:    Mark the work queue such that new threads will not be added to the
 *              work queue after we return.
 *
 * Conditions:  Called against the current process.
 */
void
workq_mark_exiting(struct proc *p)
{
        struct workqueue *wq = proc_get_wqptr(p);
        uint32_t wq_flags;
        workq_threadreq_t mgr_req;

        if (!wq) {
                return;
        }

        WQ_TRACE_WQ(TRACE_wq_pthread_exit | DBG_FUNC_START, wq, 0, 0, 0, 0);

        workq_lock_spin(wq);

        wq_flags = os_atomic_or_orig(&wq->wq_flags, WQ_EXITING, relaxed);
        if (__improbable(wq_flags & WQ_EXITING)) {
                panic("workq_mark_exiting called twice");
        }

        /*
         * Opportunistically try to cancel thread calls that are likely in flight.
         * workq_exit() will do the proper cleanup.
         */
        if (wq_flags & WQ_IMMEDIATE_CALL_SCHEDULED) {
                thread_call_cancel(wq->wq_immediate_call);
        }
        if (wq_flags & WQ_DELAYED_CALL_SCHEDULED) {
                thread_call_cancel(wq->wq_delayed_call);
        }
        if (wq_flags & WQ_DEATH_CALL_SCHEDULED) {
                thread_call_cancel(wq->wq_death_call);
        }

        mgr_req = wq->wq_event_manager_threadreq;
        wq->wq_event_manager_threadreq = NULL;
        wq->wq_reqcount = 0; /* workq_schedule_creator must not look at queues */
        wq->wq_creator = NULL;
        workq_turnstile_update_inheritor(wq, TURNSTILE_INHERITOR_NULL, 0);

        workq_unlock(wq);

        if (mgr_req) {
                kqueue_threadreq_cancel(p, mgr_req);
        }
        /*
         * No one touches the priority queues once WQ_EXITING is set.
         * It is hence safe to do the tear down without holding any lock.
         */
        priority_queue_destroy(&wq->wq_overcommit_queue,
            struct workq_threadreq_s, tr_entry, ^(workq_threadreq_t e){
                workq_threadreq_destroy(p, e);
        });
        priority_queue_destroy(&wq->wq_constrained_queue,
            struct workq_threadreq_s, tr_entry, ^(workq_threadreq_t e){
                workq_threadreq_destroy(p, e);
        });
        priority_queue_destroy(&wq->wq_special_queue,
            struct workq_threadreq_s, tr_entry, ^(workq_threadreq_t e){
                workq_threadreq_destroy(p, e);
        });

        WQ_TRACE(TRACE_wq_pthread_exit | DBG_FUNC_END, 0, 0, 0, 0, 0);
}

/*
 * Routine:     workq_exit
 *
 * Function:    clean up the work queue structure(s) now that there are no threads
 *              left running inside the work queue (except possibly current_thread).
 *
 * Conditions:  Called by the last thread in the process.
 *              Called against current process.
 */
void
workq_exit(struct proc *p)
{
        struct workqueue *wq;
        struct uthread *uth, *tmp;

        wq = os_atomic_xchg(&p->p_wqptr, NULL, relaxed);
        if (wq != NULL) {
                thread_t th = current_thread();

                WQ_TRACE_WQ(TRACE_wq_workqueue_exit | DBG_FUNC_START, wq, 0, 0, 0, 0);

                if (thread_get_tag(th) & THREAD_TAG_WORKQUEUE) {
                        /*
                         * <rdar://problem/40111515> Make sure we will no longer call the
                         * sched call, if we ever block this thread, which the cancel_wait
                         * below can do.
                         */
                        thread_sched_call(th, NULL);
                }

                /*
                 * Thread calls are always scheduled by the proc itself or under the
                 * workqueue spinlock if WQ_EXITING is not yet set.
                 *
                 * Either way, when this runs, the proc has no threads left beside
                 * the one running this very code, so we know no thread call can be
                 * dispatched anymore.
                 */
                thread_call_cancel_wait(wq->wq_delayed_call);
                thread_call_cancel_wait(wq->wq_immediate_call);
                thread_call_cancel_wait(wq->wq_death_call);
                thread_call_free(wq->wq_delayed_call);
                thread_call_free(wq->wq_immediate_call);
                thread_call_free(wq->wq_death_call);

                /*
                 * Clean up workqueue data structures for threads that exited and
                 * didn't get a chance to clean up after themselves.
                 *
                 * idle/new threads should have been interrupted and died on their own
                 */
                TAILQ_FOREACH_SAFE(uth, &wq->wq_thrunlist, uu_workq_entry, tmp) {
                        thread_sched_call(uth->uu_thread, NULL);
                        thread_deallocate(uth->uu_thread);
                }
                assert(TAILQ_EMPTY(&wq->wq_thnewlist));
                assert(TAILQ_EMPTY(&wq->wq_thidlelist));

                WQ_TRACE_WQ(TRACE_wq_destroy | DBG_FUNC_END, wq,
                    VM_KERNEL_ADDRHIDE(wq), 0, 0, 0);

                workq_deallocate(wq);

                WQ_TRACE(TRACE_wq_workqueue_exit | DBG_FUNC_END, 0, 0, 0, 0, 0);
        }
}


#pragma mark bsd thread control

static bool
_pthread_priority_to_policy(pthread_priority_t priority,
    thread_qos_policy_data_t *data)
{
        data->qos_tier = _pthread_priority_thread_qos(priority);
        data->tier_importance = _pthread_priority_relpri(priority);
        if (data->qos_tier == THREAD_QOS_UNSPECIFIED || data->tier_importance > 0 ||
            data->tier_importance < THREAD_QOS_MIN_TIER_IMPORTANCE) {
                return false;
        }
        return true;
}

static int
bsdthread_set_self(proc_t p, thread_t th, pthread_priority_t priority,
    mach_port_name_t voucher, enum workq_set_self_flags flags)
{
        struct uthread *uth = get_bsdthread_info(th);
        struct workqueue *wq = proc_get_wqptr(p);

        kern_return_t kr;
        int unbind_rv = 0, qos_rv = 0, voucher_rv = 0, fixedpri_rv = 0;
        bool is_wq_thread = (thread_get_tag(th) & THREAD_TAG_WORKQUEUE);

        if (flags & WORKQ_SET_SELF_WQ_KEVENT_UNBIND) {
                if (!is_wq_thread) {
                        unbind_rv = EINVAL;
                        goto qos;
                }

                if (uth->uu_workq_pri.qos_bucket == WORKQ_THREAD_QOS_MANAGER) {
                        unbind_rv = EINVAL;
                        goto qos;
                }

                workq_threadreq_t kqr = uth->uu_kqr_bound;
                if (kqr == NULL) {
                        unbind_rv = EALREADY;
                        goto qos;
                }

                if (kqr->tr_flags & WORKQ_TR_FLAG_WORKLOOP) {
                        unbind_rv = EINVAL;
                        goto qos;
                }

                kqueue_threadreq_unbind(p, kqr);
        }

qos:
        if (flags & WORKQ_SET_SELF_QOS_FLAG) {
                thread_qos_policy_data_t new_policy;

                if (!_pthread_priority_to_policy(priority, &new_policy)) {
                        qos_rv = EINVAL;
                        goto voucher;
                }

                if (!is_wq_thread) {
                        /*
                         * Threads opted out of QoS can't change QoS
                         */
                        if (!thread_has_qos_policy(th)) {
                                qos_rv = EPERM;
                                goto voucher;
                        }
                } else if (uth->uu_workq_pri.qos_bucket == WORKQ_THREAD_QOS_MANAGER ||
                    uth->uu_workq_pri.qos_bucket == WORKQ_THREAD_QOS_ABOVEUI) {
                        /*
                         * Workqueue manager threads or threads above UI can't change QoS
                         */
                        qos_rv = EINVAL;
                        goto voucher;
                } else {
                        /*
                         * For workqueue threads, possibly adjust buckets and redrive thread
                         * requests.
                         */
                        bool old_overcommit = uth->uu_workq_flags & UT_WORKQ_OVERCOMMIT;
                        bool new_overcommit = priority & _PTHREAD_PRIORITY_OVERCOMMIT_FLAG;
                        struct uu_workq_policy old_pri, new_pri;
                        bool force_run = false;

                        workq_lock_spin(wq);

                        if (old_overcommit != new_overcommit) {
                                uth->uu_workq_flags ^= UT_WORKQ_OVERCOMMIT;
                                if (old_overcommit) {
                                        wq->wq_constrained_threads_scheduled++;
                                } else if (wq->wq_constrained_threads_scheduled-- ==
                                    wq_max_constrained_threads) {
                                        force_run = true;
                                }
                        }

                        old_pri = new_pri = uth->uu_workq_pri;
                        new_pri.qos_req = (thread_qos_t)new_policy.qos_tier;
                        workq_thread_update_bucket(p, wq, uth, old_pri, new_pri, force_run);
                        workq_unlock(wq);
                }

                kr = thread_policy_set_internal(th, THREAD_QOS_POLICY,
                    (thread_policy_t)&new_policy, THREAD_QOS_POLICY_COUNT);
                if (kr != KERN_SUCCESS) {
                        qos_rv = EINVAL;
                }
        }

voucher:
        if (flags & WORKQ_SET_SELF_VOUCHER_FLAG) {
                kr = thread_set_voucher_name(voucher);
                if (kr != KERN_SUCCESS) {
                        voucher_rv = ENOENT;
                        goto fixedpri;
                }
        }

fixedpri:
        if (qos_rv) {
                goto done;
        }
        if (flags & WORKQ_SET_SELF_FIXEDPRIORITY_FLAG) {
                thread_extended_policy_data_t extpol = {.timeshare = 0};

                if (is_wq_thread) {
                        /* Not allowed on workqueue threads */
                        fixedpri_rv = ENOTSUP;
                        goto done;
                }

                kr = thread_policy_set_internal(th, THREAD_EXTENDED_POLICY,
                    (thread_policy_t)&extpol, THREAD_EXTENDED_POLICY_COUNT);
                if (kr != KERN_SUCCESS) {
                        fixedpri_rv = EINVAL;
                        goto done;
                }
        } else if (flags & WORKQ_SET_SELF_TIMESHARE_FLAG) {
                thread_extended_policy_data_t extpol = {.timeshare = 1};

                if (is_wq_thread) {
                        /* Not allowed on workqueue threads */
                        fixedpri_rv = ENOTSUP;
                        goto done;
                }

                kr = thread_policy_set_internal(th, THREAD_EXTENDED_POLICY,
                    (thread_policy_t)&extpol, THREAD_EXTENDED_POLICY_COUNT);
                if (kr != KERN_SUCCESS) {
                        fixedpri_rv = EINVAL;
                        goto done;
                }
        }

done:
        if (qos_rv && voucher_rv) {
                /* Both failed, give that a unique error. */
                return EBADMSG;
        }

        if (unbind_rv) {
                return unbind_rv;
        }

        if (qos_rv) {
                return qos_rv;
        }

        if (voucher_rv) {
                return voucher_rv;
        }

        if (fixedpri_rv) {
                return fixedpri_rv;
        }


        return 0;
}

static int
bsdthread_add_explicit_override(proc_t p, mach_port_name_t kport,
    pthread_priority_t pp, user_addr_t resource)
{
        thread_qos_t qos = _pthread_priority_thread_qos(pp);
        if (qos == THREAD_QOS_UNSPECIFIED) {
                return EINVAL;
        }

        thread_t th = port_name_to_thread(kport,
            PORT_TO_THREAD_IN_CURRENT_TASK);
        if (th == THREAD_NULL) {
                return ESRCH;
        }

        int rv = proc_thread_qos_add_override(p->task, th, 0, qos, TRUE,
            resource, THREAD_QOS_OVERRIDE_TYPE_PTHREAD_EXPLICIT_OVERRIDE);

        thread_deallocate(th);
        return rv;
}

static int
bsdthread_remove_explicit_override(proc_t p, mach_port_name_t kport,
    user_addr_t resource)
{
        thread_t th = port_name_to_thread(kport,
            PORT_TO_THREAD_IN_CURRENT_TASK);
        if (th == THREAD_NULL) {
                return ESRCH;
        }

        int rv = proc_thread_qos_remove_override(p->task, th, 0, resource,
            THREAD_QOS_OVERRIDE_TYPE_PTHREAD_EXPLICIT_OVERRIDE);

        thread_deallocate(th);
        return rv;
}

static int
workq_thread_add_dispatch_override(proc_t p, mach_port_name_t kport,
    pthread_priority_t pp, user_addr_t ulock_addr)
{
        struct uu_workq_policy old_pri, new_pri;
        struct workqueue *wq = proc_get_wqptr(p);

        thread_qos_t qos_override = _pthread_priority_thread_qos(pp);
        if (qos_override == THREAD_QOS_UNSPECIFIED) {
                return EINVAL;
        }

        thread_t thread = port_name_to_thread(kport,
            PORT_TO_THREAD_IN_CURRENT_TASK);
        if (thread == THREAD_NULL) {
                return ESRCH;
        }

        struct uthread *uth = get_bsdthread_info(thread);
        if ((thread_get_tag(thread) & THREAD_TAG_WORKQUEUE) == 0) {
                thread_deallocate(thread);
                return EPERM;
        }

        WQ_TRACE_WQ(TRACE_wq_override_dispatch | DBG_FUNC_NONE,
            wq, thread_tid(thread), 1, pp, 0);

        thread_mtx_lock(thread);

        if (ulock_addr) {
                uint32_t val;
                int rc;
                /*
                 * Workaround lack of explicit support for 'no-fault copyin'
                 * <rdar://problem/24999882>, as disabling preemption prevents paging in
                 */
                disable_preemption();
                rc = copyin_atomic32(ulock_addr, &val);
                enable_preemption();
                if (rc == 0 && ulock_owner_value_to_port_name(val) != kport) {
                        goto out;
                }
        }

        workq_lock_spin(wq);

        old_pri = uth->uu_workq_pri;
        if (old_pri.qos_override >= qos_override) {
                /* Nothing to do */
        } else if (thread == current_thread()) {
                new_pri = old_pri;
                new_pri.qos_override = qos_override;
                workq_thread_update_bucket(p, wq, uth, old_pri, new_pri, false);
        } else {
                uth->uu_workq_pri.qos_override = qos_override;
                if (qos_override > workq_pri_override(old_pri)) {
                        thread_set_workq_override(thread, qos_override);
                }
        }

        workq_unlock(wq);

out:
        thread_mtx_unlock(thread);
        thread_deallocate(thread);
        return 0;
}

static int
workq_thread_reset_dispatch_override(proc_t p, thread_t thread)
{
        struct uu_workq_policy old_pri, new_pri;
        struct workqueue *wq = proc_get_wqptr(p);
        struct uthread *uth = get_bsdthread_info(thread);

        if ((thread_get_tag(thread) & THREAD_TAG_WORKQUEUE) == 0) {
                return EPERM;
        }

        WQ_TRACE_WQ(TRACE_wq_override_reset | DBG_FUNC_NONE, wq, 0, 0, 0, 0);

        workq_lock_spin(wq);
        old_pri = new_pri = uth->uu_workq_pri;
        new_pri.qos_override = THREAD_QOS_UNSPECIFIED;
        workq_thread_update_bucket(p, wq, uth, old_pri, new_pri, false);
        workq_unlock(wq);
        return 0;
}

static int
workq_thread_allow_kill(__unused proc_t p, thread_t thread, bool enable)
{
        if (!(thread_get_tag(thread) & THREAD_TAG_WORKQUEUE)) {
                // If the thread isn't a workqueue thread, don't set the
                // kill_allowed bit; however, we still need to return 0
                // instead of an error code since this code is executed
                // on the abort path which needs to not depend on the
                // pthread_t (returning an error depends on pthread_t via
                // cerror_nocancel)
                return 0;
        }
        struct uthread *uth = get_bsdthread_info(thread);
        uth->uu_workq_pthread_kill_allowed = enable;
        return 0;
}

static int
bsdthread_get_max_parallelism(thread_qos_t qos, unsigned long flags,
    int *retval)
{
        static_assert(QOS_PARALLELISM_COUNT_LOGICAL ==
            _PTHREAD_QOS_PARALLELISM_COUNT_LOGICAL, "logical");
        static_assert(QOS_PARALLELISM_REALTIME ==
            _PTHREAD_QOS_PARALLELISM_REALTIME, "realtime");

        if (flags & ~(QOS_PARALLELISM_REALTIME | QOS_PARALLELISM_COUNT_LOGICAL)) {
                return EINVAL;
        }

        if (flags & QOS_PARALLELISM_REALTIME) {
                if (qos) {
                        return EINVAL;
                }
        } else if (qos == THREAD_QOS_UNSPECIFIED || qos >= THREAD_QOS_LAST) {
                return EINVAL;
        }

        *retval = qos_max_parallelism(qos, flags);
        return 0;
}

#define ENSURE_UNUSED(arg) \
                ({ if ((arg) != 0) { return EINVAL; } })

int
bsdthread_ctl(struct proc *p, struct bsdthread_ctl_args *uap, int *retval)
{
        switch (uap->cmd) {
        case BSDTHREAD_CTL_QOS_OVERRIDE_START:
                return bsdthread_add_explicit_override(p, (mach_port_name_t)uap->arg1,
                           (pthread_priority_t)uap->arg2, uap->arg3);
        case BSDTHREAD_CTL_QOS_OVERRIDE_END:
                ENSURE_UNUSED(uap->arg3);
                return bsdthread_remove_explicit_override(p, (mach_port_name_t)uap->arg1,
                           (user_addr_t)uap->arg2);

        case BSDTHREAD_CTL_QOS_OVERRIDE_DISPATCH:
                return workq_thread_add_dispatch_override(p, (mach_port_name_t)uap->arg1,
                           (pthread_priority_t)uap->arg2, uap->arg3);
        case BSDTHREAD_CTL_QOS_OVERRIDE_RESET:
                return workq_thread_reset_dispatch_override(p, current_thread());

        case BSDTHREAD_CTL_SET_SELF:
                return bsdthread_set_self(p, current_thread(),
                           (pthread_priority_t)uap->arg1, (mach_port_name_t)uap->arg2,
                           (enum workq_set_self_flags)uap->arg3);

        case BSDTHREAD_CTL_QOS_MAX_PARALLELISM:
                ENSURE_UNUSED(uap->arg3);
                return bsdthread_get_max_parallelism((thread_qos_t)uap->arg1,
                           (unsigned long)uap->arg2, retval);
        case BSDTHREAD_CTL_WORKQ_ALLOW_KILL:
                ENSURE_UNUSED(uap->arg2);
                ENSURE_UNUSED(uap->arg3);
                return workq_thread_allow_kill(p, current_thread(), (bool)uap->arg1);

        case BSDTHREAD_CTL_SET_QOS:
        case BSDTHREAD_CTL_QOS_DISPATCH_ASYNCHRONOUS_OVERRIDE_ADD:
        case BSDTHREAD_CTL_QOS_DISPATCH_ASYNCHRONOUS_OVERRIDE_RESET:
                /* no longer supported */
                return ENOTSUP;

        default:
                return EINVAL;
        }
}

#pragma mark workqueue thread manipulation

static void __dead2
workq_unpark_select_threadreq_or_park_and_unlock(proc_t p, struct workqueue *wq,
    struct uthread *uth, uint32_t setup_flags);

static void __dead2
workq_select_threadreq_or_park_and_unlock(proc_t p, struct workqueue *wq,
    struct uthread *uth, uint32_t setup_flags);

static void workq_setup_and_run(proc_t p, struct uthread *uth, int flags) __dead2;

#if KDEBUG_LEVEL >= KDEBUG_LEVEL_STANDARD
static inline uint64_t
workq_trace_req_id(workq_threadreq_t req)
{
        struct kqworkloop *kqwl;
        if (req->tr_flags & WORKQ_TR_FLAG_WORKLOOP) {
                kqwl = __container_of(req, struct kqworkloop, kqwl_request);
                return kqwl->kqwl_dynamicid;
        }

        return VM_KERNEL_ADDRHIDE(req);
}
#endif

/**
 * Entry point for libdispatch to ask for threads
 */
static int
workq_reqthreads(struct proc *p, uint32_t reqcount, pthread_priority_t pp)
{
        thread_qos_t qos = _pthread_priority_thread_qos(pp);
        struct workqueue *wq = proc_get_wqptr(p);
        uint32_t unpaced, upcall_flags = WQ_FLAG_THREAD_NEWSPI;

        if (wq == NULL || reqcount <= 0 || reqcount > UINT16_MAX ||
            qos == THREAD_QOS_UNSPECIFIED) {
                return EINVAL;
        }

        WQ_TRACE_WQ(TRACE_wq_wqops_reqthreads | DBG_FUNC_NONE,
            wq, reqcount, pp, 0, 0);

        workq_threadreq_t req = zalloc(workq_zone_threadreq);
        priority_queue_entry_init(&req->tr_entry);
        req->tr_state = WORKQ_TR_STATE_NEW;
        req->tr_flags = 0;
        req->tr_qos   = qos;

        if (pp & _PTHREAD_PRIORITY_OVERCOMMIT_FLAG) {
                req->tr_flags |= WORKQ_TR_FLAG_OVERCOMMIT;
                upcall_flags |= WQ_FLAG_THREAD_OVERCOMMIT;
        }

        WQ_TRACE_WQ(TRACE_wq_thread_request_initiate | DBG_FUNC_NONE,
            wq, workq_trace_req_id(req), req->tr_qos, reqcount, 0);

        workq_lock_spin(wq);
        do {
                if (_wq_exiting(wq)) {
                        goto exiting;
                }

                /*
                 * When userspace is asking for parallelism, wakeup up to (reqcount - 1)
                 * threads without pacing, to inform the scheduler of that workload.
                 *
                 * The last requests, or the ones that failed the admission checks are
                 * enqueued and go through the regular creator codepath.
                 *
                 * If there aren't enough threads, add one, but re-evaluate everything
                 * as conditions may now have changed.
                 */
                if (reqcount > 1 && (req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) == 0) {
                        unpaced = workq_constrained_allowance(wq, qos, NULL, false);
                        if (unpaced >= reqcount - 1) {
                                unpaced = reqcount - 1;
                        }
                } else {
                        unpaced = reqcount - 1;
                }

                /*
                 * This path does not currently handle custom workloop parameters
                 * when creating threads for parallelism.
                 */
                assert(!(req->tr_flags & WORKQ_TR_FLAG_WL_PARAMS));

                /*
                 * This is a trimmed down version of workq_threadreq_bind_and_unlock()
                 */
                while (unpaced > 0 && wq->wq_thidlecount) {
                        struct uthread *uth;
                        bool needs_wakeup;
                        uint8_t uu_flags = UT_WORKQ_EARLY_BOUND;

                        if (req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) {
                                uu_flags |= UT_WORKQ_OVERCOMMIT;
                        }

                        uth = workq_pop_idle_thread(wq, uu_flags, &needs_wakeup);

                        _wq_thactive_inc(wq, qos);
                        wq->wq_thscheduled_count[_wq_bucket(qos)]++;
                        workq_thread_reset_pri(wq, uth, req, /*unpark*/ true);
                        wq->wq_fulfilled++;

                        uth->uu_save.uus_workq_park_data.upcall_flags = upcall_flags;
                        uth->uu_save.uus_workq_park_data.thread_request = req;
                        if (needs_wakeup) {
                                workq_thread_wakeup(uth);
                        }
                        unpaced--;
                        reqcount--;
                }
        } while (unpaced && wq->wq_nthreads < wq_max_threads &&
            workq_add_new_idle_thread(p, wq));

        if (_wq_exiting(wq)) {
                goto exiting;
        }

        req->tr_count = (uint16_t)reqcount;
        if (workq_threadreq_enqueue(wq, req)) {
                /* This can drop the workqueue lock, and take it again */
                workq_schedule_creator(p, wq, WORKQ_THREADREQ_CAN_CREATE_THREADS);
        }
        workq_unlock(wq);
        return 0;

exiting:
        workq_unlock(wq);
        zfree(workq_zone_threadreq, req);
        return ECANCELED;
}

bool
workq_kern_threadreq_initiate(struct proc *p, workq_threadreq_t req,
    struct turnstile *workloop_ts, thread_qos_t qos,
    workq_kern_threadreq_flags_t flags)
{
        struct workqueue *wq = proc_get_wqptr_fast(p);
        struct uthread *uth = NULL;

        assert(req->tr_flags & (WORKQ_TR_FLAG_WORKLOOP | WORKQ_TR_FLAG_KEVENT));

        if (req->tr_flags & WORKQ_TR_FLAG_WL_OUTSIDE_QOS) {
                workq_threadreq_param_t trp = kqueue_threadreq_workloop_param(req);
                qos = thread_workq_qos_for_pri(trp.trp_pri);
                if (qos == THREAD_QOS_UNSPECIFIED) {
                        qos = WORKQ_THREAD_QOS_ABOVEUI;
                }
        }

        assert(req->tr_state == WORKQ_TR_STATE_IDLE);
        priority_queue_entry_init(&req->tr_entry);
        req->tr_count = 1;
        req->tr_state = WORKQ_TR_STATE_NEW;
        req->tr_qos   = qos;

        WQ_TRACE_WQ(TRACE_wq_thread_request_initiate | DBG_FUNC_NONE, wq,
            workq_trace_req_id(req), qos, 1, 0);

        if (flags & WORKQ_THREADREQ_ATTEMPT_REBIND) {
                /*
                 * we're called back synchronously from the context of
                 * kqueue_threadreq_unbind from within workq_thread_return()
                 * we can try to match up this thread with this request !
                 */
                uth = current_uthread();
                assert(uth->uu_kqr_bound == NULL);
        }

        workq_lock_spin(wq);
        if (_wq_exiting(wq)) {
                req->tr_state = WORKQ_TR_STATE_IDLE;
                workq_unlock(wq);
                return false;
        }

        if (uth && workq_threadreq_admissible(wq, uth, req)) {
                assert(uth != wq->wq_creator);
                if (uth->uu_workq_pri.qos_bucket != req->tr_qos) {
                        _wq_thactive_move(wq, uth->uu_workq_pri.qos_bucket, req->tr_qos);
                        workq_thread_reset_pri(wq, uth, req, /*unpark*/ false);
                }
                /*
                 * We're called from workq_kern_threadreq_initiate()
                 * due to an unbind, with the kq req held.
                 */
                WQ_TRACE_WQ(TRACE_wq_thread_logical_run | DBG_FUNC_START, wq,
                    workq_trace_req_id(req), 0, 0, 0);
                wq->wq_fulfilled++;
                kqueue_threadreq_bind(p, req, uth->uu_thread, 0);
        } else {
                if (workloop_ts) {
                        workq_perform_turnstile_operation_locked(wq, ^{
                                turnstile_update_inheritor(workloop_ts, wq->wq_turnstile,
                                TURNSTILE_IMMEDIATE_UPDATE | TURNSTILE_INHERITOR_TURNSTILE);
                                turnstile_update_inheritor_complete(workloop_ts,
                                TURNSTILE_INTERLOCK_HELD);
                        });
                }
                if (workq_threadreq_enqueue(wq, req)) {
                        workq_schedule_creator(p, wq, flags);
                }
        }

        workq_unlock(wq);

        return true;
}

void
workq_kern_threadreq_modify(struct proc *p, workq_threadreq_t req,
    thread_qos_t qos, workq_kern_threadreq_flags_t flags)
{
        struct workqueue *wq = proc_get_wqptr_fast(p);
        bool make_overcommit = false;

        if (req->tr_flags & WORKQ_TR_FLAG_WL_OUTSIDE_QOS) {
                /* Requests outside-of-QoS shouldn't accept modify operations */
                return;
        }

        workq_lock_spin(wq);

        assert(req->tr_qos != WORKQ_THREAD_QOS_MANAGER);
        assert(req->tr_flags & (WORKQ_TR_FLAG_KEVENT | WORKQ_TR_FLAG_WORKLOOP));

        if (req->tr_state == WORKQ_TR_STATE_BINDING) {
                kqueue_threadreq_bind(p, req, req->tr_thread, 0);
                workq_unlock(wq);
                return;
        }

        if (flags & WORKQ_THREADREQ_MAKE_OVERCOMMIT) {
                make_overcommit = (req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) == 0;
        }

        if (_wq_exiting(wq) || (req->tr_qos == qos && !make_overcommit)) {
                workq_unlock(wq);
                return;
        }

        assert(req->tr_count == 1);
        if (req->tr_state != WORKQ_TR_STATE_QUEUED) {
                panic("Invalid thread request (%p) state %d", req, req->tr_state);
        }

        WQ_TRACE_WQ(TRACE_wq_thread_request_modify | DBG_FUNC_NONE, wq,
            workq_trace_req_id(req), qos, 0, 0);

        struct priority_queue_sched_max *pq = workq_priority_queue_for_req(wq, req);
        workq_threadreq_t req_max;

        /*
         * Stage 1: Dequeue the request from its priority queue.
         *
         * If we dequeue the root item of the constrained priority queue,
         * maintain the best constrained request qos invariant.
         */
        if (priority_queue_remove(pq, &req->tr_entry)) {
                if ((req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) == 0) {
                        _wq_thactive_refresh_best_constrained_req_qos(wq);
                }
        }

        /*
         * Stage 2: Apply changes to the thread request
         *
         * If the item will not become the root of the priority queue it belongs to,
         * then we need to wait in line, just enqueue and return quickly.
         */
        if (__improbable(make_overcommit)) {
                req->tr_flags ^= WORKQ_TR_FLAG_OVERCOMMIT;
                pq = workq_priority_queue_for_req(wq, req);
        }
        req->tr_qos = qos;

        req_max = priority_queue_max(pq, struct workq_threadreq_s, tr_entry);
        if (req_max && req_max->tr_qos >= qos) {
                priority_queue_entry_set_sched_pri(pq, &req->tr_entry,
                    workq_priority_for_req(req), false);
                priority_queue_insert(pq, &req->tr_entry);
                workq_unlock(wq);
                return;
        }

        /*
         * Stage 3: Reevaluate whether we should run the thread request.
         *
         * Pretend the thread request is new again:
         * - adjust wq_reqcount to not count it anymore.
         * - make its state WORKQ_TR_STATE_NEW (so that workq_threadreq_bind_and_unlock
         *   properly attempts a synchronous bind)
         */
        wq->wq_reqcount--;
        req->tr_state = WORKQ_TR_STATE_NEW;
        if (workq_threadreq_enqueue(wq, req)) {
                workq_schedule_creator(p, wq, flags);
        }
        workq_unlock(wq);
}

void
workq_kern_threadreq_lock(struct proc *p)
{
        workq_lock_spin(proc_get_wqptr_fast(p));
}

void
workq_kern_threadreq_unlock(struct proc *p)
{
        workq_unlock(proc_get_wqptr_fast(p));
}

void
workq_kern_threadreq_update_inheritor(struct proc *p, workq_threadreq_t req,
    thread_t owner, struct turnstile *wl_ts,
    turnstile_update_flags_t flags)
{
        struct workqueue *wq = proc_get_wqptr_fast(p);
        turnstile_inheritor_t inheritor;

        assert(req->tr_qos != WORKQ_THREAD_QOS_MANAGER);
        assert(req->tr_flags & WORKQ_TR_FLAG_WORKLOOP);
        workq_lock_held(wq);

        if (req->tr_state == WORKQ_TR_STATE_BINDING) {
                kqueue_threadreq_bind(p, req, req->tr_thread,
                    KQUEUE_THREADERQ_BIND_NO_INHERITOR_UPDATE);
                return;
        }

        if (_wq_exiting(wq)) {
                inheritor = TURNSTILE_INHERITOR_NULL;
        } else {
                if (req->tr_state != WORKQ_TR_STATE_QUEUED) {
                        panic("Invalid thread request (%p) state %d", req, req->tr_state);
                }

                if (owner) {
                        inheritor = owner;
                        flags |= TURNSTILE_INHERITOR_THREAD;
                } else {
                        inheritor = wq->wq_turnstile;
                        flags |= TURNSTILE_INHERITOR_TURNSTILE;
                }
        }

        workq_perform_turnstile_operation_locked(wq, ^{
                turnstile_update_inheritor(wl_ts, inheritor, flags);
        });
}

void
workq_kern_threadreq_redrive(struct proc *p, workq_kern_threadreq_flags_t flags)
{
        struct workqueue *wq = proc_get_wqptr_fast(p);

        workq_lock_spin(wq);
        workq_schedule_creator(p, wq, flags);
        workq_unlock(wq);
}

void
workq_schedule_creator_turnstile_redrive(struct workqueue *wq, bool locked)
{
        if (locked) {
                workq_schedule_creator(NULL, wq, WORKQ_THREADREQ_NONE);
        } else {
                workq_schedule_immediate_thread_creation(wq);
        }
}

static int
workq_thread_return(struct proc *p, struct workq_kernreturn_args *uap,
    struct workqueue *wq)
{
        thread_t th = current_thread();
        struct uthread *uth = get_bsdthread_info(th);
        workq_threadreq_t kqr = uth->uu_kqr_bound;
        workq_threadreq_param_t trp = { };
        int nevents = uap->affinity, error;
        user_addr_t eventlist = uap->item;

        if (((thread_get_tag(th) & THREAD_TAG_WORKQUEUE) == 0) ||
            (uth->uu_workq_flags & UT_WORKQ_DYING)) {
                return EINVAL;
        }

        if (eventlist && nevents && kqr == NULL) {
                return EINVAL;
        }

        /* reset signal mask on the workqueue thread to default state */
        if (uth->uu_sigmask != (sigset_t)(~workq_threadmask)) {
                proc_lock(p);
                uth->uu_sigmask = ~workq_threadmask;
                proc_unlock(p);
        }

        if (kqr && kqr->tr_flags & WORKQ_TR_FLAG_WL_PARAMS) {
                /*
                 * Ensure we store the threadreq param before unbinding
                 * the kqr from this thread.
                 */
                trp = kqueue_threadreq_workloop_param(kqr);
        }

        /*
         * Freeze thee base pri while we decide the fate of this thread.
         *
         * Either:
         * - we return to user and kevent_cleanup will have unfrozen the base pri,
         * - or we proceed to workq_select_threadreq_or_park_and_unlock() who will.
         */
        thread_freeze_base_pri(th);

        if (kqr) {
                uint32_t upcall_flags = WQ_FLAG_THREAD_NEWSPI | WQ_FLAG_THREAD_REUSE;
                if (kqr->tr_flags & WORKQ_TR_FLAG_WORKLOOP) {
                        upcall_flags |= WQ_FLAG_THREAD_WORKLOOP | WQ_FLAG_THREAD_KEVENT;
                } else {
                        upcall_flags |= WQ_FLAG_THREAD_KEVENT;
                }
                if (uth->uu_workq_pri.qos_bucket == WORKQ_THREAD_QOS_MANAGER) {
                        upcall_flags |= WQ_FLAG_THREAD_EVENT_MANAGER;
                } else {
                        if (uth->uu_workq_flags & UT_WORKQ_OVERCOMMIT) {
                                upcall_flags |= WQ_FLAG_THREAD_OVERCOMMIT;
                        }
                        if (uth->uu_workq_flags & UT_WORKQ_OUTSIDE_QOS) {
                                upcall_flags |= WQ_FLAG_THREAD_OUTSIDEQOS;
                        } else {
                                upcall_flags |= uth->uu_workq_pri.qos_req |
                                    WQ_FLAG_THREAD_PRIO_QOS;
                        }
                }

                error = pthread_functions->workq_handle_stack_events(p, th,
                    get_task_map(p->task), uth->uu_workq_stackaddr,
                    uth->uu_workq_thport, eventlist, nevents, upcall_flags);
                if (error) {
                        assert(uth->uu_kqr_bound == kqr);
                        return error;
                }

                // pthread is supposed to pass KEVENT_FLAG_PARKING here
                // which should cause the above call to either:
                // - not return
                // - return an error
                // - return 0 and have unbound properly
                assert(uth->uu_kqr_bound == NULL);
        }

        WQ_TRACE_WQ(TRACE_wq_runthread | DBG_FUNC_END, wq, uap->options, 0, 0, 0);

        thread_sched_call(th, NULL);
        thread_will_park_or_terminate(th);
#if CONFIG_WORKLOOP_DEBUG
        UU_KEVENT_HISTORY_WRITE_ENTRY(uth, { .uu_error = -1, });
#endif

        workq_lock_spin(wq);
        WQ_TRACE_WQ(TRACE_wq_thread_logical_run | DBG_FUNC_END, wq, 0, 0, 0, 0);
        uth->uu_save.uus_workq_park_data.workloop_params = trp.trp_value;
        workq_select_threadreq_or_park_and_unlock(p, wq, uth,
            WQ_SETUP_CLEAR_VOUCHER);
        __builtin_unreachable();
}

/**
 * Multiplexed call to interact with the workqueue mechanism
 */
int
workq_kernreturn(struct proc *p, struct workq_kernreturn_args *uap, int32_t *retval)
{
        int options = uap->options;
        int arg2 = uap->affinity;
        int arg3 = uap->prio;
        struct workqueue *wq = proc_get_wqptr(p);
        int error = 0;

        if ((p->p_lflag & P_LREGISTER) == 0) {
                return EINVAL;
        }

        switch (options) {
        case WQOPS_QUEUE_NEWSPISUPP: {
                /*
                 * arg2 = offset of serialno into dispatch queue
                 * arg3 = kevent support
                 */
                int offset = arg2;
                if (arg3 & 0x01) {
                        // If we get here, then userspace has indicated support for kevent delivery.
                }

                p->p_dispatchqueue_serialno_offset = (uint64_t)offset;
                break;
        }
        case WQOPS_QUEUE_REQTHREADS: {
                /*
                 * arg2 = number of threads to start
                 * arg3 = priority
                 */
                error = workq_reqthreads(p, arg2, arg3);
                break;
        }
        case WQOPS_SET_EVENT_MANAGER_PRIORITY: {
                /*
                 * arg2 = priority for the manager thread
                 *
                 * if _PTHREAD_PRIORITY_SCHED_PRI_FLAG is set,
                 * the low bits of the value contains a scheduling priority
                 * instead of a QOS value
                 */
                pthread_priority_t pri = arg2;

                if (wq == NULL) {
                        error = EINVAL;
                        break;
                }

                /*
                 * Normalize the incoming priority so that it is ordered numerically.
                 */
                if (pri & _PTHREAD_PRIORITY_SCHED_PRI_FLAG) {
                        pri &= (_PTHREAD_PRIORITY_SCHED_PRI_MASK |
                            _PTHREAD_PRIORITY_SCHED_PRI_FLAG);
                } else {
                        thread_qos_t qos = _pthread_priority_thread_qos(pri);
                        int relpri = _pthread_priority_relpri(pri);
                        if (relpri > 0 || relpri < THREAD_QOS_MIN_TIER_IMPORTANCE ||
                            qos == THREAD_QOS_UNSPECIFIED) {
                                error = EINVAL;
                                break;
                        }
                        pri &= ~_PTHREAD_PRIORITY_FLAGS_MASK;
                }

                /*
                 * If userspace passes a scheduling priority, that wins over any QoS.
                 * Userspace should takes care not to lower the priority this way.
                 */
                workq_lock_spin(wq);
                if (wq->wq_event_manager_priority < (uint32_t)pri) {
                        wq->wq_event_manager_priority = (uint32_t)pri;
                }
                workq_unlock(wq);
                break;
        }
        case WQOPS_THREAD_KEVENT_RETURN:
        case WQOPS_THREAD_WORKLOOP_RETURN:
        case WQOPS_THREAD_RETURN: {
                error = workq_thread_return(p, uap, wq);
                break;
        }

        case WQOPS_SHOULD_NARROW: {
                /*
                 * arg2 = priority to test
                 * arg3 = unused
                 */
                thread_t th = current_thread();
                struct uthread *uth = get_bsdthread_info(th);
                if (((thread_get_tag(th) & THREAD_TAG_WORKQUEUE) == 0) ||
                    (uth->uu_workq_flags & (UT_WORKQ_DYING | UT_WORKQ_OVERCOMMIT))) {
                        error = EINVAL;
                        break;
                }

                thread_qos_t qos = _pthread_priority_thread_qos(arg2);
                if (qos == THREAD_QOS_UNSPECIFIED) {
                        error = EINVAL;
                        break;
                }
                workq_lock_spin(wq);
                bool should_narrow = !workq_constrained_allowance(wq, qos, uth, false);
                workq_unlock(wq);

                *retval = should_narrow;
                break;
        }
        case WQOPS_SETUP_DISPATCH: {
                /*
                 * item = pointer to workq_dispatch_config structure
                 * arg2 = sizeof(item)
                 */
                struct workq_dispatch_config cfg;
                bzero(&cfg, sizeof(cfg));

                error = copyin(uap->item, &cfg, MIN(sizeof(cfg), (unsigned long) arg2));
                if (error) {
                        break;
                }

                if (cfg.wdc_flags & ~WORKQ_DISPATCH_SUPPORTED_FLAGS ||
                    cfg.wdc_version < WORKQ_DISPATCH_MIN_SUPPORTED_VERSION) {
                        error = ENOTSUP;
                        break;
                }

                /* Load fields from version 1 */
                p->p_dispatchqueue_serialno_offset = cfg.wdc_queue_serialno_offs;

                /* Load fields from version 2 */
                if (cfg.wdc_version >= 2) {
                        p->p_dispatchqueue_label_offset = cfg.wdc_queue_label_offs;
                }

                break;
        }
        default:
                error = EINVAL;
                break;
        }

        return error;
}

/*
 * We have no work to do, park ourselves on the idle list.
 *
 * Consumes the workqueue lock and does not return.
 */
__attribute__((noreturn, noinline))
static void
workq_park_and_unlock(proc_t p, struct workqueue *wq, struct uthread *uth,
    uint32_t setup_flags)
{
        assert(uth == current_uthread());
        assert(uth->uu_kqr_bound == NULL);
        workq_push_idle_thread(p, wq, uth, setup_flags); // may not return

        workq_thread_reset_cpupercent(NULL, uth);

        if ((uth->uu_workq_flags & UT_WORKQ_IDLE_CLEANUP) &&
            !(uth->uu_workq_flags & UT_WORKQ_DYING)) {
                workq_unlock(wq);

                /*
                 * workq_push_idle_thread() will unset `has_stack`
                 * if it wants us to free the stack before parking.
                 */
                if (!uth->uu_save.uus_workq_park_data.has_stack) {
                        pthread_functions->workq_markfree_threadstack(p, uth->uu_thread,
                            get_task_map(p->task), uth->uu_workq_stackaddr);
                }

                /*
                 * When we remove the voucher from the thread, we may lose our importance
                 * causing us to get preempted, so we do this after putting the thread on
                 * the idle list.  Then, when we get our importance back we'll be able to
                 * use this thread from e.g. the kevent call out to deliver a boosting
                 * message.
                 */
                __assert_only kern_return_t kr;
                kr = thread_set_voucher_name(MACH_PORT_NULL);
                assert(kr == KERN_SUCCESS);

                workq_lock_spin(wq);
                uth->uu_workq_flags &= ~UT_WORKQ_IDLE_CLEANUP;
                setup_flags &= ~WQ_SETUP_CLEAR_VOUCHER;
        }

        WQ_TRACE_WQ(TRACE_wq_thread_logical_run | DBG_FUNC_END, wq, 0, 0, 0, 0);

        if (uth->uu_workq_flags & UT_WORKQ_RUNNING) {
                /*
                 * While we'd dropped the lock to unset our voucher, someone came
                 * around and made us runnable.  But because we weren't waiting on the
                 * event their thread_wakeup() was ineffectual.  To correct for that,
                 * we just run the continuation ourselves.
                 */
                workq_unpark_select_threadreq_or_park_and_unlock(p, wq, uth, setup_flags);
                __builtin_unreachable();
        }

        if (uth->uu_workq_flags & UT_WORKQ_DYING) {
                workq_unpark_for_death_and_unlock(p, wq, uth,
                    WORKQ_UNPARK_FOR_DEATH_WAS_IDLE, setup_flags);
                __builtin_unreachable();
        }

        thread_set_pending_block_hint(uth->uu_thread, kThreadWaitParkedWorkQueue);
        assert_wait(workq_parked_wait_event(uth), THREAD_INTERRUPTIBLE);
        workq_unlock(wq);
        thread_block(workq_unpark_continue);
        __builtin_unreachable();
}

static inline bool
workq_may_start_event_mgr_thread(struct workqueue *wq, struct uthread *uth)
{
        /*
         * There's an event manager request and either:
         * - no event manager currently running
         * - we are re-using the event manager
         */
        return wq->wq_thscheduled_count[_wq_bucket(WORKQ_THREAD_QOS_MANAGER)] == 0 ||
               (uth && uth->uu_workq_pri.qos_bucket == WORKQ_THREAD_QOS_MANAGER);
}

static uint32_t
workq_constrained_allowance(struct workqueue *wq, thread_qos_t at_qos,
    struct uthread *uth, bool may_start_timer)
{
        assert(at_qos != WORKQ_THREAD_QOS_MANAGER);
        uint32_t count = 0;

        uint32_t max_count = wq->wq_constrained_threads_scheduled;
        if (uth && (uth->uu_workq_flags & UT_WORKQ_OVERCOMMIT) == 0) {
                /*
                 * don't count the current thread as scheduled
                 */
                assert(max_count > 0);
                max_count--;
        }
        if (max_count >= wq_max_constrained_threads) {
                WQ_TRACE_WQ(TRACE_wq_constrained_admission | DBG_FUNC_NONE, wq, 1,
                    wq->wq_constrained_threads_scheduled,
                    wq_max_constrained_threads, 0);
                /*
                 * we need 1 or more constrained threads to return to the kernel before
                 * we can dispatch additional work
                 */
                return 0;
        }
        max_count -= wq_max_constrained_threads;

        /*
         * Compute a metric for many how many threads are active.  We find the
         * highest priority request outstanding and then add up the number of
         * active threads in that and all higher-priority buckets.  We'll also add
         * any "busy" threads which are not active but blocked recently enough that
         * we can't be sure they've gone idle yet.  We'll then compare this metric
         * to our max concurrency to decide whether to add a new thread.
         */

        uint32_t busycount, thactive_count;

        thactive_count = _wq_thactive_aggregate_downto_qos(wq, _wq_thactive(wq),
            at_qos, &busycount, NULL);

        if (uth && uth->uu_workq_pri.qos_bucket != WORKQ_THREAD_QOS_MANAGER &&
            at_qos <= uth->uu_workq_pri.qos_bucket) {
                /*
                 * Don't count this thread as currently active, but only if it's not
                 * a manager thread, as _wq_thactive_aggregate_downto_qos ignores active
                 * managers.
                 */
                assert(thactive_count > 0);
                thactive_count--;
        }

        count = wq_max_parallelism[_wq_bucket(at_qos)];
        if (count > thactive_count + busycount) {
                count -= thactive_count + busycount;
                WQ_TRACE_WQ(TRACE_wq_constrained_admission | DBG_FUNC_NONE, wq, 2,
                    thactive_count, busycount, 0);
                return MIN(count, max_count);
        } else {
                WQ_TRACE_WQ(TRACE_wq_constrained_admission | DBG_FUNC_NONE, wq, 3,
                    thactive_count, busycount, 0);
        }

        if (busycount && may_start_timer) {
                /*
                 * If this is called from the add timer, we won't have another timer
                 * fire when the thread exits the "busy" state, so rearm the timer.
                 */
                workq_schedule_delayed_thread_creation(wq, 0);
        }

        return 0;
}

static bool
workq_threadreq_admissible(struct workqueue *wq, struct uthread *uth,
    workq_threadreq_t req)
{
        if (req->tr_qos == WORKQ_THREAD_QOS_MANAGER) {
                return workq_may_start_event_mgr_thread(wq, uth);
        }
        if ((req->tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) == 0) {
                return workq_constrained_allowance(wq, req->tr_qos, uth, true);
        }
        return true;
}

static workq_threadreq_t
workq_threadreq_select_for_creator(struct workqueue *wq)
{
        workq_threadreq_t req_qos, req_pri, req_tmp, req_mgr;
        thread_qos_t qos = THREAD_QOS_UNSPECIFIED;
        uint8_t pri = 0;

        /*
         * Compute the best priority request, and ignore the turnstile for now
         */

        req_pri = priority_queue_max(&wq->wq_special_queue,
            struct workq_threadreq_s, tr_entry);
        if (req_pri) {
                pri = (uint8_t)priority_queue_entry_sched_pri(&wq->wq_special_queue,
                    &req_pri->tr_entry);
        }

        /*
         * Handle the manager thread request. The special queue might yield
         * a higher priority, but the manager always beats the QoS world.
         */

        req_mgr = wq->wq_event_manager_threadreq;
        if (req_mgr && workq_may_start_event_mgr_thread(wq, NULL)) {
                uint32_t mgr_pri = wq->wq_event_manager_priority;

                if (mgr_pri & _PTHREAD_PRIORITY_SCHED_PRI_FLAG) {
                        mgr_pri &= _PTHREAD_PRIORITY_SCHED_PRI_MASK;
                } else {
                        mgr_pri = thread_workq_pri_for_qos(
                                _pthread_priority_thread_qos(mgr_pri));
                }

                return mgr_pri >= pri ? req_mgr : req_pri;
        }

        /*
         * Compute the best QoS Request, and check whether it beats the "pri" one
         */

        req_qos = priority_queue_max(&wq->wq_overcommit_queue,
            struct workq_threadreq_s, tr_entry);
        if (req_qos) {
                qos = req_qos->tr_qos;
        }

        req_tmp = priority_queue_max(&wq->wq_constrained_queue,
            struct workq_threadreq_s, tr_entry);

        if (req_tmp && qos < req_tmp->tr_qos) {
                if (pri && pri >= thread_workq_pri_for_qos(req_tmp->tr_qos)) {
                        return req_pri;
                }

                if (workq_constrained_allowance(wq, req_tmp->tr_qos, NULL, true)) {
                        /*
                         * If the constrained thread request is the best one and passes
                         * the admission check, pick it.
                         */
                        return req_tmp;
                }
        }

        if (pri && (!qos || pri >= thread_workq_pri_for_qos(qos))) {
                return req_pri;
        }

        if (req_qos) {
                return req_qos;
        }

        /*
         * If we had no eligible request but we have a turnstile push,
         * it must be a non overcommit thread request that failed
         * the admission check.
         *
         * Just fake a BG thread request so that if the push stops the creator
         * priority just drops to 4.
         */
        if (turnstile_workq_proprietor_of_max_turnstile(wq->wq_turnstile, NULL)) {
                static struct workq_threadreq_s workq_sync_push_fake_req = {
                        .tr_qos = THREAD_QOS_BACKGROUND,
                };

                return &workq_sync_push_fake_req;
        }

        return NULL;
}

static workq_threadreq_t
workq_threadreq_select(struct workqueue *wq, struct uthread *uth)
{
        workq_threadreq_t req_qos, req_pri, req_tmp, req_mgr;
        uintptr_t proprietor;
        thread_qos_t qos = THREAD_QOS_UNSPECIFIED;
        uint8_t pri = 0;

        if (uth == wq->wq_creator) {
                uth = NULL;
        }

        /*
         * Compute the best priority request (special or turnstile)
         */

        pri = (uint8_t)turnstile_workq_proprietor_of_max_turnstile(wq->wq_turnstile,
            &proprietor);
        if (pri) {
                struct kqworkloop *kqwl = (struct kqworkloop *)proprietor;
                req_pri = &kqwl->kqwl_request;
                if (req_pri->tr_state != WORKQ_TR_STATE_QUEUED) {
                        panic("Invalid thread request (%p) state %d",
                            req_pri, req_pri->tr_state);
                }
        } else {
                req_pri = NULL;
        }

        req_tmp = priority_queue_max(&wq->wq_special_queue,
            struct workq_threadreq_s, tr_entry);
        if (req_tmp && pri < priority_queue_entry_sched_pri(&wq->wq_special_queue,
            &req_tmp->tr_entry)) {
                req_pri = req_tmp;
                pri = (uint8_t)priority_queue_entry_sched_pri(&wq->wq_special_queue,
                    &req_tmp->tr_entry);
        }

        /*
         * Handle the manager thread request. The special queue might yield
         * a higher priority, but the manager always beats the QoS world.
         */

        req_mgr = wq->wq_event_manager_threadreq;
        if (req_mgr && workq_may_start_event_mgr_thread(wq, uth)) {
                uint32_t mgr_pri = wq->wq_event_manager_priority;

                if (mgr_pri & _PTHREAD_PRIORITY_SCHED_PRI_FLAG) {
                        mgr_pri &= _PTHREAD_PRIORITY_SCHED_PRI_MASK;
                } else {
                        mgr_pri = thread_workq_pri_for_qos(
                                _pthread_priority_thread_qos(mgr_pri));
                }

                return mgr_pri >= pri ? req_mgr : req_pri;
        }

        /*
         * Compute the best QoS Request, and check whether it beats the "pri" one
         */

        req_qos = priority_queue_max(&wq->wq_overcommit_queue,
            struct workq_threadreq_s, tr_entry);
        if (req_qos) {
                qos = req_qos->tr_qos;
        }

        req_tmp = priority_queue_max(&wq->wq_constrained_queue,
            struct workq_threadreq_s, tr_entry);

        if (req_tmp && qos < req_tmp->tr_qos) {
                if (pri && pri >= thread_workq_pri_for_qos(req_tmp->tr_qos)) {
                        return req_pri;
                }

                if (workq_constrained_allowance(wq, req_tmp->tr_qos, uth, true)) {
                        /*
                         * If the constrained thread request is the best one and passes
                         * the admission check, pick it.
                         */
                        return req_tmp;
                }
        }

        if (req_pri && (!qos || pri >= thread_workq_pri_for_qos(qos))) {
                return req_pri;
        }

        return req_qos;
}

/*
 * The creator is an anonymous thread that is counted as scheduled,
 * but otherwise without its scheduler callback set or tracked as active
 * that is used to make other threads.
 *
 * When more requests are added or an existing one is hurried along,
 * a creator is elected and setup, or the existing one overridden accordingly.
 *
 * While this creator is in flight, because no request has been dequeued,
 * already running threads have a chance at stealing thread requests avoiding
 * useless context switches, and the creator once scheduled may not find any
 * work to do and will then just park again.
 *
 * The creator serves the dual purpose of informing the scheduler of work that
 * hasn't be materialized as threads yet, and also as a natural pacing mechanism
 * for thread creation.
 *
 * By being anonymous (and not bound to anything) it means that thread requests
 * can be stolen from this creator by threads already on core yielding more
 * efficient scheduling and reduced context switches.
 */
static void
workq_schedule_creator(proc_t p, struct workqueue *wq,
    workq_kern_threadreq_flags_t flags)
{
        workq_threadreq_t req;
        struct uthread *uth;
        bool needs_wakeup;

        workq_lock_held(wq);
        assert(p || (flags & WORKQ_THREADREQ_CAN_CREATE_THREADS) == 0);

again:
        uth = wq->wq_creator;

        if (!wq->wq_reqcount) {
                /*
                 * There is no thread request left.
                 *
                 * If there is a creator, leave everything in place, so that it cleans
                 * up itself in workq_push_idle_thread().
                 *
                 * Else, make sure the turnstile state is reset to no inheritor.
                 */
                if (uth == NULL) {
                        workq_turnstile_update_inheritor(wq, TURNSTILE_INHERITOR_NULL, 0);
                }
                return;
        }

        req = workq_threadreq_select_for_creator(wq);
        if (req == NULL) {
                /*
                 * There isn't a thread request that passes the admission check.
                 *
                 * If there is a creator, do not touch anything, the creator will sort
                 * it out when it runs.
                 *
                 * Else, set the inheritor to "WORKQ" so that the turnstile propagation
                 * code calls us if anything changes.
                 */
                if (uth == NULL) {
                        workq_turnstile_update_inheritor(wq, wq, TURNSTILE_INHERITOR_WORKQ);
                }
                return;
        }

        if (uth) {
                /*
                 * We need to maybe override the creator we already have
                 */
                if (workq_thread_needs_priority_change(req, uth)) {
                        WQ_TRACE_WQ(TRACE_wq_creator_select | DBG_FUNC_NONE,
                            wq, 1, thread_tid(uth->uu_thread), req->tr_qos, 0);
                        workq_thread_reset_pri(wq, uth, req, /*unpark*/ true);
                }
                assert(wq->wq_inheritor == uth->uu_thread);
        } else if (wq->wq_thidlecount) {
                /*
                 * We need to unpark a creator thread
                 */
                wq->wq_creator = uth = workq_pop_idle_thread(wq, UT_WORKQ_OVERCOMMIT,
                    &needs_wakeup);
                /* Always reset the priorities on the newly chosen creator */
                workq_thread_reset_pri(wq, uth, req, /*unpark*/ true);
                workq_turnstile_update_inheritor(wq, uth->uu_thread,
                    TURNSTILE_INHERITOR_THREAD);
                WQ_TRACE_WQ(TRACE_wq_creator_select | DBG_FUNC_NONE,
                    wq, 2, thread_tid(uth->uu_thread), req->tr_qos, 0);
                uth->uu_save.uus_workq_park_data.fulfilled_snapshot = wq->wq_fulfilled;
                uth->uu_save.uus_workq_park_data.yields = 0;
                if (needs_wakeup) {
                        workq_thread_wakeup(uth);
                }
        } else {
                /*
                 * We need to allocate a thread...
                 */
                if (__improbable(wq->wq_nthreads >= wq_max_threads)) {
                        /* out of threads, just go away */
                        flags = WORKQ_THREADREQ_NONE;
                } else if (flags & WORKQ_THREADREQ_SET_AST_ON_FAILURE) {
                        act_set_astkevent(current_thread(), AST_KEVENT_REDRIVE_THREADREQ);
                } else if (!(flags & WORKQ_THREADREQ_CAN_CREATE_THREADS)) {
                        /* This can drop the workqueue lock, and take it again */
                        workq_schedule_immediate_thread_creation(wq);
                } else if (workq_add_new_idle_thread(p, wq)) {
                        goto again;
                } else {
                        workq_schedule_delayed_thread_creation(wq, 0);
                }

                /*
                 * If the current thread is the inheritor:
                 *
                 * If we set the AST, then the thread will stay the inheritor until
                 * either the AST calls workq_kern_threadreq_redrive(), or it parks
                 * and calls workq_push_idle_thread().
                 *
                 * Else, the responsibility of the thread creation is with a thread-call
                 * and we need to clear the inheritor.
                 */
                if ((flags & WORKQ_THREADREQ_SET_AST_ON_FAILURE) == 0 &&
                    wq->wq_inheritor == current_thread()) {
                        workq_turnstile_update_inheritor(wq, TURNSTILE_INHERITOR_NULL, 0);
                }
        }
}

/**
 * Same as workq_unpark_select_threadreq_or_park_and_unlock,
 * but do not allow early binds.
 *
 * Called with the base pri frozen, will unfreeze it.
 */
__attribute__((noreturn, noinline))
static void
workq_select_threadreq_or_park_and_unlock(proc_t p, struct workqueue *wq,
    struct uthread *uth, uint32_t setup_flags)
{
        workq_threadreq_t req = NULL;
        bool is_creator = (wq->wq_creator == uth);
        bool schedule_creator = false;

        if (__improbable(_wq_exiting(wq))) {
                WQ_TRACE_WQ(TRACE_wq_select_threadreq | DBG_FUNC_NONE, wq, 0, 0, 0, 0);
                goto park;
        }

        if (wq->wq_reqcount == 0) {
                WQ_TRACE_WQ(TRACE_wq_select_threadreq | DBG_FUNC_NONE, wq, 1, 0, 0, 0);
                goto park;
        }

        req = workq_threadreq_select(wq, uth);
        if (__improbable(req == NULL)) {
                WQ_TRACE_WQ(TRACE_wq_select_threadreq | DBG_FUNC_NONE, wq, 2, 0, 0, 0);
                goto park;
        }

        uint8_t tr_flags = req->tr_flags;
        struct turnstile *req_ts = kqueue_threadreq_get_turnstile(req);

        /*
         * Attempt to setup ourselves as the new thing to run, moving all priority
         * pushes to ourselves.
         *
         * If the current thread is the creator, then the fact that we are presently
         * running is proof that we'll do something useful, so keep going.
         *
         * For other cases, peek at the AST to know whether the scheduler wants
         * to preempt us, if yes, park instead, and move the thread request
         * turnstile back to the workqueue.
         */
        if (req_ts) {
                workq_perform_turnstile_operation_locked(wq, ^{
                        turnstile_update_inheritor(req_ts, uth->uu_thread,
                        TURNSTILE_IMMEDIATE_UPDATE | TURNSTILE_INHERITOR_THREAD);
                        turnstile_update_inheritor_complete(req_ts,
                        TURNSTILE_INTERLOCK_HELD);
                });
        }

        if (is_creator) {
                WQ_TRACE_WQ(TRACE_wq_creator_select, wq, 4, 0,
                    uth->uu_save.uus_workq_park_data.yields, 0);
                wq->wq_creator = NULL;
                _wq_thactive_inc(wq, req->tr_qos);
                wq->wq_thscheduled_count[_wq_bucket(req->tr_qos)]++;
        } else if (uth->uu_workq_pri.qos_bucket != req->tr_qos) {
                _wq_thactive_move(wq, uth->uu_workq_pri.qos_bucket, req->tr_qos);
        }

        workq_thread_reset_pri(wq, uth, req, /*unpark*/ true);

        thread_unfreeze_base_pri(uth->uu_thread);
#if 0 // <rdar://problem/55259863> to turn this back on
        if (__improbable(thread_unfreeze_base_pri(uth->uu_thread) && !is_creator)) {
                if (req_ts) {
                        workq_perform_turnstile_operation_locked(wq, ^{
                                turnstile_update_inheritor(req_ts, wq->wq_turnstile,
                                TURNSTILE_IMMEDIATE_UPDATE | TURNSTILE_INHERITOR_TURNSTILE);
                                turnstile_update_inheritor_complete(req_ts,
                                TURNSTILE_INTERLOCK_HELD);
                        });
                }
                WQ_TRACE_WQ(TRACE_wq_select_threadreq | DBG_FUNC_NONE, wq, 3, 0, 0, 0);
                goto park_thawed;
        }
#endif

        /*
         * We passed all checks, dequeue the request, bind to it, and set it up
         * to return to user.
         */
        WQ_TRACE_WQ(TRACE_wq_thread_logical_run | DBG_FUNC_START, wq,
            workq_trace_req_id(req), 0, 0, 0);
        wq->wq_fulfilled++;
        schedule_creator = workq_threadreq_dequeue(wq, req);

        if (tr_flags & (WORKQ_TR_FLAG_KEVENT | WORKQ_TR_FLAG_WORKLOOP)) {
                kqueue_threadreq_bind_prepost(p, req, uth);
                req = NULL;
        } else if (req->tr_count > 0) {
                req = NULL;
        }

        workq_thread_reset_cpupercent(req, uth);
        if (uth->uu_workq_flags & UT_WORKQ_NEW) {
                uth->uu_workq_flags ^= UT_WORKQ_NEW;
                setup_flags |= WQ_SETUP_FIRST_USE;
        }
        if (tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) {
                if ((uth->uu_workq_flags & UT_WORKQ_OVERCOMMIT) == 0) {
                        uth->uu_workq_flags |= UT_WORKQ_OVERCOMMIT;
                        wq->wq_constrained_threads_scheduled--;
                }
        } else {
                if ((uth->uu_workq_flags & UT_WORKQ_OVERCOMMIT) != 0) {
                        uth->uu_workq_flags &= ~UT_WORKQ_OVERCOMMIT;
                        wq->wq_constrained_threads_scheduled++;
                }
        }

        if (is_creator || schedule_creator) {
                /* This can drop the workqueue lock, and take it again */
                workq_schedule_creator(p, wq, WORKQ_THREADREQ_CAN_CREATE_THREADS);
        }

        workq_unlock(wq);

        if (req) {
                zfree(workq_zone_threadreq, req);
        }

        /*
         * Run Thread, Run!
         */
        uint32_t upcall_flags = WQ_FLAG_THREAD_NEWSPI;
        if (uth->uu_workq_pri.qos_bucket == WORKQ_THREAD_QOS_MANAGER) {
                upcall_flags |= WQ_FLAG_THREAD_EVENT_MANAGER;
        } else if (tr_flags & WORKQ_TR_FLAG_OVERCOMMIT) {
                upcall_flags |= WQ_FLAG_THREAD_OVERCOMMIT;
        }
        if (tr_flags & WORKQ_TR_FLAG_KEVENT) {
                upcall_flags |= WQ_FLAG_THREAD_KEVENT;
        }
        if (tr_flags & WORKQ_TR_FLAG_WORKLOOP) {
                upcall_flags |= WQ_FLAG_THREAD_WORKLOOP | WQ_FLAG_THREAD_KEVENT;
        }
        uth->uu_save.uus_workq_park_data.upcall_flags = upcall_flags;

        if (tr_flags & (WORKQ_TR_FLAG_KEVENT | WORKQ_TR_FLAG_WORKLOOP)) {
                kqueue_threadreq_bind_commit(p, uth->uu_thread);
        }
        workq_setup_and_run(p, uth, setup_flags);
        __builtin_unreachable();

park:
        thread_unfreeze_base_pri(uth->uu_thread);
#if 0 // <rdar://problem/55259863>
park_thawed:
#endif
        workq_park_and_unlock(p, wq, uth, setup_flags);
}

/**
 * Runs a thread request on a thread
 *
 * - if thread is THREAD_NULL, will find a thread and run the request there.
 *   Otherwise, the thread must be the current thread.
 *
 * - if req is NULL, will find the highest priority request and run that.  If
 *   it is not NULL, it must be a threadreq object in state NEW.  If it can not
 *   be run immediately, it will be enqueued and moved to state QUEUED.
 *
 *   Either way, the thread request object serviced will be moved to state
 *   BINDING and attached to the uthread.
 *
 * Should be called with the workqueue lock held.  Will drop it.
 * Should be called with the base pri not frozen.
 */
__attribute__((noreturn, noinline))
static void
workq_unpark_select_threadreq_or_park_and_unlock(proc_t p, struct workqueue *wq,
    struct uthread *uth, uint32_t setup_flags)
{
        if (uth->uu_workq_flags & UT_WORKQ_EARLY_BOUND) {
                if (uth->uu_workq_flags & UT_WORKQ_NEW) {
                        setup_flags |= WQ_SETUP_FIRST_USE;
                }
                uth->uu_workq_flags &= ~(UT_WORKQ_NEW | UT_WORKQ_EARLY_BOUND);
                /*
                 * This pointer is possibly freed and only used for tracing purposes.
                 */
                workq_threadreq_t req = uth->uu_save.uus_workq_park_data.thread_request;
                workq_unlock(wq);
                WQ_TRACE_WQ(TRACE_wq_thread_logical_run | DBG_FUNC_START, wq,
                    VM_KERNEL_ADDRHIDE(req), 0, 0, 0);
                (void)req;
                workq_setup_and_run(p, uth, setup_flags);
                __builtin_unreachable();
        }

        thread_freeze_base_pri(uth->uu_thread);
        workq_select_threadreq_or_park_and_unlock(p, wq, uth, setup_flags);
}

static bool
workq_creator_should_yield(struct workqueue *wq, struct uthread *uth)
{
        thread_qos_t qos = workq_pri_override(uth->uu_workq_pri);

        if (qos >= THREAD_QOS_USER_INTERACTIVE) {
                return false;
        }

        uint32_t snapshot = uth->uu_save.uus_workq_park_data.fulfilled_snapshot;
        if (wq->wq_fulfilled == snapshot) {
                return false;
        }

        uint32_t cnt = 0, conc = wq_max_parallelism[_wq_bucket(qos)];
        if (wq->wq_fulfilled - snapshot > conc) {
                /* we fulfilled more than NCPU requests since being dispatched */
                WQ_TRACE_WQ(TRACE_wq_creator_yield, wq, 1,
                    wq->wq_fulfilled, snapshot, 0);
                return true;
        }

        for (int i = _wq_bucket(qos); i < WORKQ_NUM_QOS_BUCKETS; i++) {
                cnt += wq->wq_thscheduled_count[i];
        }
        if (conc <= cnt) {
                /* We fulfilled requests and have more than NCPU scheduled threads */
                WQ_TRACE_WQ(TRACE_wq_creator_yield, wq, 2,
                    wq->wq_fulfilled, snapshot, 0);
                return true;
        }

        return false;
}

/**
 * parked thread wakes up
 */
__attribute__((noreturn, noinline))
static void
workq_unpark_continue(void *parameter __unused, wait_result_t wr __unused)
{
        thread_t th = current_thread();
        struct uthread *uth = get_bsdthread_info(th);
        proc_t p = current_proc();
        struct workqueue *wq = proc_get_wqptr_fast(p);

        workq_lock_spin(wq);

        if (wq->wq_creator == uth && workq_creator_should_yield(wq, uth)) {
                /*
                 * If the number of threads we have out are able to keep up with the
                 * demand, then we should avoid sending this creator thread to
                 * userspace.
                 */
                uth->uu_save.uus_workq_park_data.fulfilled_snapshot = wq->wq_fulfilled;
                uth->uu_save.uus_workq_park_data.yields++;
                workq_unlock(wq);
                thread_yield_with_continuation(workq_unpark_continue, NULL);
                __builtin_unreachable();
        }

        if (__probable(uth->uu_workq_flags & UT_WORKQ_RUNNING)) {
                workq_unpark_select_threadreq_or_park_and_unlock(p, wq, uth, WQ_SETUP_NONE);
                __builtin_unreachable();
        }

        if (__probable(wr == THREAD_AWAKENED)) {
                /*
                 * We were set running, but for the purposes of dying.
                 */
                assert(uth->uu_workq_flags & UT_WORKQ_DYING);
                assert((uth->uu_workq_flags & UT_WORKQ_NEW) == 0);
        } else {
                /*
                 * workaround for <rdar://problem/38647347>,
                 * in case we do hit userspace, make sure calling
                 * workq_thread_terminate() does the right thing here,
                 * and if we never call it, that workq_exit() will too because it sees
                 * this thread on the runlist.
                 */
                assert(wr == THREAD_INTERRUPTED);
                wq->wq_thdying_count++;
                uth->uu_workq_flags |= UT_WORKQ_DYING;
        }

        workq_unpark_for_death_and_unlock(p, wq, uth,
            WORKQ_UNPARK_FOR_DEATH_WAS_IDLE, WQ_SETUP_NONE);
        __builtin_unreachable();
}

__attribute__((noreturn, noinline))
static void
workq_setup_and_run(proc_t p, struct uthread *uth, int setup_flags)
{
        thread_t th = uth->uu_thread;
        vm_map_t vmap = get_task_map(p->task);

        if (setup_flags & WQ_SETUP_CLEAR_VOUCHER) {
                /*
                 * For preemption reasons, we want to reset the voucher as late as
                 * possible, so we do it in two places:
                 *   - Just before parking (i.e. in workq_park_and_unlock())
                 *   - Prior to doing the setup for the next workitem (i.e. here)
                 *
                 * Those two places are sufficient to ensure we always reset it before
                 * it goes back out to user space, but be careful to not break that
                 * guarantee.
                 */
                __assert_only kern_return_t kr;
                kr = thread_set_voucher_name(MACH_PORT_NULL);
                assert(kr == KERN_SUCCESS);
        }

        uint32_t upcall_flags = uth->uu_save.uus_workq_park_data.upcall_flags;
        if (!(setup_flags & WQ_SETUP_FIRST_USE)) {
                upcall_flags |= WQ_FLAG_THREAD_REUSE;
        }

        if (uth->uu_workq_flags & UT_WORKQ_OUTSIDE_QOS) {
                /*
                 * For threads that have an outside-of-QoS thread priority, indicate
                 * to userspace that setting QoS should only affect the TSD and not
                 * change QOS in the kernel.
                 */
                upcall_flags |= WQ_FLAG_THREAD_OUTSIDEQOS;
        } else {
                /*
                 * Put the QoS class value into the lower bits of the reuse_thread
                 * register, this is where the thread priority used to be stored
                 * anyway.
                 */
                upcall_flags |= uth->uu_save.uus_workq_park_data.qos |
                    WQ_FLAG_THREAD_PRIO_QOS;
        }

        if (uth->uu_workq_thport == MACH_PORT_NULL) {
                /* convert_thread_to_port() consumes a reference */
                thread_reference(th);
                ipc_port_t port = convert_thread_to_port(th);
                uth->uu_workq_thport = ipc_port_copyout_send(port, get_task_ipcspace(p->task));
        }

        /*
         * Call out to pthread, this sets up the thread, pulls in kevent structs
         * onto the stack, sets up the thread state and then returns to userspace.
         */
        WQ_TRACE_WQ(TRACE_wq_runthread | DBG_FUNC_START,
            proc_get_wqptr_fast(p), 0, 0, 0, 0);
        thread_sched_call(th, workq_sched_callback);
        pthread_functions->workq_setup_thread(p, th, vmap, uth->uu_workq_stackaddr,
            uth->uu_workq_thport, 0, setup_flags, upcall_flags);

        __builtin_unreachable();
}

#pragma mark misc

int
fill_procworkqueue(proc_t p, struct proc_workqueueinfo * pwqinfo)
{
        struct workqueue *wq = proc_get_wqptr(p);
        int error = 0;
        int     activecount;

        if (wq == NULL) {
                return EINVAL;
        }

        /*
         * This is sometimes called from interrupt context by the kperf sampler.
         * In that case, it's not safe to spin trying to take the lock since we
         * might already hold it.  So, we just try-lock it and error out if it's
         * already held.  Since this is just a debugging aid, and all our callers
         * are able to handle an error, that's fine.
         */
        bool locked = workq_lock_try(wq);
        if (!locked) {
                return EBUSY;
        }

        wq_thactive_t act = _wq_thactive(wq);
        activecount = _wq_thactive_aggregate_downto_qos(wq, act,
            WORKQ_THREAD_QOS_MIN, NULL, NULL);
        if (act & _wq_thactive_offset_for_qos(WORKQ_THREAD_QOS_MANAGER)) {
                activecount++;
        }
        pwqinfo->pwq_nthreads = wq->wq_nthreads;
        pwqinfo->pwq_runthreads = activecount;
        pwqinfo->pwq_blockedthreads = wq->wq_threads_scheduled - activecount;
        pwqinfo->pwq_state = 0;

        if (wq->wq_constrained_threads_scheduled >= wq_max_constrained_threads) {
                pwqinfo->pwq_state |= WQ_EXCEEDED_CONSTRAINED_THREAD_LIMIT;
        }

        if (wq->wq_nthreads >= wq_max_threads) {
                pwqinfo->pwq_state |= WQ_EXCEEDED_TOTAL_THREAD_LIMIT;
        }

        workq_unlock(wq);
        return error;
}

boolean_t
workqueue_get_pwq_exceeded(void *v, boolean_t *exceeded_total,
    boolean_t *exceeded_constrained)
{
        proc_t p = v;
        struct proc_workqueueinfo pwqinfo;
        int err;

        assert(p != NULL);
        assert(exceeded_total != NULL);
        assert(exceeded_constrained != NULL);

        err = fill_procworkqueue(p, &pwqinfo);
        if (err) {
                return FALSE;
        }
        if (!(pwqinfo.pwq_state & WQ_FLAGS_AVAILABLE)) {
                return FALSE;
        }

        *exceeded_total = (pwqinfo.pwq_state & WQ_EXCEEDED_TOTAL_THREAD_LIMIT);
        *exceeded_constrained = (pwqinfo.pwq_state & WQ_EXCEEDED_CONSTRAINED_THREAD_LIMIT);

        return TRUE;
}

uint32_t
workqueue_get_pwq_state_kdp(void * v)
{
        static_assert((WQ_EXCEEDED_CONSTRAINED_THREAD_LIMIT << 17) ==
            kTaskWqExceededConstrainedThreadLimit);
        static_assert((WQ_EXCEEDED_TOTAL_THREAD_LIMIT << 17) ==
            kTaskWqExceededTotalThreadLimit);
        static_assert((WQ_FLAGS_AVAILABLE << 17) == kTaskWqFlagsAvailable);
        static_assert((WQ_FLAGS_AVAILABLE | WQ_EXCEEDED_TOTAL_THREAD_LIMIT |
            WQ_EXCEEDED_CONSTRAINED_THREAD_LIMIT) == 0x7);

        if (v == NULL) {
                return 0;
        }

        proc_t p = v;
        struct workqueue *wq = proc_get_wqptr(p);

        if (wq == NULL || workq_lock_spin_is_acquired_kdp(wq)) {
                return 0;
        }

        uint32_t pwq_state = WQ_FLAGS_AVAILABLE;

        if (wq->wq_constrained_threads_scheduled >= wq_max_constrained_threads) {
                pwq_state |= WQ_EXCEEDED_CONSTRAINED_THREAD_LIMIT;
        }

        if (wq->wq_nthreads >= wq_max_threads) {
                pwq_state |= WQ_EXCEEDED_TOTAL_THREAD_LIMIT;
        }

        return pwq_state;
}

void
workq_init(void)
{
        clock_interval_to_absolutetime_interval(wq_stalled_window.usecs,
            NSEC_PER_USEC, &wq_stalled_window.abstime);
        clock_interval_to_absolutetime_interval(wq_reduce_pool_window.usecs,
            NSEC_PER_USEC, &wq_reduce_pool_window.abstime);
        clock_interval_to_absolutetime_interval(wq_max_timer_interval.usecs,
            NSEC_PER_USEC, &wq_max_timer_interval.abstime);

        thread_deallocate_daemon_register_queue(&workq_deallocate_queue,
            workq_deallocate_queue_invoke);
}