xnu-6153.61.1.tar.gz

[apple/xnu.git] / osfmk / kern / priority.c

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/*
 * @OSF_COPYRIGHT@
 */
/*
 * Mach Operating System
 * Copyright (c) 1991,1990,1989,1988,1987 Carnegie Mellon University
 * All Rights Reserved.
 *
 * Permission to use, copy, modify and distribute this software and its
 * documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 *
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
 * ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 *
 * Carnegie Mellon requests users of this software to return to
 *
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 *
 * any improvements or extensions that they make and grant Carnegie Mellon
 * the rights to redistribute these changes.
 */
/*
 */
/*
 *      File:   priority.c
 *      Author: Avadis Tevanian, Jr.
 *      Date:   1986
 *
 *      Priority related scheduler bits.
 */

#include <mach/boolean.h>
#include <mach/kern_return.h>
#include <mach/machine.h>
#include <kern/host.h>
#include <kern/mach_param.h>
#include <kern/sched.h>
#include <sys/kdebug.h>
#include <kern/spl.h>
#include <kern/thread.h>
#include <kern/processor.h>
#include <kern/ledger.h>
#include <machine/machparam.h>
#include <kern/machine.h>
#include <kern/policy_internal.h>
#include <kern/sched_clutch.h>

#ifdef CONFIG_MACH_APPROXIMATE_TIME
#include <machine/commpage.h>  /* for commpage_update_mach_approximate_time */
#endif

#if MONOTONIC
#include <kern/monotonic.h>
#endif /* MONOTONIC */

/*
 *      thread_quantum_expire:
 *
 *      Recalculate the quantum and priority for a thread.
 *
 *      Called at splsched.
 */

void
thread_quantum_expire(
        timer_call_param_t      p0,
        timer_call_param_t      p1)
{
        processor_t                     processor = p0;
        thread_t                        thread = p1;
        ast_t                           preempt;
        uint64_t                        ctime;

        assert(processor == current_processor());
        assert(thread == current_thread());

        KERNEL_DEBUG_CONSTANT(MACHDBG_CODE(DBG_MACH_SCHED, MACH_SCHED_QUANTUM_EXPIRED) | DBG_FUNC_START, 0, 0, 0, 0, 0);

        SCHED_STATS_QUANTUM_TIMER_EXPIRATION(processor);

        /*
         * We bill CPU time to both the individual thread and its task.
         *
         * Because this balance adjustment could potentially attempt to wake this
         * very thread, we must credit the ledger before taking the thread lock.
         * The ledger pointers are only manipulated by the thread itself at the ast
         * boundary.
         *
         * TODO: This fails to account for the time between when the timer was
         * armed and when it fired.  It should be based on the system_timer and
         * running a timer_update operation here.
         */
        ledger_credit(thread->t_ledger, task_ledgers.cpu_time, thread->quantum_remaining);
        ledger_credit(thread->t_threadledger, thread_ledgers.cpu_time, thread->quantum_remaining);
        if (thread->t_bankledger) {
                ledger_credit(thread->t_bankledger, bank_ledgers.cpu_time,
                    (thread->quantum_remaining - thread->t_deduct_bank_ledger_time));
        }
        thread->t_deduct_bank_ledger_time = 0;

        ctime = mach_absolute_time();

#ifdef CONFIG_MACH_APPROXIMATE_TIME
        commpage_update_mach_approximate_time(ctime);
#endif

#if MONOTONIC
        mt_sched_update(thread);
#endif /* MONOTONIC */

        thread_lock(thread);

        /*
         * We've run up until our quantum expiration, and will (potentially)
         * continue without re-entering the scheduler, so update this now.
         */
        processor->last_dispatch = ctime;
        thread->last_run_time = ctime;

        /*
         *      Check for fail-safe trip.
         */
        if ((thread->sched_mode == TH_MODE_REALTIME || thread->sched_mode == TH_MODE_FIXED) &&
            !(thread->sched_flags & TH_SFLAG_PROMOTED) &&
            !(thread->kern_promotion_schedpri != 0) &&
            !(thread->sched_flags & TH_SFLAG_PROMOTE_REASON_MASK) &&
            !(thread->options & TH_OPT_SYSTEM_CRITICAL)) {
                uint64_t new_computation;

                new_computation = ctime - thread->computation_epoch;
                new_computation += thread->computation_metered;
                if (new_computation > max_unsafe_computation) {
                        KERNEL_DEBUG_CONSTANT(MACHDBG_CODE(DBG_MACH_SCHED, MACH_FAILSAFE) | DBG_FUNC_NONE,
                            (uintptr_t)thread->sched_pri, (uintptr_t)thread->sched_mode, 0, 0, 0);

                        thread->safe_release = ctime + sched_safe_duration;

                        sched_thread_mode_demote(thread, TH_SFLAG_FAILSAFE);
                }
        }

        /*
         *      Recompute scheduled priority if appropriate.
         */
        if (SCHED(can_update_priority)(thread)) {
                SCHED(update_priority)(thread);
        } else {
                SCHED(lightweight_update_priority)(thread);
        }

        if (thread->sched_mode != TH_MODE_REALTIME) {
                SCHED(quantum_expire)(thread);
        }

        /*
         *      This quantum is up, give this thread another.
         */
        processor->first_timeslice = FALSE;

        thread_quantum_init(thread);

        /* Reload precise timing global policy to thread-local policy */
        thread->precise_user_kernel_time = use_precise_user_kernel_time(thread);

        /*
         * Since non-precise user/kernel time doesn't update the state/thread timer
         * during privilege transitions, synthesize an event now.
         */
        if (!thread->precise_user_kernel_time) {
                timer_update(PROCESSOR_DATA(processor, current_state), ctime);
                timer_update(PROCESSOR_DATA(processor, thread_timer), ctime);
                timer_update(&thread->runnable_timer, ctime);
        }


        processor->quantum_end = ctime + thread->quantum_remaining;

        /*
         * Context switch check
         *
         * non-urgent flags don't affect kernel threads, so upgrade to urgent
         * to ensure that rebalancing and non-recommendation kick in quickly.
         */

        ast_t check_reason = AST_QUANTUM;
        if (thread->task == kernel_task) {
                check_reason |= AST_URGENT;
        }

        if ((preempt = csw_check(thread, processor, check_reason)) != AST_NONE) {
                ast_on(preempt);
        }

        /*
         * AST_KEVENT does not send an IPI when setting the AST,
         * to avoid waiting for the next context switch to propagate the AST,
         * the AST is propagated here at quantum expiration.
         */
        ast_propagate(thread);

        thread_unlock(thread);

        timer_call_quantum_timer_enter(&processor->quantum_timer, thread,
            processor->quantum_end, ctime);

        /* Tell platform layer that we are still running this thread */
        thread_urgency_t urgency = thread_get_urgency(thread, NULL, NULL);
        machine_thread_going_on_core(thread, urgency, 0, 0, ctime);
        machine_switch_perfcontrol_state_update(QUANTUM_EXPIRY, ctime,
            0, thread);

#if defined(CONFIG_SCHED_TIMESHARE_CORE)
        sched_timeshare_consider_maintenance(ctime);
#endif /* CONFIG_SCHED_TIMESHARE_CORE */

#if __arm__ || __arm64__
        if (thread->sched_mode == TH_MODE_REALTIME) {
                sched_consider_recommended_cores(ctime, thread);
        }
#endif /* __arm__ || __arm64__ */

        KERNEL_DEBUG_CONSTANT(MACHDBG_CODE(DBG_MACH_SCHED, MACH_SCHED_QUANTUM_EXPIRED) | DBG_FUNC_END, preempt, 0, 0, 0, 0);
}

/*
 *      sched_set_thread_base_priority:
 *
 *      Set the base priority of the thread
 *      and reset its scheduled priority.
 *
 *      This is the only path to change base_pri.
 *
 *      Called with the thread locked.
 */
void
sched_set_thread_base_priority(thread_t thread, int priority)
{
        assert(priority >= MINPRI);
        uint64_t ctime = 0;

        if (thread->sched_mode == TH_MODE_REALTIME) {
                assert(priority <= BASEPRI_RTQUEUES);
        } else {
                assert(priority < BASEPRI_RTQUEUES);
        }

        int old_base_pri = thread->base_pri;
        thread->req_base_pri = priority;
        if (thread->sched_flags & TH_SFLAG_BASE_PRI_FROZEN) {
                priority = MAX(priority, old_base_pri);
        }
        thread->base_pri = priority;

        if ((thread->state & TH_RUN) == TH_RUN) {
                assert(thread->last_made_runnable_time != THREAD_NOT_RUNNABLE);
                ctime = mach_approximate_time();
                thread->last_basepri_change_time = ctime;
        } else {
                assert(thread->last_basepri_change_time == THREAD_NOT_RUNNABLE);
                assert(thread->last_made_runnable_time == THREAD_NOT_RUNNABLE);
        }

        /*
         * Currently the perfcontrol_attr depends on the base pri of the
         * thread. Therefore, we use this function as the hook for the
         * perfcontrol callout.
         */
        if (thread == current_thread() && old_base_pri != priority) {
                if (!ctime) {
                        ctime = mach_approximate_time();
                }
                machine_switch_perfcontrol_state_update(PERFCONTROL_ATTR_UPDATE,
                    ctime, PERFCONTROL_CALLOUT_WAKE_UNSAFE, thread);
        }
#if !CONFIG_SCHED_CLUTCH
        /* For the clutch scheduler, this operation is done in set_sched_pri() */
        SCHED(update_thread_bucket)(thread);
#endif /* !CONFIG_SCHED_CLUTCH */

        thread_recompute_sched_pri(thread, SETPRI_DEFAULT);
}

/*
 *      sched_set_kernel_thread_priority:
 *
 *      Set the absolute base priority of the thread
 *      and reset its scheduled priority.
 *
 *      Called with the thread unlocked.
 */
void
sched_set_kernel_thread_priority(thread_t thread, int new_priority)
{
        spl_t s = splsched();

        thread_lock(thread);

        assert(thread->sched_mode != TH_MODE_REALTIME);
        assert(thread->effective_policy.thep_qos == THREAD_QOS_UNSPECIFIED);

        if (new_priority > thread->max_priority) {
                new_priority = thread->max_priority;
        }
#if CONFIG_EMBEDDED
        if (new_priority < MAXPRI_THROTTLE) {
                new_priority = MAXPRI_THROTTLE;
        }
#endif /* CONFIG_EMBEDDED */

        thread->importance = new_priority - thread->task_priority;

        sched_set_thread_base_priority(thread, new_priority);

        thread_unlock(thread);
        splx(s);
}

/*
 *      thread_recompute_sched_pri:
 *
 *      Reset the scheduled priority of the thread
 *      according to its base priority if the
 *      thread has not been promoted or depressed.
 *
 *      This is the only way to push base_pri changes into sched_pri,
 *      or to recalculate the appropriate sched_pri after changing
 *      a promotion or depression.
 *
 *      Called at splsched with the thread locked.
 *
 *      TODO: Add an 'update urgency' flag to avoid urgency callouts on every rwlock operation
 */
void
thread_recompute_sched_pri(thread_t thread, set_sched_pri_options_t options)
{
        uint32_t     sched_flags = thread->sched_flags;
        sched_mode_t sched_mode  = thread->sched_mode;

        int priority = thread->base_pri;

        if (sched_mode == TH_MODE_TIMESHARE) {
                priority = SCHED(compute_timeshare_priority)(thread);
        }

        if (sched_flags & TH_SFLAG_DEPRESS) {
                /* thread_yield_internal overrides kernel mutex promotion */
                priority = DEPRESSPRI;
        } else {
                /* poll-depress is overridden by mutex promotion and promote-reasons */
                if ((sched_flags & TH_SFLAG_POLLDEPRESS)) {
                        priority = DEPRESSPRI;
                }

                if (thread->kern_promotion_schedpri > 0) {
                        priority = MAX(priority, thread->kern_promotion_schedpri);

                        if (sched_mode != TH_MODE_REALTIME) {
                                priority = MIN(priority, MAXPRI_PROMOTE);
                        }
                }

                if (sched_flags & TH_SFLAG_PROMOTED) {
                        priority = MAX(priority, thread->promotion_priority);

                        if (sched_mode != TH_MODE_REALTIME) {
                                priority = MIN(priority, MAXPRI_PROMOTE);
                        }
                }

                if (sched_flags & TH_SFLAG_PROMOTE_REASON_MASK) {
                        if (sched_flags & TH_SFLAG_RW_PROMOTED) {
                                priority = MAX(priority, MINPRI_RWLOCK);
                        }

                        if (sched_flags & TH_SFLAG_WAITQ_PROMOTED) {
                                priority = MAX(priority, MINPRI_WAITQ);
                        }

                        if (sched_flags & TH_SFLAG_EXEC_PROMOTED) {
                                priority = MAX(priority, MINPRI_EXEC);
                        }
                }
        }

        set_sched_pri(thread, priority, options);
}

void
sched_default_quantum_expire(thread_t thread __unused)
{
        /*
         * No special behavior when a timeshare, fixed, or realtime thread
         * uses up its entire quantum
         */
}

#if defined(CONFIG_SCHED_TIMESHARE_CORE)

/*
 *      lightweight_update_priority:
 *
 *      Update the scheduled priority for
 *      a timesharing thread.
 *
 *      Only for use on the current thread.
 *
 *      Called with the thread locked.
 */
void
lightweight_update_priority(thread_t thread)
{
        assert(thread->runq == PROCESSOR_NULL);
        assert(thread == current_thread());

        if (thread->sched_mode == TH_MODE_TIMESHARE) {
                int priority;
                uint32_t delta;

                thread_timer_delta(thread, delta);

                /*
                 *      Accumulate timesharing usage only
                 *      during contention for processor
                 *      resources.
                 */
                if (thread->pri_shift < INT8_MAX) {
                        thread->sched_usage += delta;
                }

                thread->cpu_delta += delta;

#if CONFIG_SCHED_CLUTCH
                /*
                 * Update the CPU usage for the thread group to which the thread belongs.
                 * The implementation assumes that the thread ran for the entire delta
                 * as part of the same thread group.
                 */
                sched_clutch_cpu_usage_update(thread, delta);
#endif /* CONFIG_SCHED_CLUTCH */

                priority = sched_compute_timeshare_priority(thread);

                if (priority != thread->sched_pri) {
                        thread_recompute_sched_pri(thread, SETPRI_LAZY);
                }
        }
}

/*
 *      Define shifts for simulating (5/8) ** n
 *
 *      Shift structures for holding update shifts.  Actual computation
 *      is  usage = (usage >> shift1) +/- (usage >> abs(shift2))  where the
 *      +/- is determined by the sign of shift 2.
 */

const struct shift_data        sched_decay_shifts[SCHED_DECAY_TICKS] = {
        { .shift1 = 1, .shift2 = 1 },
        { .shift1 = 1, .shift2 = 3 },
        { .shift1 = 1, .shift2 = -3 },
        { .shift1 = 2, .shift2 = -7 },
        { .shift1 = 3, .shift2 = 5 },
        { .shift1 = 3, .shift2 = -5 },
        { .shift1 = 4, .shift2 = -8 },
        { .shift1 = 5, .shift2 = 7 },
        { .shift1 = 5, .shift2 = -7 },
        { .shift1 = 6, .shift2 = -10 },
        { .shift1 = 7, .shift2 = 10 },
        { .shift1 = 7, .shift2 = -9 },
        { .shift1 = 8, .shift2 = -11 },
        { .shift1 = 9, .shift2 = 12 },
        { .shift1 = 9, .shift2 = -11 },
        { .shift1 = 10, .shift2 = -13 },
        { .shift1 = 11, .shift2 = 14 },
        { .shift1 = 11, .shift2 = -13 },
        { .shift1 = 12, .shift2 = -15 },
        { .shift1 = 13, .shift2 = 17 },
        { .shift1 = 13, .shift2 = -15 },
        { .shift1 = 14, .shift2 = -17 },
        { .shift1 = 15, .shift2 = 19 },
        { .shift1 = 16, .shift2 = 18 },
        { .shift1 = 16, .shift2 = -19 },
        { .shift1 = 17, .shift2 = 22 },
        { .shift1 = 18, .shift2 = 20 },
        { .shift1 = 18, .shift2 = -20 },
        { .shift1 = 19, .shift2 = 26 },
        { .shift1 = 20, .shift2 = 22 },
        { .shift1 = 20, .shift2 = -22 },
        { .shift1 = 21, .shift2 = -27 }
};

/*
 *      sched_compute_timeshare_priority:
 *
 *      Calculate the timesharing priority based upon usage and load.
 */
extern int sched_pri_decay_band_limit;


/* Only use the decay floor logic on embedded non-clutch schedulers */
#if CONFIG_EMBEDDED && !CONFIG_SCHED_CLUTCH

int
sched_compute_timeshare_priority(thread_t thread)
{
        int decay_amount = (thread->sched_usage >> thread->pri_shift);
        int decay_limit = sched_pri_decay_band_limit;

        if (thread->base_pri > BASEPRI_FOREGROUND) {
                decay_limit += (thread->base_pri - BASEPRI_FOREGROUND);
        }

        if (decay_amount > decay_limit) {
                decay_amount = decay_limit;
        }

        /* start with base priority */
        int priority = thread->base_pri - decay_amount;

        if (priority < MAXPRI_THROTTLE) {
                if (thread->task->max_priority > MAXPRI_THROTTLE) {
                        priority = MAXPRI_THROTTLE;
                } else if (priority < MINPRI_USER) {
                        priority = MINPRI_USER;
                }
        } else if (priority > MAXPRI_KERNEL) {
                priority = MAXPRI_KERNEL;
        }

        return priority;
}

#else /* CONFIG_EMBEDDED && !CONFIG_SCHED_CLUTCH */

int
sched_compute_timeshare_priority(thread_t thread)
{
        /* start with base priority */
        int priority = thread->base_pri - (thread->sched_usage >> thread->pri_shift);

        if (priority < MINPRI_USER) {
                priority = MINPRI_USER;
        } else if (priority > MAXPRI_KERNEL) {
                priority = MAXPRI_KERNEL;
        }

        return priority;
}

#endif /* CONFIG_EMBEDDED && !CONFIG_SCHED_CLUTCH */

/*
 *      can_update_priority
 *
 *      Make sure we don't do re-dispatches more frequently than a scheduler tick.
 *
 *      Called with the thread locked.
 */
boolean_t
can_update_priority(
        thread_t        thread)
{
        if (sched_tick == thread->sched_stamp) {
                return FALSE;
        } else {
                return TRUE;
        }
}

/*
 *      update_priority
 *
 *      Perform housekeeping operations driven by scheduler tick.
 *
 *      Called with the thread locked.
 */
void
update_priority(
        thread_t        thread)
{
        uint32_t ticks, delta;

        ticks = sched_tick - thread->sched_stamp;
        assert(ticks != 0);

        thread->sched_stamp += ticks;

        /* If requested, accelerate aging of sched_usage */
        if (sched_decay_usage_age_factor > 1) {
                ticks *= sched_decay_usage_age_factor;
        }

        /*
         *      Gather cpu usage data.
         */
        thread_timer_delta(thread, delta);
        if (ticks < SCHED_DECAY_TICKS) {
                /*
                 *      Accumulate timesharing usage only during contention for processor
                 *      resources. Use the pri_shift from the previous tick window to
                 *      determine if the system was in a contended state.
                 */
                if (thread->pri_shift < INT8_MAX) {
                        thread->sched_usage += delta;
                }

                thread->cpu_usage += delta + thread->cpu_delta;
                thread->cpu_delta = 0;

#if CONFIG_SCHED_CLUTCH
                /*
                 * Update the CPU usage for the thread group to which the thread belongs.
                 * The implementation assumes that the thread ran for the entire delta
                 * as part of the same thread group.
                 */
                sched_clutch_cpu_usage_update(thread, delta);
#endif /* CONFIG_SCHED_CLUTCH */

                const struct shift_data *shiftp = &sched_decay_shifts[ticks];

                if (shiftp->shift2 > 0) {
                        thread->cpu_usage =   (thread->cpu_usage >> shiftp->shift1) +
                            (thread->cpu_usage >> shiftp->shift2);
                        thread->sched_usage = (thread->sched_usage >> shiftp->shift1) +
                            (thread->sched_usage >> shiftp->shift2);
                } else {
                        thread->cpu_usage =   (thread->cpu_usage >>   shiftp->shift1) -
                            (thread->cpu_usage >> -(shiftp->shift2));
                        thread->sched_usage = (thread->sched_usage >>   shiftp->shift1) -
                            (thread->sched_usage >> -(shiftp->shift2));
                }
        } else {
                thread->cpu_usage = thread->cpu_delta = 0;
                thread->sched_usage = 0;
        }

        /*
         *      Check for fail-safe release.
         */
        if ((thread->sched_flags & TH_SFLAG_FAILSAFE) &&
            mach_absolute_time() >= thread->safe_release) {
                sched_thread_mode_undemote(thread, TH_SFLAG_FAILSAFE);
        }

        /*
         * Now that the thread's CPU usage has been accumulated and aged
         * based on contention of the previous tick window, update the
         * pri_shift of the thread to match the current global load/shift
         * values. The updated pri_shift would be used to calculate the
         * new priority of the thread.
         */
#if CONFIG_SCHED_CLUTCH
        thread->pri_shift = sched_clutch_thread_pri_shift(thread, thread->th_sched_bucket);
#else /* CONFIG_SCHED_CLUTCH */
        thread->pri_shift = sched_pri_shifts[thread->th_sched_bucket];
#endif /* CONFIG_SCHED_CLUTCH */

        /* Recompute scheduled priority if appropriate. */
        if (thread->sched_mode == TH_MODE_TIMESHARE) {
                thread_recompute_sched_pri(thread, SETPRI_LAZY);
        }
}

#endif /* CONFIG_SCHED_TIMESHARE_CORE */


/*
 * TH_BUCKET_RUN is a count of *all* runnable non-idle threads.
 * Each other bucket is a count of the runnable non-idle threads
 * with that property. All updates to these counts should be
 * performed with os_atomic_* operations.
 *
 * For the clutch scheduler, this global bucket is used only for
 * keeping the total global run count.
 */
uint32_t       sched_run_buckets[TH_BUCKET_MAX];

static void
sched_incr_bucket(sched_bucket_t bucket)
{
        assert(bucket >= TH_BUCKET_FIXPRI &&
            bucket <= TH_BUCKET_SHARE_BG);

        os_atomic_inc(&sched_run_buckets[bucket], relaxed);
}

static void
sched_decr_bucket(sched_bucket_t bucket)
{
        assert(bucket >= TH_BUCKET_FIXPRI &&
            bucket <= TH_BUCKET_SHARE_BG);

        assert(os_atomic_load(&sched_run_buckets[bucket], relaxed) > 0);

        os_atomic_dec(&sched_run_buckets[bucket], relaxed);
}

uint32_t
sched_run_incr(thread_t thread)
{
        assert((thread->state & (TH_RUN | TH_IDLE)) == TH_RUN);

        uint32_t new_count = os_atomic_inc(&sched_run_buckets[TH_BUCKET_RUN], relaxed);

        sched_incr_bucket(thread->th_sched_bucket);

        return new_count;
}

uint32_t
sched_run_decr(thread_t thread)
{
        assert((thread->state & (TH_RUN | TH_IDLE)) != TH_RUN);

        sched_decr_bucket(thread->th_sched_bucket);

        uint32_t new_count = os_atomic_dec(&sched_run_buckets[TH_BUCKET_RUN], relaxed);

        return new_count;
}

void
sched_update_thread_bucket(thread_t thread)
{
        sched_bucket_t old_bucket = thread->th_sched_bucket;
        sched_bucket_t new_bucket = TH_BUCKET_RUN;

        switch (thread->sched_mode) {
        case TH_MODE_FIXED:
        case TH_MODE_REALTIME:
                new_bucket = TH_BUCKET_FIXPRI;
                break;

        case TH_MODE_TIMESHARE:
                if (thread->base_pri > BASEPRI_DEFAULT) {
                        new_bucket = TH_BUCKET_SHARE_FG;
                } else if (thread->base_pri > BASEPRI_UTILITY) {
                        new_bucket = TH_BUCKET_SHARE_DF;
                } else if (thread->base_pri > MAXPRI_THROTTLE) {
                        new_bucket = TH_BUCKET_SHARE_UT;
                } else {
                        new_bucket = TH_BUCKET_SHARE_BG;
                }
                break;

        default:
                panic("unexpected mode: %d", thread->sched_mode);
                break;
        }

        if (old_bucket != new_bucket) {
                thread->th_sched_bucket = new_bucket;
                thread->pri_shift = sched_pri_shifts[new_bucket];

                if ((thread->state & (TH_RUN | TH_IDLE)) == TH_RUN) {
                        sched_decr_bucket(old_bucket);
                        sched_incr_bucket(new_bucket);
                }
        }
}

/*
 * Set the thread's true scheduling mode
 * Called with thread mutex and thread locked
 * The thread has already been removed from the runqueue.
 *
 * (saved_mode is handled before this point)
 */
void
sched_set_thread_mode(thread_t thread, sched_mode_t new_mode)
{
        assert(thread->runq == PROCESSOR_NULL);

        switch (new_mode) {
        case TH_MODE_FIXED:
        case TH_MODE_REALTIME:
        case TH_MODE_TIMESHARE:
                break;

        default:
                panic("unexpected mode: %d", new_mode);
                break;
        }

        thread->sched_mode = new_mode;

        SCHED(update_thread_bucket)(thread);
}

/*
 * Demote the true scheduler mode to timeshare (called with the thread locked)
 */
void
sched_thread_mode_demote(thread_t thread, uint32_t reason)
{
        assert(reason & TH_SFLAG_DEMOTED_MASK);
        assert((thread->sched_flags & reason) != reason);

        if (thread->policy_reset) {
                return;
        }

        if (thread->sched_flags & TH_SFLAG_DEMOTED_MASK) {
                /* Another demotion reason is already active */
                thread->sched_flags |= reason;
                return;
        }

        assert(thread->saved_mode == TH_MODE_NONE);

        boolean_t removed = thread_run_queue_remove(thread);

        thread->sched_flags |= reason;

        thread->saved_mode = thread->sched_mode;

        sched_set_thread_mode(thread, TH_MODE_TIMESHARE);

        thread_recompute_priority(thread);

        if (removed) {
                thread_run_queue_reinsert(thread, SCHED_TAILQ);
        }
}

/*
 * Un-demote the true scheduler mode back to the saved mode (called with the thread locked)
 */
void
sched_thread_mode_undemote(thread_t thread, uint32_t reason)
{
        assert(reason & TH_SFLAG_DEMOTED_MASK);
        assert((thread->sched_flags & reason) == reason);
        assert(thread->saved_mode != TH_MODE_NONE);
        assert(thread->sched_mode == TH_MODE_TIMESHARE);
        assert(thread->policy_reset == 0);

        thread->sched_flags &= ~reason;

        if (thread->sched_flags & TH_SFLAG_DEMOTED_MASK) {
                /* Another demotion reason is still active */
                return;
        }

        boolean_t removed = thread_run_queue_remove(thread);

        sched_set_thread_mode(thread, thread->saved_mode);

        thread->saved_mode = TH_MODE_NONE;

        thread_recompute_priority(thread);

        if (removed) {
                thread_run_queue_reinsert(thread, SCHED_TAILQ);
        }
}

/*
 * Promote thread to have a sched pri floor for a specific reason
 *
 * Promotion must not last past syscall boundary
 * Clients must always pair promote and demote 1:1,
 * Handling nesting of the same promote reason is the client's responsibility
 *
 * Called at splsched with thread locked
 */
void
sched_thread_promote_reason(thread_t    thread,
    uint32_t    reason,
    __kdebug_only uintptr_t   trace_obj /* already unslid */)
{
        assert(reason & TH_SFLAG_PROMOTE_REASON_MASK);
        assert((thread->sched_flags & reason) != reason);

        switch (reason) {
        case TH_SFLAG_RW_PROMOTED:
                KDBG(MACHDBG_CODE(DBG_MACH_SCHED, MACH_RW_PROMOTE),
                    thread_tid(thread), thread->sched_pri,
                    thread->base_pri, trace_obj);
                break;
        case TH_SFLAG_WAITQ_PROMOTED:
                KDBG(MACHDBG_CODE(DBG_MACH_SCHED, MACH_WAITQ_PROMOTE),
                    thread_tid(thread), thread->sched_pri,
                    thread->base_pri, trace_obj);
                break;
        case TH_SFLAG_EXEC_PROMOTED:
                KDBG(MACHDBG_CODE(DBG_MACH_SCHED, MACH_EXEC_PROMOTE),
                    thread_tid(thread), thread->sched_pri,
                    thread->base_pri, trace_obj);
                break;
        }

        thread->sched_flags |= reason;

        thread_recompute_sched_pri(thread, SETPRI_DEFAULT);
}

/*
 * End a specific promotion reason
 * Demotes a thread back to its expected priority without the promotion in place
 *
 * Called at splsched with thread locked
 */
void
sched_thread_unpromote_reason(thread_t  thread,
    uint32_t  reason,
    __kdebug_only uintptr_t trace_obj /* already unslid */)
{
        assert(reason & TH_SFLAG_PROMOTE_REASON_MASK);
        assert((thread->sched_flags & reason) == reason);

        switch (reason) {
        case TH_SFLAG_RW_PROMOTED:
                KDBG(MACHDBG_CODE(DBG_MACH_SCHED, MACH_RW_DEMOTE),
                    thread_tid(thread), thread->sched_pri,
                    thread->base_pri, trace_obj);
                break;
        case TH_SFLAG_WAITQ_PROMOTED:
                KDBG(MACHDBG_CODE(DBG_MACH_SCHED, MACH_WAITQ_DEMOTE),
                    thread_tid(thread), thread->sched_pri,
                    thread->base_pri, trace_obj);
                break;
        case TH_SFLAG_EXEC_PROMOTED:
                KDBG(MACHDBG_CODE(DBG_MACH_SCHED, MACH_EXEC_DEMOTE),
                    thread_tid(thread), thread->sched_pri,
                    thread->base_pri, trace_obj);
                break;
        }

        thread->sched_flags &= ~reason;

        thread_recompute_sched_pri(thread, SETPRI_DEFAULT);
}