xnu-344.23.tar.gz

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

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * The contents of this file constitute Original Code as defined in and
 * are subject to the Apple Public Source License Version 1.1 (the
 * "License").  You may not use this file except in compliance with the
 * License.  Please obtain a copy of the License at
 * http://www.apple.com/publicsource and read it before using this file.
 * 
 * This 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 OR NON-INFRINGEMENT.  Please see the
 * License for the specific language governing rights and limitations
 * under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */
/*
 * @OSF_COPYRIGHT@
 */
/*      
 *              File:   kern/task_swap.c
 *      
 *      Task residency management primitives implementation.
 */
#include <mach_assert.h>
#include <task_swapper.h>

#include <kern/spl.h>
#include <kern/lock.h>
#include <kern/queue.h>
#include <kern/host.h>
#include <kern/task.h>
#include <kern/task_swap.h>
#include <kern/thread.h>
#include <kern/thread_swap.h>
#include <kern/host_statistics.h>
#include <kern/misc_protos.h>
#include <kern/assert.h>
#include <mach/policy.h>

#include <ipc/ipc_port.h>       /* We use something from in here */

/*
 * Note:  if TASK_SWAPPER is disabled, then this file defines only
 * a stub version of task_swappable(), so that the service can always
 * be defined, even if swapping has been configured out of the kernel.
 */
#if TASK_SWAPPER

/* temporary debug flags */
#define TASK_SW_DEBUG 1
#define TASK_SW_STATS 1

int task_swap_debug = 0;
int task_swap_stats = 0;
int task_swap_enable = 1;
int task_swap_on = 1;

queue_head_t    swapped_tasks;          /* completely swapped out tasks */
queue_head_t    swapout_thread_q;       /* threads to be swapped out */
mutex_t         task_swapper_lock;      /* protects above queue */

#define task_swapper_lock()     mutex_lock(&task_swapper_lock)
#define task_swapper_unlock()   mutex_unlock(&task_swapper_lock)
#define task_swapper_wakeup()   thread_wakeup((event_t)&swapout_thread_q)
#define task_swapper_sleep()    thread_sleep_mutex((event_t)&swapout_thread_q, \
                                                        &task_swapper_lock,    \
                                                        THREAD_UNINT)


queue_head_t    eligible_tasks;         /* tasks eligible for swapout */
mutex_t         task_swapout_list_lock; /* protects above queue */
#define task_swapout_lock()     mutex_lock(&task_swapout_list_lock)
#define task_swapout_unlock()   mutex_unlock(&task_swapout_list_lock)

/*
 * The next section of constants and globals are tunable parameters
 * used in making swapping decisions.  They may be changed dynamically
 * without adversely affecting the robustness of the system; however,
 * the policy will change, one way or the other.
 */

#define SHORT_AVG_INTERVAL      5       /* in seconds */
#define LONG_AVG_INTERVAL       30      /* in seconds */
#define AVE_SCALE               1024

unsigned int short_avg_interval = SHORT_AVG_INTERVAL;
unsigned int long_avg_interval = LONG_AVG_INTERVAL;

#ifndef MIN_SWAP_PAGEOUT_RATE
#define MIN_SWAP_PAGEOUT_RATE   10
#endif

/*
 * The following are all stored in fixed-point representation (the actual
 * value times AVE_SCALE), to allow more accurate computing of decaying
 * averages.  So all variables that end with "avg" must be divided by
 * AVE_SCALE to convert them or compare them to ints.
 */
unsigned int vm_grab_rate_avg;
unsigned int vm_pageout_rate_avg = MIN_SWAP_PAGEOUT_RATE * AVE_SCALE;
unsigned int vm_pageout_rate_longavg = MIN_SWAP_PAGEOUT_RATE * AVE_SCALE;
unsigned int vm_pageout_rate_peakavg = MIN_SWAP_PAGEOUT_RATE * AVE_SCALE;
unsigned int vm_page_free_avg;  /* average free pages over short_avg_interval */
unsigned int vm_page_free_longavg; /* avg free pages over long_avg_interval */

/*
 * Trigger task swapping when paging activity reaches
 * SWAP_HIGH_WATER_MARK per cent of the maximum paging activity ever observed.
 * Turn off task swapping when paging activity goes back down to below
 * SWAP_PAGEOUT_LOW_WATER_MARK per cent of the maximum.
 * These numbers have been found empirically and might need some tuning...
 */
#ifndef SWAP_PAGEOUT_HIGH_WATER_MARK
#define SWAP_PAGEOUT_HIGH_WATER_MARK    30
#endif
#ifndef SWAP_PAGEOUT_LOW_WATER_MARK
#define SWAP_PAGEOUT_LOW_WATER_MARK     10
#endif

#ifndef MAX_GRAB_RATE
#define MAX_GRAB_RATE   ((unsigned int) -1)     /* XXX no maximum */
#endif

/*
 * swap_{start,stop}_pageout_rate start at the minimum value, then increase
 * to adjust to the hardware's performance, following the paging rate peaks.
 */
unsigned int swap_pageout_high_water_mark = SWAP_PAGEOUT_HIGH_WATER_MARK;
unsigned int swap_pageout_low_water_mark = SWAP_PAGEOUT_LOW_WATER_MARK;
unsigned int swap_start_pageout_rate = MIN_SWAP_PAGEOUT_RATE * AVE_SCALE *
                                        SWAP_PAGEOUT_HIGH_WATER_MARK / 100;
unsigned int swap_stop_pageout_rate = MIN_SWAP_PAGEOUT_RATE * AVE_SCALE *
                                        SWAP_PAGEOUT_LOW_WATER_MARK / 100;
#if     TASK_SW_DEBUG
unsigned int fixed_swap_start_pageout_rate = 0; /* only for testing purpose */
unsigned int fixed_swap_stop_pageout_rate = 0;  /* only for testing purpose */
#endif  /* TASK_SW_DEBUG */
unsigned int max_grab_rate = MAX_GRAB_RATE;

#ifndef MIN_SWAP_TIME
#define MIN_SWAP_TIME   1
#endif

int min_swap_time = MIN_SWAP_TIME;                      /* in seconds */

#ifndef MIN_RES_TIME
#define MIN_RES_TIME    6
#endif

int min_res_time = MIN_RES_TIME;                        /* in seconds */

#ifndef MIN_ACTIVE_TASKS
#define MIN_ACTIVE_TASKS        4
#endif

int min_active_tasks = MIN_ACTIVE_TASKS;

#ifndef TASK_SWAP_CYCLE_TIME
#define TASK_SWAP_CYCLE_TIME    2
#endif

int task_swap_cycle_time = TASK_SWAP_CYCLE_TIME;        /* in seconds */

int last_task_swap_cycle = 0;

/* temporary statistics */
int task_swapouts = 0;
int task_swapins = 0;
int task_swaprss_out = 0;       /* total rss at swapout time */
int task_swaprss_in = 0;        /* total rss at swapin time */
int task_swap_total_time = 0;   /* total time spent swapped out */
int tasks_swapped_out = 0;      /* number of tasks swapped out now */

#ifdef  TASK_SW_STATS
#define TASK_STATS_INCR(cnt)    (cnt)++
#else
#define TASK_STATS_INCR(cnt)
#endif  /* TASK_SW_STATS */

#if     TASK_SW_DEBUG
boolean_t on_swapped_list(task_t task); /* forward */
/*
 * Debug function to determine if a task is already on the
 * swapped out tasks list.  It also checks for tasks on the list
 * that are in an illegal state (i.e. swapped in).
 */
boolean_t
on_swapped_list(task_t task)
{
        task_t ltask;
        /* task_swapper_lock is locked. */

        if (queue_empty(&swapped_tasks)) {
                return(FALSE);
        }
        ltask = (task_t)queue_first(&swapped_tasks);
        while (!queue_end(&swapped_tasks, (queue_entry_t)ltask)) {
                /* check for illegal state */
                if (ltask->swap_state == TASK_SW_IN) {
                        printf("on_swapped_list and in: 0x%X\n",ltask);
                        Debugger("");
                }
                if (ltask == task)
                        return(TRUE);
                ltask = (task_t)queue_next(&ltask->swapped_tasks);
        }
        return(FALSE);
}
#endif  /* TASK_SW_DEBUG */

/*
 *      task_swapper_init: [exported]
 */
void
task_swapper_init()
{
        queue_init(&swapped_tasks);
        queue_init(&eligible_tasks);
        queue_init(&swapout_thread_q);
        mutex_init(&task_swapper_lock, ETAP_THREAD_TASK_SWAP);
        mutex_init(&task_swapout_list_lock, ETAP_THREAD_TASK_SWAPOUT);
        vm_page_free_avg = vm_page_free_count * AVE_SCALE;
        vm_page_free_longavg = vm_page_free_count * AVE_SCALE;
}

#endif  /* TASK_SWAPPER */

/*
 *      task_swappable: [exported]
 *
 *      Make a task swappable or non-swappable. If made non-swappable,
 *      it will be swapped in.
 *
 *      Locking: task_swapout_lock is taken before task lock.
 */
kern_return_t
task_swappable(
        host_priv_t host_priv,
        task_t task,
        boolean_t make_swappable)
{
        if (host_priv == HOST_PRIV_NULL)
                return(KERN_INVALID_ARGUMENT);

        if (task == TASK_NULL)
                return(KERN_INVALID_ARGUMENT);

#if     !TASK_SWAPPER

        /*
         * If we don't support swapping, this call is purely advisory.
         */
        return(KERN_SUCCESS);

#else   /* TASK_SWAPPER */
        
        task_lock(task);
        if (make_swappable) {
                /* make task swappable */
                if (task->swap_state == TASK_SW_UNSWAPPABLE) {
                        task->swap_state = TASK_SW_IN; 
                        task_unlock(task);
                        task_swapout_eligible(task);
                }
        } else {
            switch (task->swap_state) {
                case TASK_SW_IN:
                        task->swap_state = TASK_SW_UNSWAPPABLE;
                        task_unlock(task);
                        task_swapout_ineligible(task);
                        break;
                case TASK_SW_UNSWAPPABLE:
                        task_unlock(task);
                        break;
                default:
                        /*
                         * swap_state could be TASK_SW_OUT, TASK_SW_GOING_OUT,
                         * or TASK_SW_COMING_IN.  task_swapin handles all
                         * three, and its default case will catch any bad
                         * states.
                         */
                        task_unlock(task);
                        task_swapin(task, TRUE);
                        break;
            }
        }
        return(KERN_SUCCESS);

#endif  /* TASK_SWAPPER */

}

#if     TASK_SWAPPER

/*
 *      task_swapout:
 *      A reference to the task must be held.
 *
 *      Start swapping out a task by sending an AST_SWAPOUT to each thread.
 *      When the threads reach a clean point, they queue themselves up on the
 *      swapout_thread_q to be swapped out by the task_swap_swapout_thread.
 *      The task can be swapped in at any point in this process.
 *
 *      A task will not be fully swapped out (i.e. its map residence count
 *      at zero) until all currently-swapped threads run and reach
 *      a clean point, at which time they will be swapped again,
 *      decrementing the swap_ast_waiting count on the task.
 *
 *      Locking: no locks held upon entry and exit.
 *               Task_lock is held throughout this function.
 */
kern_return_t
task_swapout(task_t task)
{
        thread_act_t thr_act;
        thread_t thread;
        queue_head_t *list;
        int s;

        task_swapout_lock();
        task_lock(task);
        /*
         * NOTE: look into turning these into assertions if they
         * are invariants.
         */
        if ((task->swap_state != TASK_SW_IN) || (!task->active)) {
                task_unlock(task);
                task_swapout_unlock();
                return(KERN_FAILURE);
        }
        if (task->swap_flags & TASK_SW_ELIGIBLE) {
                queue_remove(&eligible_tasks, task, task_t, swapped_tasks);
                task->swap_flags &= ~TASK_SW_ELIGIBLE;
        }
        task_swapout_unlock();

        /* set state to avoid races with task_swappable(FALSE) */
        task->swap_state = TASK_SW_GOING_OUT;
        task->swap_rss = pmap_resident_count(task->map->pmap);
        task_swaprss_out += task->swap_rss;
        task->swap_ast_waiting = task->thr_act_count;

        /*
         * halt all threads in this task:
         * We don't need the thread list lock for traversal.
         */
        list = &task->thr_acts;
        thr_act = (thread_act_t) queue_first(list);
        while (!queue_end(list, (queue_entry_t) thr_act)) {
                boolean_t swappable;
                thread_act_t ract;

                thread = act_lock_thread(thr_act);
                s = splsched();
                if (!thread)
                        swappable = (thr_act->swap_state != TH_SW_UNSWAPPABLE);
                else {
                        thread_lock(thread);
                        swappable = TRUE;
                        for (ract = thread->top_act; ract; ract = ract->lower)
                                if (ract->swap_state == TH_SW_UNSWAPPABLE) {
                                        swappable = FALSE;
                                        break;
                                }
                }
                if (swappable)
                        thread_ast_set(thr_act, AST_SWAPOUT);
                if (thread)
                        thread_unlock(thread);
                splx(s);
                assert((thr_act->ast & AST_TERMINATE) == 0);
                act_unlock_thread(thr_act);
                thr_act = (thread_act_t) queue_next(&thr_act->thr_acts);
        }

        task->swap_stamp = sched_tick;
        task->swap_nswap++;
        assert((task->swap_flags&TASK_SW_WANT_IN) == 0);
        /* put task on the queue of swapped out tasks */
        task_swapper_lock();
#if     TASK_SW_DEBUG
        if (task_swap_debug && on_swapped_list(task)) {
                printf("task 0x%X already on list\n", task);
                Debugger("");
        }
#endif  /* TASK_SW_DEBUG */
        queue_enter(&swapped_tasks, task, task_t, swapped_tasks);
        tasks_swapped_out++;
        task_swapouts++;
        task_swapper_unlock();
        task_unlock(task);

        return(KERN_SUCCESS);
}

#ifdef  TASK_SW_STATS
int     task_sw_race_in = 0;
int     task_sw_race_coming_in = 0;
int     task_sw_race_going_out = 0;
int     task_sw_before_ast = 0;
int     task_sw_before_swap = 0;
int     task_sw_after_swap = 0;
int     task_sw_race_in_won = 0;
int     task_sw_unswappable = 0;
int     task_sw_act_inactive = 0;
#endif  /* TASK_SW_STATS */

/*
 *      thread_swapout_enqueue is called by thread_halt_self when it 
 *      processes AST_SWAPOUT to enqueue threads to be swapped out.
 *      It must be called at normal interrupt priority for the
 *      sake of the task_swapper_lock.
 *
 *      There can be races with task swapin here.
 *      First lock task and decrement swap_ast_waiting count, and if
 *      it's 0, we can decrement the residence count on the task's map
 *      and set the task's swap state to TASK_SW_OUT.
 */
void
thread_swapout_enqueue(thread_act_t thr_act)
{
        task_t task = thr_act->task;
        task_lock(task);
        /*
         * If the swap_state is not TASK_SW_GOING_OUT, then
         * task_swapin has beaten us to this operation, and
         * we have nothing to do.
         */
        if (task->swap_state != TASK_SW_GOING_OUT) {
                task_unlock(task);
                return;
        }
        if (--task->swap_ast_waiting == 0) {
                vm_map_t map = task->map;
                task->swap_state = TASK_SW_OUT;
                task_unlock(task);
                mutex_lock(&map->s_lock);
                vm_map_res_deallocate(map);
                mutex_unlock(&map->s_lock);
        } else
                task_unlock(task);

        task_swapper_lock();
        act_lock(thr_act);
        if (! (thr_act->swap_state & TH_SW_TASK_SWAPPING)) {
                /*
                 * We lost a race with task_swapin(): don't enqueue.
                 */
        } else {
                queue_enter(&swapout_thread_q, thr_act,
                            thread_act_t, swap_queue);
                task_swapper_wakeup();
        }
        act_unlock(thr_act);
        task_swapper_unlock();
}

/*
 *      task_swap_swapout_thread: [exported]
 *
 *      Executes as a separate kernel thread.
 *      Its job is to swap out threads that have been halted by AST_SWAPOUT.
 */
void
task_swap_swapout_thread(void)
{
        thread_act_t thr_act;
        thread_t thread, nthread;
        task_t task;
        int s;

        thread_swappable(current_act(), FALSE);
        stack_privilege(current_thread());

        spllo();

        task_swapper_lock();
        while (TRUE) {
                while (! queue_empty(&swapout_thread_q)) {

                        queue_remove_first(&swapout_thread_q, thr_act,
                                           thread_act_t, swap_queue);
                        /*
                         * If we're racing with task_swapin, we need
                         * to make it safe for it to do remque on the
                         * thread, so make its links point to itself.
                         * Allowing this ugliness is cheaper than 
                         * making task_swapin search the entire queue.
                         */
                        act_lock(thr_act);
                        queue_init((queue_t) &thr_act->swap_queue);
                        act_unlock(thr_act);
                        task_swapper_unlock();
                        /*
                         * Wait for thread's RUN bit to be deasserted.
                         */
                        thread = act_lock_thread(thr_act);
                        if (thread == THREAD_NULL)
                                act_unlock_thread(thr_act);
                        else {
                                boolean_t r;

                                thread_reference(thread);
                                thread_hold(thr_act);
                                act_unlock_thread(thr_act);
                                r = thread_stop_wait(thread);
                                nthread = act_lock_thread(thr_act);
                                thread_release(thr_act);
                                thread_deallocate(thread);
                                act_unlock_thread(thr_act);
                                if (!r || nthread != thread) {
                                        task_swapper_lock();
                                        continue;
                                }
                        }
                        task = thr_act->task;
                        task_lock(task);
                        /* 
                         * we can race with swapin, which would set the
                         * state to TASK_SW_IN. 
                         */
                        if ((task->swap_state != TASK_SW_OUT) &&
                            (task->swap_state != TASK_SW_GOING_OUT)) {
                                task_unlock(task);
                                task_swapper_lock();
                                TASK_STATS_INCR(task_sw_race_in_won);
                                if (thread != THREAD_NULL)
                                        thread_unstop(thread);
                                continue;
                        }
                        nthread = act_lock_thread(thr_act);
                        if (nthread != thread || thr_act->active == FALSE) {
                                act_unlock_thread(thr_act);
                                task_unlock(task);
                                task_swapper_lock();
                                TASK_STATS_INCR(task_sw_act_inactive);
                                if (thread != THREAD_NULL)
                                        thread_unstop(thread);
                                continue;
                        }
                        s = splsched();
                        if (thread != THREAD_NULL)
                                thread_lock(thread);
                        /* 
                         * Thread cannot have been swapped out yet because
                         * TH_SW_TASK_SWAPPING was set in AST.  If task_swapin
                         * beat us here, we either wouldn't have found it on
                         * the queue, or the task->swap_state would have
                         * changed.  The synchronization is on the
                         * task's swap_state and the task_lock.
                         * The thread can't be swapped in any other way
                         * because its task has been swapped.
                         */
                        assert(thr_act->swap_state & TH_SW_TASK_SWAPPING);
                        assert(thread == THREAD_NULL ||
                               !(thread->state & (TH_SWAPPED_OUT|TH_RUN)));
                        assert((thr_act->swap_state & TH_SW_STATE) == TH_SW_IN);
                        /* assert(thread->state & TH_HALTED); */
                        /* this also clears TH_SW_TASK_SWAPPING flag */
                        thr_act->swap_state = TH_SW_GOING_OUT;
                        if (thread != THREAD_NULL) {
                                if (thread->top_act == thr_act) {
                                        thread->state |= TH_SWAPPED_OUT;
                                        /*
                                         * Once we unlock the task, things can happen
                                         * to the thread, so make sure it's consistent
                                         * for thread_swapout.
                                         */
                                }
                                thread->ref_count++;
                                thread_unlock(thread);
                                thread_unstop(thread);
                        }
                        splx(s);
                        act_locked_act_reference(thr_act);
                        act_unlock_thread(thr_act);
                        task_unlock(task);

                        thread_swapout(thr_act);        /* do the work */

                        if (thread != THREAD_NULL)
                                thread_deallocate(thread);
                        act_deallocate(thr_act);
                        task_swapper_lock();
                }
                task_swapper_sleep();
        }
}

/*
 *      task_swapin:
 *
 *      Make a task resident.
 *      Performs all of the work to make a task resident and possibly
 *      non-swappable.  If we race with a competing task_swapin call,
 *      we wait for its completion, then return.
 *
 *      Locking: no locks held upon entry and exit.
 *
 *      Note that TASK_SW_MAKE_UNSWAPPABLE can only be set when the
 *      state is TASK_SW_COMING_IN.
 */

kern_return_t
task_swapin(task_t task, boolean_t make_unswappable)
{
        register queue_head_t   *list;
        register thread_act_t   thr_act, next;
        thread_t                thread;
        int                     s;
        boolean_t               swappable = TRUE;

        task_lock(task);
        switch (task->swap_state) {
            case TASK_SW_OUT:
                        {
                        vm_map_t map = task->map;
                        /*
                         * Task has made it all the way out, which means
                         * that vm_map_res_deallocate has been done; set 
                         * state to TASK_SW_COMING_IN, then bring map
                         * back in.  We could actually be racing with
                         * the thread_swapout_enqueue, which does the
                         * vm_map_res_deallocate, but that race is covered.
                         */
                        task->swap_state = TASK_SW_COMING_IN;
                        assert(task->swap_ast_waiting == 0);
                        assert(map->res_count >= 0);
                        task_unlock(task);
                        mutex_lock(&map->s_lock);
                        vm_map_res_reference(map);
                        mutex_unlock(&map->s_lock);
                        task_lock(task);
                        assert(task->swap_state == TASK_SW_COMING_IN);
                        }
                        break;

            case TASK_SW_GOING_OUT:
                        /*
                         * Task isn't all the way out yet.  There is
                         * still at least one thread not swapped, and
                         * vm_map_res_deallocate has not been done.
                         */
                        task->swap_state = TASK_SW_COMING_IN;
                        assert(task->swap_ast_waiting > 0 ||
                               (task->swap_ast_waiting == 0 &&
                                task->thr_act_count == 0));
                        assert(task->map->res_count > 0);
                        TASK_STATS_INCR(task_sw_race_going_out);
                        break;
            case TASK_SW_IN:
                        assert(task->map->res_count > 0);
#if     TASK_SW_DEBUG
                        task_swapper_lock();
                        if (task_swap_debug && on_swapped_list(task)) {
                                printf("task 0x%X on list, state is SW_IN\n",
                                        task);
                                Debugger("");
                        }
                        task_swapper_unlock();
#endif  /* TASK_SW_DEBUG */
                        TASK_STATS_INCR(task_sw_race_in);
                        if (make_unswappable) {
                                task->swap_state = TASK_SW_UNSWAPPABLE;
                                task_unlock(task);
                                task_swapout_ineligible(task);
                        } else
                                task_unlock(task);
                        return(KERN_SUCCESS);
            case TASK_SW_COMING_IN:
                        /* 
                         * Raced with another task_swapin and lost;
                         * wait for other one to complete first
                         */
                        assert(task->map->res_count >= 0);
                        /*
                         * set MAKE_UNSWAPPABLE so that whoever is swapping
                         * the task in will make it unswappable, and return
                         */
                        if (make_unswappable)
                                task->swap_flags |= TASK_SW_MAKE_UNSWAPPABLE;
                        task->swap_flags |= TASK_SW_WANT_IN;
                        assert_wait((event_t)&task->swap_state, THREAD_UNINT);
                        task_unlock(task);
                        thread_block(THREAD_CONTINUE_NULL);
                        TASK_STATS_INCR(task_sw_race_coming_in);
                        return(KERN_SUCCESS);
            case TASK_SW_UNSWAPPABLE:
                        /* 
                         * This can happen, since task_terminate 
                         * unconditionally calls task_swapin.
                         */
                        task_unlock(task);
                        return(KERN_SUCCESS);
            default:
                        panic("task_swapin bad state");
                        break;
        }
        if (make_unswappable)
                task->swap_flags |= TASK_SW_MAKE_UNSWAPPABLE;
        assert(task->swap_state == TASK_SW_COMING_IN);
        task_swapper_lock();
#if     TASK_SW_DEBUG
        if (task_swap_debug && !on_swapped_list(task)) {
                printf("task 0x%X not on list\n", task);
                Debugger("");
        }
#endif  /* TASK_SW_DEBUG */
        queue_remove(&swapped_tasks, task, task_t, swapped_tasks);
        tasks_swapped_out--;
        task_swapins++;
        task_swapper_unlock();

        /*
         * Iterate through all threads for this task and 
         * release them, as required.  They may not have been swapped
         * out yet.  The task remains locked throughout.
         */
        list = &task->thr_acts;
        thr_act = (thread_act_t) queue_first(list);
        while (!queue_end(list, (queue_entry_t) thr_act)) {
                boolean_t need_to_release;
                next = (thread_act_t) queue_next(&thr_act->thr_acts);
                /*
                 * Keep task_swapper_lock across thread handling
                 * to synchronize with task_swap_swapout_thread
                 */
                task_swapper_lock();
                thread = act_lock_thread(thr_act);
                s = splsched();
                if (thr_act->ast & AST_SWAPOUT) {
                        /* thread hasn't gotten the AST yet, just clear it */
                        thread_ast_clear(thr_act, AST_SWAPOUT);
                        need_to_release = FALSE;
                        TASK_STATS_INCR(task_sw_before_ast);
                        splx(s);
                        act_unlock_thread(thr_act);
                } else {
                        /*
                         * If AST_SWAPOUT was cleared, then thread_hold,
                         * or equivalent was done.
                         */
                        need_to_release = TRUE;
                        /*
                         * Thread has hit AST, but it may not have
                         * been dequeued yet, so we need to check.
                         * NOTE: the thread may have been dequeued, but
                         * has not yet been swapped (the task_swapper_lock
                         * has been dropped, but the thread is not yet
                         * locked), and the TH_SW_TASK_SWAPPING flag may 
                         * not have been cleared.  In this case, we will do 
                         * an extra remque, which the task_swap_swapout_thread
                         * has made safe, and clear the flag, which is also
                         * checked by the t_s_s_t before doing the swapout.
                         */
                        if (thread)
                                thread_lock(thread);
                        if (thr_act->swap_state & TH_SW_TASK_SWAPPING) {
                                /* 
                                 * hasn't yet been dequeued for swapout,
                                 * so clear flags and dequeue it first.
                                 */
                                thr_act->swap_state &= ~TH_SW_TASK_SWAPPING;
                                assert(thr_act->thread == THREAD_NULL || 
                                       !(thr_act->thread->state &
                                         TH_SWAPPED_OUT));
                                queue_remove(&swapout_thread_q, thr_act,
                                             thread_act_t, swap_queue);
                                TASK_STATS_INCR(task_sw_before_swap);
                        } else {
                                TASK_STATS_INCR(task_sw_after_swap);
                                /*
                                 * It's possible that the thread was
                                 * made unswappable before hitting the
                                 * AST, in which case it's still running.
                                 */
                                if (thr_act->swap_state == TH_SW_UNSWAPPABLE) {
                                        need_to_release = FALSE;
                                        TASK_STATS_INCR(task_sw_unswappable);
                                }
                        }
                        if (thread)
                                thread_unlock(thread);
                        splx(s);
                        act_unlock_thread(thr_act);
                }
                task_swapper_unlock();

                /* 
                 * thread_release will swap in the thread if it's been
                 * swapped out.
                 */
                if (need_to_release) {
                        act_lock_thread(thr_act);
                        thread_release(thr_act);
                        act_unlock_thread(thr_act);
                }
                thr_act = next;
        }

        if (task->swap_flags & TASK_SW_MAKE_UNSWAPPABLE) {
                task->swap_flags &= ~TASK_SW_MAKE_UNSWAPPABLE;
                task->swap_state = TASK_SW_UNSWAPPABLE;
                swappable = FALSE;
        } else {
                task->swap_state = TASK_SW_IN;
        }

        task_swaprss_in += pmap_resident_count(task->map->pmap);
        task_swap_total_time += sched_tick - task->swap_stamp;
        /* note when task came back in */
        task->swap_stamp = sched_tick;
        if (task->swap_flags & TASK_SW_WANT_IN) {
                task->swap_flags &= ~TASK_SW_WANT_IN;
                thread_wakeup((event_t)&task->swap_state);
        }
        assert((task->swap_flags & TASK_SW_ELIGIBLE) == 0);
        task_unlock(task);
#if     TASK_SW_DEBUG
        task_swapper_lock();
        if (task_swap_debug && on_swapped_list(task)) {
                printf("task 0x%X on list at end of swap in\n", task);
                Debugger("");
        }
        task_swapper_unlock();
#endif  /* TASK_SW_DEBUG */
        /*
         * Make the task eligible to be swapped again
         */
        if (swappable)
                task_swapout_eligible(task);
        return(KERN_SUCCESS);
}

void wake_task_swapper(boolean_t now);  /* forward */

/*
 *      wake_task_swapper: [exported]
 *
 *      Wakes up task swapper if now == TRUE or if at least
 *      task_swap_cycle_time has elapsed since the last call.
 *
 *      NOTE: this function is not multithreaded, so if there is
 *      more than one caller, it must be modified.
 */
void
wake_task_swapper(boolean_t now)
{
        /* last_task_swap_cycle may require locking */
        if (now ||
            (sched_tick > (last_task_swap_cycle + task_swap_cycle_time))) {
                last_task_swap_cycle = sched_tick;
                if (task_swap_debug)
                        printf("wake_task_swapper: waking swapper\n");
                thread_wakeup((event_t)&swapped_tasks); /* poke swapper */
        }
}

task_t pick_intask(void);       /* forward */
/*
 * pick_intask:
 * returns a task to be swapped in, or TASK_NULL if nothing suitable is found.
 *
 * current algorithm: Return the task that has been swapped out the 
 *      longest, as long as it is > min_swap_time.  It will be dequeued
 *      if actually swapped in.
 *
 * NOTE:**********************************************
 * task->swap_rss (the size when the task was swapped out) could be used to
 * further refine the selection.  Another possibility would be to look at
 * the state of the thread(s) to see if the task/threads would run if they
 * were swapped in.
 * ***************************************************
 *
 * Locking:  no locks held upon entry and exit.
 */
task_t
pick_intask(void)
{
        register task_t         task = TASK_NULL;

        task_swapper_lock();
        /* the oldest task is the first one */
        if (!queue_empty(&swapped_tasks)) {
                task = (task_t) queue_first(&swapped_tasks);
                assert(task != TASK_NULL);
                /* Make sure it's been out min_swap_time */
                if ((sched_tick - task->swap_stamp) < min_swap_time)
                        task = TASK_NULL;
        }
        task_swapper_unlock();
        return(task);
#if     0
        /*
         * This code looks at the entire list of swapped tasks, but since
         * it does not yet do anything but look at time swapped, we 
         * can simply use the fact that the queue is ordered, and take 
         * the first one off the queue.
         */
        task = (task_t)queue_first(&swapped_tasks);
        while (!queue_end(&swapped_tasks, (queue_entry_t)task)) {
                task_lock(task);
                tmp_time = sched_tick - task->swap_stamp;
                if (tmp_time > min_swap_time && tmp_time > time_swapped) {
                        target_task = task;
                        time_swapped = tmp_time;
                }
                task_unlock(task);
                task = (task_t)queue_next(&task->swapped_tasks);
        }
        task_swapper_unlock();
        return(target_task);
#endif
}

task_t pick_outtask(void);      /* forward */
/*
 * pick_outtask:
 * returns a task to be swapped out, with a reference on the task,
 * or NULL if no suitable task is found.
 *
 * current algorithm:
 * 
 * Examine all eligible tasks.  While looking, use the first thread in 
 * each task as an indication of the task's activity.  Count up 
 * "active" threads (those either runnable or sleeping).  If the task 
 * is active (by these criteria), swapped in, and resident 
 * for at least min_res_time, then select the task with the largest 
 * number of pages in memory.  If there are less 
 * than min_active_tasks active tasks in the system, then don't 
 * swap anything out (this avoids swapping out the only running task 
 * in the system, for example).
 *
 * NOTE:  the task selected will not be removed from the eligible list.
 *        This means that it will be selected again if it is not swapped
 *        out, where it is removed from the list.
 *
 * Locking: no locks held upon entry and exit.  Task_swapout_lock must be
 *          taken before task locks.
 *
 * ***************************************************
 * TBD:
 * This algorithm only examines the first thread in the task.  Currently, since
 * most swappable tasks in the system are single-threaded, this generalization
 * works reasonably well.  However, the algorithm should be changed
 * to consider all threads in the task if more multi-threaded tasks were used.  
 * ***************************************************
 */

#ifdef  TASK_SW_STATS
int inactive_task_count = 0;
int empty_task_count = 0;
#endif  /* TASK_SW_STATS */

task_t
pick_outtask(void)
{
        register task_t         task;
        register task_t         target_task = TASK_NULL;
        unsigned long           task_rss;
        unsigned long           target_rss = 0;
        boolean_t               wired;
        boolean_t               active;
        int                     nactive = 0;

        task_swapout_lock();
        if (queue_empty(&eligible_tasks)) {
                /* not likely to happen */
                task_swapout_unlock();
                return(TASK_NULL);
        }
        task = (task_t)queue_first(&eligible_tasks);
        while (!queue_end(&eligible_tasks, (queue_entry_t)task)) {
                int s;
                register thread_act_t thr_act;
                thread_t th;
                

                task_lock(task);
                /*
                 * Don't swap real-time tasks.
                 * XXX Should we enforce that or can we let really critical
                 * tasks use task_swappable() to make sure they never end up
                 * n the eligible list ?
                 */
                if (task->policy & POLICYCLASS_FIXEDPRI) {
                        goto tryagain;
                }
                if (!task->active) {
                        TASK_STATS_INCR(inactive_task_count);
                        goto tryagain;
                }
                if (task->res_act_count == 0) {
                        TASK_STATS_INCR(empty_task_count);
                        goto tryagain;
                }
                assert(!queue_empty(&task->thr_acts));
                thr_act = (thread_act_t)queue_first(&task->thr_acts);
                active = FALSE;
                th = act_lock_thread(thr_act);
                s = splsched();
                if (th != THREAD_NULL)
                        thread_lock(th);
                if ((th == THREAD_NULL) ||
                    (th->state == TH_RUN) ||
                    (th->state & TH_WAIT)) {
                        /*
                         * thread is "active": either runnable 
                         * or sleeping.  Count it and examine 
                         * it further below.
                         */
                        nactive++;
                        active = TRUE;
                }
                if (th != THREAD_NULL)
                        thread_unlock(th);
                splx(s);
                act_unlock_thread(thr_act);
                if (active &&
                    (task->swap_state == TASK_SW_IN) &&
                    ((sched_tick - task->swap_stamp) > min_res_time)) {
                        long rescount = pmap_resident_count(task->map->pmap);
                        /*
                         * thread must be "active", task must be swapped
                         * in and resident for at least min_res_time
                         */
#if 0
/* DEBUG Test round-robin strategy.  Picking biggest task could cause extreme
 * unfairness to such large interactive programs as xterm.  Instead, pick the
 * first task that has any pages resident:
 */
                        if (rescount > 1) {
                                task->ref_count++;
                                target_task = task;
                                task_unlock(task);
                                task_swapout_unlock();
                                return(target_task);
                        }
#else
                        if (rescount > target_rss) {
                                /*
                                 * task is not swapped, and it has the
                                 * largest rss seen so far.
                                 */
                                task->ref_count++;
                                target_rss = rescount;
                                assert(target_task != task);
                                if (target_task != TASK_NULL)
                                        task_deallocate(target_task);
                                target_task = task;
                        }
#endif
                }
tryagain:
                task_unlock(task);
                task = (task_t)queue_next(&task->swapped_tasks);
        }
        task_swapout_unlock();
        /* only swap out if there are at least min_active_tasks */
        if (nactive < min_active_tasks) {
                if (target_task != TASK_NULL) {
                        task_deallocate(target_task);
                        target_task = TASK_NULL;
                }
        }
        return(target_task);
}

#if     TASK_SW_DEBUG
void print_pid(task_t task, unsigned long n1, unsigned long n2,
               const char *comp, const char *inout);    /* forward */
void
print_pid(
        task_t task,
        unsigned long n1,
        unsigned long n2,
        const char *comp,
        const char *inout)
{
        long rescount;
        task_lock(task);
        rescount = pmap_resident_count(task->map->pmap);
        task_unlock(task);
        printf("task_swapper: swapped %s task %x; %d %s %d; res=%d\n",
                inout, task, n1, comp, n2, rescount);
}
#endif

/*
 *      task_swapper: [exported]
 *
 *      Executes as a separate kernel thread.
 */
#define MAX_LOOP 3
void
task_swapper(void)
{
        task_t  outtask, intask;
        int timeout;
        int loopcnt = 0;
        boolean_t start_swapping;
        boolean_t stop_swapping;
        int local_page_free_avg;
        extern int hz;

        thread_swappable(current_act(), FALSE);
        stack_privilege(current_thread());

        spllo();

        for (;;) {
        local_page_free_avg = vm_page_free_avg;
        while (TRUE) {
#if     0
                if (task_swap_debug)
                        printf("task_swapper: top of loop; cnt = %d\n",loopcnt);
#endif
                intask = pick_intask();

                start_swapping = ((vm_pageout_rate_avg > swap_start_pageout_rate) ||
                                  (vm_grab_rate_avg > max_grab_rate));
                stop_swapping = (vm_pageout_rate_avg < swap_stop_pageout_rate);

                /*
                 * If a lot of paging is going on, or another task should come
                 * in but memory is tight, find something to swap out and start
                 * it.  Don't swap any task out if task swapping is disabled.
                 * vm_page_queue_free_lock protects the vm globals.
                 */
                outtask = TASK_NULL;
                if (start_swapping ||
                    (!stop_swapping && intask &&
                     ((local_page_free_avg / AVE_SCALE) < vm_page_free_target))
                   ) {
                        if (task_swap_enable &&
                            (outtask = pick_outtask()) &&
                            (task_swapout(outtask) == KERN_SUCCESS)) {
                                unsigned long rss;
#if     TASK_SW_DEBUG
                                if (task_swap_debug)
                                    print_pid(outtask, local_page_free_avg / AVE_SCALE,
                                              vm_page_free_target, "<",
                                              "out");
#endif
                                rss = outtask->swap_rss;
                                if (outtask->swap_nswap == 1)
                                        rss /= 2; /* divide by 2 if never out */
                                local_page_free_avg += (rss/short_avg_interval) * AVE_SCALE;
                        }
                        if (outtask != TASK_NULL)
                                task_deallocate(outtask);
                }

                /*
                 * If there is an eligible task to bring in and there are at
                 * least vm_page_free_target free pages, swap it in.  If task
                 * swapping has been disabled, bring the task in anyway.
                 */
                if (intask && ((local_page_free_avg / AVE_SCALE) >=
                                                        vm_page_free_target ||
                                stop_swapping || !task_swap_enable)) {
                        if (task_swapin(intask, FALSE) == KERN_SUCCESS) {
                                unsigned long rss;
#if     TASK_SW_DEBUG
                                if (task_swap_debug)
                                    print_pid(intask, local_page_free_avg / AVE_SCALE,
                                              vm_page_free_target, ">=",
                                              "in");
#endif
                                rss = intask->swap_rss;
                                if (intask->swap_nswap == 1)
                                        rss /= 2; /* divide by 2 if never out */
                                local_page_free_avg -= (rss/short_avg_interval) * AVE_SCALE;
                        }
                }
                /*
                 * XXX
                 * Here we have to decide whether to continue swapping
                 * in and/or out before sleeping.  The decision should
                 * be made based on the previous action (swapin/out) and
                 * current system parameters, such as paging rates and
                 * demand.
                 * The function, compute_vm_averages, which does these
                 * calculations, depends on being called every second,
                 * so we can't just do the same thing.
                 */
                if (++loopcnt < MAX_LOOP)
                        continue;

                /*
                 * Arrange to be awakened if paging is still heavy or there are
                 * any tasks partially or completely swapped out.  (Otherwise,
                 * the wakeup will come from the external trigger(s).)
                 */
                timeout = 0;
                if (start_swapping)
                        timeout = task_swap_cycle_time;
                else {
                        task_swapper_lock();
                        if (!queue_empty(&swapped_tasks))
                                timeout = min_swap_time;
                        task_swapper_unlock();
                }
                assert_wait((event_t)&swapped_tasks, THREAD_UNINT);
                if (timeout) {
                        if (task_swap_debug)
                                printf("task_swapper: set timeout of %d\n",
                                                                timeout);
                        thread_set_timeout(timeout, NSEC_PER_SEC);
                }
                if (task_swap_debug)
                        printf("task_swapper: blocking\n");
                thread_block(THREAD_CONTINUE_NULL);
                if (timeout) {
                        thread_cancel_timeout(current_thread());
                }
                /* reset locals */
                loopcnt = 0;
                local_page_free_avg = vm_page_free_avg;
        }
        }
}

/* from BSD */
#define ave(smooth, cnt, time) \
        smooth = ((time - 1) * (smooth) + ((cnt) * AVE_SCALE)) / (time)

/*
 * We estimate the system paging load in more than one metric: 
 *      1) the total number of calls into the function, vm_page_grab, 
 *         which allocates all page frames for real pages.
 *      2) the total number of pages paged in and out of paging files.
 *         This is a measure of page cleaning and faulting from backing
 *         store.
 *
 * When either metric passes a threshold, tasks are swapped out.
 */
long last_grab_count = 0;
long last_pageout_count = 0;

/*
 * compute_vm_averages: [exported]
 *
 * This function is to be called once a second to calculate average paging
 * demand and average numbers of free pages for use by the task swapper.
 * Can also be used to wake up task swapper at desired thresholds.
 *
 * NOTE: this function is single-threaded, and requires locking if
 *      ever there are multiple callers.
 */
void
compute_vm_averages(void)
{
        extern unsigned long vm_page_grab_count;
        long grab_count, pageout_count;
        int i;

        ave(vm_page_free_avg, vm_page_free_count, short_avg_interval);
        ave(vm_page_free_longavg, vm_page_free_count, long_avg_interval);

        /* 
         * NOTE: the vm_page_grab_count and vm_stat structure are 
         * under control of vm_page_queue_free_lock.  We're simply reading
         * memory here, and the numbers don't depend on each other, so
         * no lock is taken.
         */

        grab_count = vm_page_grab_count;
        pageout_count = 0;
        for (i = 0; i < NCPUS; i++) {
                pageout_count += vm_stat[i].pageouts;
        }

        ave(vm_pageout_rate_avg, pageout_count - last_pageout_count,
            short_avg_interval);
        ave(vm_pageout_rate_longavg, pageout_count - last_pageout_count,
            long_avg_interval);
        ave(vm_grab_rate_avg, grab_count - last_grab_count,
            short_avg_interval);
        last_grab_count = grab_count;
        last_pageout_count = pageout_count;

        /*
         * Adjust swap_{start,stop}_pageout_rate to the paging rate peak.
         * This is an attempt to find the optimum paging rates at which
         * to trigger task swapping on or off to regulate paging activity,
         * depending on the hardware capacity.
         */
        if (vm_pageout_rate_avg > vm_pageout_rate_peakavg) {
                unsigned int desired_max;

                vm_pageout_rate_peakavg = vm_pageout_rate_avg;
                swap_start_pageout_rate =
                        vm_pageout_rate_peakavg * swap_pageout_high_water_mark / 100;
                swap_stop_pageout_rate = 
                        vm_pageout_rate_peakavg * swap_pageout_low_water_mark / 100;
        }

#if     TASK_SW_DEBUG
        /*
         * For measurements, allow fixed values.
         */
        if (fixed_swap_start_pageout_rate)
                swap_start_pageout_rate = fixed_swap_start_pageout_rate;
        if (fixed_swap_stop_pageout_rate)
                swap_stop_pageout_rate = fixed_swap_stop_pageout_rate;
#endif  /* TASK_SW_DEBUG */

#if     TASK_SW_DEBUG
        if (task_swap_stats)
                printf("vm_avgs: pageout_rate: %d %d (on/off: %d/%d); page_free: %d %d (tgt: %d)\n",
                       vm_pageout_rate_avg / AVE_SCALE,
                       vm_pageout_rate_longavg / AVE_SCALE,
                       swap_start_pageout_rate / AVE_SCALE,
                       swap_stop_pageout_rate / AVE_SCALE,
                       vm_page_free_avg / AVE_SCALE,
                       vm_page_free_longavg / AVE_SCALE,
                       vm_page_free_target);
#endif  /* TASK_SW_DEBUG */
        
        if (vm_page_free_avg / AVE_SCALE <= vm_page_free_target) {
                if (task_swap_on) {
                        /* The following is a delicate attempt to balance the
                         * need for reasonably rapid response to system
                         * thrashing, with the equally important desire to
                         * prevent the onset of swapping simply because of a
                         * short burst of paging activity.
                         */
                        if ((vm_pageout_rate_longavg > swap_stop_pageout_rate) &&
                            (vm_pageout_rate_avg > swap_start_pageout_rate) ||
                            (vm_pageout_rate_avg > vm_pageout_rate_peakavg) ||
                            (vm_grab_rate_avg > max_grab_rate))
                                wake_task_swapper(FALSE);
                }
        } else /* page demand is low; should consider swapin */ {
                if (tasks_swapped_out != 0)
                        wake_task_swapper(TRUE);
        }
}

void
task_swapout_eligible(task_t task)
{
#if     TASK_SW_DEBUG
        task_swapper_lock();
        if (task_swap_debug && on_swapped_list(task)) {
                printf("swapout_eligible: task 0x%X on swapped list\n", task);
                Debugger("");
        }
        task_swapper_unlock();
#endif
        task_swapout_lock();
        task_lock(task);
#if     TASK_SW_DEBUG
        if (task->swap_flags & TASK_SW_ELIGIBLE) {
                printf("swapout_eligible: task 0x%X already eligible\n", task);
        }
#endif  /* TASK_SW_DEBUG */
        if ((task->swap_state == TASK_SW_IN) &&
            ((task->swap_flags & TASK_SW_ELIGIBLE) == 0)) {
                queue_enter(&eligible_tasks,task,task_t,swapped_tasks);
                task->swap_flags |= TASK_SW_ELIGIBLE;
        }
        task_unlock(task);
        task_swapout_unlock();
}

void
task_swapout_ineligible(task_t task)
{
#if     TASK_SW_DEBUG
        task_swapper_lock();
        if (task_swap_debug && on_swapped_list(task)) {
                printf("swapout_ineligible: task 0x%X on swapped list\n", task);
                Debugger("");
        }
        task_swapper_unlock();
#endif
        task_swapout_lock();
        task_lock(task);
#if     TASK_SW_DEBUG
        if (!(task->swap_flags & TASK_SW_ELIGIBLE))
                printf("swapout_ineligible: task 0x%X already inel.\n", task);
#endif  /* TASK_SW_DEBUG */
        if ((task->swap_state != TASK_SW_IN) && 
            (task->swap_flags & TASK_SW_ELIGIBLE)) {
                queue_remove(&eligible_tasks, task, task_t, swapped_tasks);
                task->swap_flags &= ~TASK_SW_ELIGIBLE;
        }
        task_unlock(task);
        task_swapout_unlock();
}

int task_swap_ast_aborted = 0;

/*
 *      Process an AST_SWAPOUT.
 */
void
swapout_ast()
{
        spl_t           s;
        thread_act_t    act;
        thread_t        thread;

        act = current_act();

        /*
         * Task is being swapped out.  First mark it as suspended
         * and halted, then call thread_swapout_enqueue to put
         * the thread on the queue for task_swap_swapout_threads
         * to swap out the thread.
         */
        /*
         * Don't swap unswappable threads
         */
        thread = act_lock_thread(act);
        s = splsched();
        if (thread)
                thread_lock(thread);
        if ((act->ast & AST_SWAPOUT) == 0) {
                /*
                 * Race with task_swapin. Abort swapout.
                 */
                task_swap_ast_aborted++;        /* not locked XXX */
                if (thread)
                        thread_unlock(thread);
                splx(s);
                act_unlock_thread(act);
        } else if (act->swap_state == TH_SW_IN) {
                /*
                 * Mark swap_state as TH_SW_TASK_SWAPPING to avoid
                 * race with thread swapper, which will only
                 * swap thread if swap_state is TH_SW_IN.
                 * This way, the thread can only be swapped by
                 * the task swapping mechanism.
                 */
                act->swap_state |= TH_SW_TASK_SWAPPING;
                /* assert(act->suspend_count == 0); XXX ? */
                if (thread)
                        thread_unlock(thread);
                if (act->suspend_count++ == 0)  /* inline thread_hold */
                        install_special_handler(act);
                /* self->state |= TH_HALTED; */
                thread_ast_clear(act, AST_SWAPOUT);
                /*
                 * Initialize the swap_queue fields to allow an extra
                 * queue_remove() in task_swapin if we lose the race
                 * (task_swapin can be called before we complete
                 * thread_swapout_enqueue).
                 */
                queue_init((queue_t) &act->swap_queue);
                splx(s);
                act_unlock_thread(act);
                /* this must be called at normal interrupt level */
                thread_swapout_enqueue(act);
        } else {
                /* thread isn't swappable; continue running */
                assert(act->swap_state == TH_SW_UNSWAPPABLE);
                if (thread)
                        thread_unlock(thread);
                thread_ast_clear(act, AST_SWAPOUT);
                splx(s);
                act_unlock_thread(act);
        }
}

#endif  /* TASK_SWAPPER */