sys/kern/kern_synch.c

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 1982, 1986, 1990, 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)kern_synch.c        8.9 (Berkeley) 5/19/95
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#include "opt_ktrace.h"
#include "opt_sched.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/blockcount.h>
#include <sys/condvar.h>
#include <sys/kdb.h>
#include <sys/kernel.h>
#include <sys/ktr.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/resourcevar.h>
#include <sys/sched.h>
#include <sys/sdt.h>
#include <sys/signalvar.h>
#include <sys/sleepqueue.h>
#include <sys/smp.h>
#include <sys/sx.h>
#include <sys/sysctl.h>
#include <sys/sysproto.h>
#include <sys/vmmeter.h>
#ifdef KTRACE
#include <sys/uio.h>
#include <sys/ktrace.h>
#endif
#ifdef EPOCH_TRACE
#include <sys/epoch.h>
#endif

#include <machine/cpu.h>

static void synch_setup(void *dummy);
SYSINIT(synch_setup, SI_SUB_KICK_SCHEDULER, SI_ORDER_FIRST, synch_setup,
    NULL);

int     hogticks;
static const char pause_wchan[MAXCPU];

static struct callout loadav_callout;

struct loadavg averunnable =
        { {0, 0, 0}, FSCALE };  /* load average, of runnable procs */
/*
 * Constants for averages over 1, 5, and 15 minutes
 * when sampling at 5 second intervals.
 */
static fixpt_t cexp[3] = {
        0.9200444146293232 * FSCALE,    /* exp(-1/12) */
        0.9834714538216174 * FSCALE,    /* exp(-1/60) */
        0.9944598480048967 * FSCALE,    /* exp(-1/180) */
};

/* kernel uses `FSCALE', userland (SHOULD) use kern.fscale */
SYSCTL_INT(_kern, OID_AUTO, fscale, CTLFLAG_RD, SYSCTL_NULL_INT_PTR, FSCALE,
    "Fixed-point scale factor used for calculating load average values");

static void     loadav(void *arg);

SDT_PROVIDER_DECLARE(sched);
SDT_PROBE_DEFINE(sched, , , preempt);

static void
sleepinit(void *unused)
{

        hogticks = (hz / 10) * 2;       /* Default only. */
        init_sleepqueues();
}

/*
 * vmem tries to lock the sleepq mutexes when free'ing kva, so make sure
 * it is available.
 */
SYSINIT(sleepinit, SI_SUB_KMEM, SI_ORDER_ANY, sleepinit, NULL);

/*
 * General sleep call.  Suspends the current thread until a wakeup is
 * performed on the specified identifier.  The thread will then be made
 * runnable with the specified priority.  Sleeps at most sbt units of time
 * (0 means no timeout).  If pri includes the PCATCH flag, let signals
 * interrupt the sleep, otherwise ignore them while sleeping.  Returns 0 if
 * awakened, EWOULDBLOCK if the timeout expires.  If PCATCH is set and a
 * signal becomes pending, ERESTART is returned if the current system
 * call should be restarted if possible, and EINTR is returned if the system
 * call should be interrupted by the signal (return EINTR).
 *
 * The lock argument is unlocked before the caller is suspended, and
 * re-locked before _sleep() returns.  If priority includes the PDROP
 * flag the lock is not re-locked before returning.
 */
int
_sleep(const void *ident, struct lock_object *lock, int priority,
    const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
{
        struct thread *td;
        struct lock_class *class;
        uintptr_t lock_state;
        int catch, pri, rval, sleepq_flags;
        WITNESS_SAVE_DECL(lock_witness);

        td = curthread;
#ifdef KTRACE
        if (KTRPOINT(td, KTR_CSW))
                ktrcsw(1, 0, wmesg);
#endif
        WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
            "Sleeping on \"%s\"", wmesg);
        KASSERT(sbt != 0 || mtx_owned(&Giant) || lock != NULL,
            ("sleeping without a lock"));
        KASSERT(ident != NULL, ("_sleep: NULL ident"));
        KASSERT(TD_IS_RUNNING(td), ("_sleep: curthread not running"));
        if (priority & PDROP)
                KASSERT(lock != NULL && lock != &Giant.lock_object,
                    ("PDROP requires a non-Giant lock"));
        if (lock != NULL)
                class = LOCK_CLASS(lock);
        else
                class = NULL;

        if (SCHEDULER_STOPPED_TD(td)) {
                if (lock != NULL && priority & PDROP)
                        class->lc_unlock(lock);
                return (0);
        }
        catch = priority & PCATCH;
        pri = priority & PRIMASK;

        KASSERT(!TD_ON_SLEEPQ(td), ("recursive sleep"));

        if ((uintptr_t)ident >= (uintptr_t)&pause_wchan[0] &&
            (uintptr_t)ident <= (uintptr_t)&pause_wchan[MAXCPU - 1])
                sleepq_flags = SLEEPQ_PAUSE;
        else
                sleepq_flags = SLEEPQ_SLEEP;
        if (catch)
                sleepq_flags |= SLEEPQ_INTERRUPTIBLE;

        sleepq_lock(ident);
        CTR5(KTR_PROC, "sleep: thread %ld (pid %ld, %s) on %s (%p)",
            td->td_tid, td->td_proc->p_pid, td->td_name, wmesg, ident);

        if (lock == &Giant.lock_object)
                mtx_assert(&Giant, MA_OWNED);
        DROP_GIANT();
        if (lock != NULL && lock != &Giant.lock_object &&
            !(class->lc_flags & LC_SLEEPABLE)) {
                WITNESS_SAVE(lock, lock_witness);
                lock_state = class->lc_unlock(lock);
        } else
                /* GCC needs to follow the Yellow Brick Road */
                lock_state = -1;

        /*
         * We put ourselves on the sleep queue and start our timeout
         * before calling thread_suspend_check, as we could stop there,
         * and a wakeup or a SIGCONT (or both) could occur while we were
         * stopped without resuming us.  Thus, we must be ready for sleep
         * when cursig() is called.  If the wakeup happens while we're
         * stopped, then td will no longer be on a sleep queue upon
         * return from cursig().
         */
        sleepq_add(ident, lock, wmesg, sleepq_flags, 0);
        if (sbt != 0)
                sleepq_set_timeout_sbt(ident, sbt, pr, flags);
        if (lock != NULL && class->lc_flags & LC_SLEEPABLE) {
                sleepq_release(ident);
                WITNESS_SAVE(lock, lock_witness);
                lock_state = class->lc_unlock(lock);
                sleepq_lock(ident);
        }
        if (sbt != 0 && catch)
                rval = sleepq_timedwait_sig(ident, pri);
        else if (sbt != 0)
                rval = sleepq_timedwait(ident, pri);
        else if (catch)
                rval = sleepq_wait_sig(ident, pri);
        else {
                sleepq_wait(ident, pri);
                rval = 0;
        }
#ifdef KTRACE
        if (KTRPOINT(td, KTR_CSW))
                ktrcsw(0, 0, wmesg);
#endif
        PICKUP_GIANT();
        if (lock != NULL && lock != &Giant.lock_object && !(priority & PDROP)) {
                class->lc_lock(lock, lock_state);
                WITNESS_RESTORE(lock, lock_witness);
        }
        return (rval);
}

int
msleep_spin_sbt(const void *ident, struct mtx *mtx, const char *wmesg,
    sbintime_t sbt, sbintime_t pr, int flags)
{
        struct thread *td;
        int rval;
        WITNESS_SAVE_DECL(mtx);

        td = curthread;
        KASSERT(mtx != NULL, ("sleeping without a mutex"));
        KASSERT(ident != NULL, ("msleep_spin_sbt: NULL ident"));
        KASSERT(TD_IS_RUNNING(td), ("msleep_spin_sbt: curthread not running"));

        if (SCHEDULER_STOPPED_TD(td))
                return (0);

        sleepq_lock(ident);
        CTR5(KTR_PROC, "msleep_spin: thread %ld (pid %ld, %s) on %s (%p)",
            td->td_tid, td->td_proc->p_pid, td->td_name, wmesg, ident);

        DROP_GIANT();
        mtx_assert(mtx, MA_OWNED | MA_NOTRECURSED);
        WITNESS_SAVE(&mtx->lock_object, mtx);
        mtx_unlock_spin(mtx);

        /*
         * We put ourselves on the sleep queue and start our timeout.
         */
        sleepq_add(ident, &mtx->lock_object, wmesg, SLEEPQ_SLEEP, 0);
        if (sbt != 0)
                sleepq_set_timeout_sbt(ident, sbt, pr, flags);

        /*
         * Can't call ktrace with any spin locks held so it can lock the
         * ktrace_mtx lock, and WITNESS_WARN considers it an error to hold
         * any spin lock.  Thus, we have to drop the sleepq spin lock while
         * we handle those requests.  This is safe since we have placed our
         * thread on the sleep queue already.
         */
#ifdef KTRACE
        if (KTRPOINT(td, KTR_CSW)) {
                sleepq_release(ident);
                ktrcsw(1, 0, wmesg);
                sleepq_lock(ident);
        }
#endif
#ifdef WITNESS
        sleepq_release(ident);
        WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, "Sleeping on \"%s\"",
            wmesg);
        sleepq_lock(ident);
#endif
        if (sbt != 0)
                rval = sleepq_timedwait(ident, 0);
        else {
                sleepq_wait(ident, 0);
                rval = 0;
        }
#ifdef KTRACE
        if (KTRPOINT(td, KTR_CSW))
                ktrcsw(0, 0, wmesg);
#endif
        PICKUP_GIANT();
        mtx_lock_spin(mtx);
        WITNESS_RESTORE(&mtx->lock_object, mtx);
        return (rval);
}

/*
 * pause_sbt() delays the calling thread by the given signed binary
 * time. During cold bootup, pause_sbt() uses the DELAY() function
 * instead of the _sleep() function to do the waiting. The "sbt"
 * argument must be greater than or equal to zero. A "sbt" value of
 * zero is equivalent to a "sbt" value of one tick.
 */
int
pause_sbt(const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
{
        KASSERT(sbt >= 0, ("pause_sbt: timeout must be >= 0"));

        /* silently convert invalid timeouts */
        if (sbt == 0)
                sbt = tick_sbt;

        if ((cold && curthread == &thread0) || kdb_active ||
            SCHEDULER_STOPPED()) {
                /*
                 * We delay one second at a time to avoid overflowing the
                 * system specific DELAY() function(s):
                 */
                while (sbt >= SBT_1S) {
                        DELAY(1000000);
                        sbt -= SBT_1S;
                }
                /* Do the delay remainder, if any */
                sbt = howmany(sbt, SBT_1US);
                if (sbt > 0)
                        DELAY(sbt);
                return (EWOULDBLOCK);
        }
        return (_sleep(&pause_wchan[curcpu], NULL,
            (flags & C_CATCH) ? PCATCH : 0, wmesg, sbt, pr, flags));
}

/*
 * Make all threads sleeping on the specified identifier runnable.
 */
void
wakeup(const void *ident)
{
        int wakeup_swapper;

        sleepq_lock(ident);
        wakeup_swapper = sleepq_broadcast(ident, SLEEPQ_SLEEP, 0, 0);
        sleepq_release(ident);
        if (wakeup_swapper) {
                KASSERT(ident != &proc0,
                    ("wakeup and wakeup_swapper and proc0"));
                kick_proc0();
        }
}

/*
 * Make a thread sleeping on the specified identifier runnable.
 * May wake more than one thread if a target thread is currently
 * swapped out.
 */
void
wakeup_one(const void *ident)
{
        int wakeup_swapper;

        sleepq_lock(ident);
        wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP, 0, 0);
        sleepq_release(ident);
        if (wakeup_swapper)
                kick_proc0();
}

void
wakeup_any(const void *ident)
{
        int wakeup_swapper;

        sleepq_lock(ident);
        wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP | SLEEPQ_UNFAIR,
            0, 0);
        sleepq_release(ident);
        if (wakeup_swapper)
                kick_proc0();
}

/*
 * Signal sleeping waiters after the counter has reached zero.
 */
void
_blockcount_wakeup(blockcount_t *bc, u_int old)
{

        KASSERT(_BLOCKCOUNT_WAITERS(old),
            ("%s: no waiters on %p", __func__, bc));

        if (atomic_cmpset_int(&bc->__count, _BLOCKCOUNT_WAITERS_FLAG, 0))
                wakeup(bc);
}

/*
 * Wait for a wakeup or a signal.  This does not guarantee that the count is
 * still zero on return.  Callers wanting a precise answer should use
 * blockcount_wait() with an interlock.
 *
 * If there is no work to wait for, return 0.  If the sleep was interrupted by a
 * signal, return EINTR or ERESTART, and return EAGAIN otherwise.
 */
int
_blockcount_sleep(blockcount_t *bc, struct lock_object *lock, const char *wmesg,
    int prio)
{
        void *wchan;
        uintptr_t lock_state;
        u_int old;
        int ret;
        bool catch, drop;

        KASSERT(lock != &Giant.lock_object,
            ("%s: cannot use Giant as the interlock", __func__));

        catch = (prio & PCATCH) != 0;
        drop = (prio & PDROP) != 0;
        prio &= PRIMASK;

        /*
         * Synchronize with the fence in blockcount_release().  If we end up
         * waiting, the sleepqueue lock acquisition will provide the required
         * side effects.
         *
         * If there is no work to wait for, but waiters are present, try to put
         * ourselves to sleep to avoid jumping ahead.
         */
        if (atomic_load_acq_int(&bc->__count) == 0) {
                if (lock != NULL && drop)
                        LOCK_CLASS(lock)->lc_unlock(lock);
                return (0);
        }
        lock_state = 0;
        wchan = bc;
        sleepq_lock(wchan);
        DROP_GIANT();
        if (lock != NULL)
                lock_state = LOCK_CLASS(lock)->lc_unlock(lock);
        old = blockcount_read(bc);
        ret = 0;
        do {
                if (_BLOCKCOUNT_COUNT(old) == 0) {
                        sleepq_release(wchan);
                        goto out;
                }
                if (_BLOCKCOUNT_WAITERS(old))
                        break;
        } while (!atomic_fcmpset_int(&bc->__count, &old,
            old | _BLOCKCOUNT_WAITERS_FLAG));
        sleepq_add(wchan, NULL, wmesg, catch ? SLEEPQ_INTERRUPTIBLE : 0, 0);
        if (catch)
                ret = sleepq_wait_sig(wchan, prio);
        else
                sleepq_wait(wchan, prio);
        if (ret == 0)
                ret = EAGAIN;

out:
        PICKUP_GIANT();
        if (lock != NULL && !drop)
                LOCK_CLASS(lock)->lc_lock(lock, lock_state);

        return (ret);
}

static void
kdb_switch(void)
{
        thread_unlock(curthread);
        kdb_backtrace();
        kdb_reenter();
        panic("%s: did not reenter debugger", __func__);
}

/*
 * The machine independent parts of context switching.
 *
 * The thread lock is required on entry and is no longer held on return.
 */
void
mi_switch(int flags)
{
        uint64_t runtime, new_switchtime;
        struct thread *td;

        td = curthread;                 /* XXX */
        THREAD_LOCK_ASSERT(td, MA_OWNED | MA_NOTRECURSED);
        KASSERT(!TD_ON_RUNQ(td), ("mi_switch: called by old code"));
#ifdef INVARIANTS
        if (!TD_ON_LOCK(td) && !TD_IS_RUNNING(td))
                mtx_assert(&Giant, MA_NOTOWNED);
#endif
        KASSERT(td->td_critnest == 1 || KERNEL_PANICKED(),
                ("mi_switch: switch in a critical section"));
        KASSERT((flags & (SW_INVOL | SW_VOL)) != 0,
            ("mi_switch: switch must be voluntary or involuntary"));

        /*
         * Don't perform context switches from the debugger.
         */
        if (kdb_active)
                kdb_switch();
        if (SCHEDULER_STOPPED_TD(td))
                return;
        if (flags & SW_VOL) {
                td->td_ru.ru_nvcsw++;
                td->td_swvoltick = ticks;
        } else {
                td->td_ru.ru_nivcsw++;
                td->td_swinvoltick = ticks;
        }
#ifdef SCHED_STATS
        SCHED_STAT_INC(sched_switch_stats[flags & SW_TYPE_MASK]);
#endif
        /*
         * Compute the amount of time during which the current
         * thread was running, and add that to its total so far.
         */
        new_switchtime = cpu_ticks();
        runtime = new_switchtime - PCPU_GET(switchtime);
        td->td_runtime += runtime;
        td->td_incruntime += runtime;
        PCPU_SET(switchtime, new_switchtime);
        td->td_generation++;    /* bump preempt-detect counter */
        VM_CNT_INC(v_swtch);
        PCPU_SET(switchticks, ticks);
        CTR4(KTR_PROC, "mi_switch: old thread %ld (td_sched %p, pid %ld, %s)",
            td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name);
#ifdef KDTRACE_HOOKS
        if (SDT_PROBES_ENABLED() &&
            ((flags & SW_PREEMPT) != 0 || ((flags & SW_INVOL) != 0 &&
            (flags & SW_TYPE_MASK) == SWT_NEEDRESCHED)))
                SDT_PROBE0(sched, , , preempt);
#endif
        sched_switch(td, flags);
        CTR4(KTR_PROC, "mi_switch: new thread %ld (td_sched %p, pid %ld, %s)",
            td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name);

        /* 
         * If the last thread was exiting, finish cleaning it up.
         */
        if ((td = PCPU_GET(deadthread))) {
                PCPU_SET(deadthread, NULL);
                thread_stash(td);
        }
        spinlock_exit();
}

/*
 * Change thread state to be runnable, placing it on the run queue if
 * it is in memory.  If it is swapped out, return true so our caller
 * will know to awaken the swapper.
 *
 * Requires the thread lock on entry, drops on exit.
 */
int
setrunnable(struct thread *td, int srqflags)
{
        int swapin;

        THREAD_LOCK_ASSERT(td, MA_OWNED);
        KASSERT(td->td_proc->p_state != PRS_ZOMBIE,
            ("setrunnable: pid %d is a zombie", td->td_proc->p_pid));

        swapin = 0;
        switch (td->td_state) {
        case TDS_RUNNING:
        case TDS_RUNQ:
                break;
        case TDS_CAN_RUN:
                KASSERT((td->td_flags & TDF_INMEM) != 0,
                    ("setrunnable: td %p not in mem, flags 0x%X inhibit 0x%X",
                    td, td->td_flags, td->td_inhibitors));
                /* unlocks thread lock according to flags */
                sched_wakeup(td, srqflags);
                return (0);
        case TDS_INHIBITED:
                /*
                 * If we are only inhibited because we are swapped out
                 * arrange to swap in this process.
                 */
                if (td->td_inhibitors == TDI_SWAPPED &&
                    (td->td_flags & TDF_SWAPINREQ) == 0) {
                        td->td_flags |= TDF_SWAPINREQ;
                        swapin = 1;
                }
                break;
        default:
                panic("setrunnable: state 0x%x", td->td_state);
        }
        if ((srqflags & (SRQ_HOLD | SRQ_HOLDTD)) == 0)
                thread_unlock(td);

        return (swapin);
}

/*
 * Compute a tenex style load average of a quantity on
 * 1, 5 and 15 minute intervals.
 */
static void
loadav(void *arg)
{
        int i, nrun;
        struct loadavg *avg;

        nrun = sched_load();
        avg = &averunnable;

        for (i = 0; i < 3; i++)
                avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
                    nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;

        /*
         * Schedule the next update to occur after 5 seconds, but add a
         * random variation to avoid synchronisation with processes that
         * run at regular intervals.
         */
        callout_reset_sbt(&loadav_callout,
            SBT_1US * (4000000 + (int)(random() % 2000001)), SBT_1US,
            loadav, NULL, C_DIRECT_EXEC | C_PREL(32));
}

/* ARGSUSED */
static void
synch_setup(void *dummy)
{
        callout_init(&loadav_callout, 1);

        /* Kick off timeout driven events by calling first time. */
        loadav(NULL);
}

int
should_yield(void)
{

        return ((u_int)ticks - (u_int)curthread->td_swvoltick >= hogticks);
}

void
maybe_yield(void)
{

        if (should_yield())
                kern_yield(PRI_USER);
}

void
kern_yield(int prio)
{
        struct thread *td;

        td = curthread;
        DROP_GIANT();
        thread_lock(td);
        if (prio == PRI_USER)
                prio = td->td_user_pri;
        if (prio >= 0)
                sched_prio(td, prio);
        mi_switch(SW_VOL | SWT_RELINQUISH);
        PICKUP_GIANT();
}

/*
 * General purpose yield system call.
 */
int
sys_yield(struct thread *td, struct yield_args *uap)
{

        thread_lock(td);
        if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE)
                sched_prio(td, PRI_MAX_TIMESHARE);
        mi_switch(SW_VOL | SWT_RELINQUISH);
        td->td_retval[0] = 0;
        return (0);
}
1	/*-
2	* SPDX-License-Identifier: BSD-3-Clause
3	*
4	* Copyright (c) 1982, 1986, 1990, 1991, 1993
5	* The Regents of the University of California. All rights reserved.
6	* (c) UNIX System Laboratories, Inc.
7	* All or some portions of this file are derived from material licensed
8	* to the University of California by American Telephone and Telegraph
9	* Co. or Unix System Laboratories, Inc. and are reproduced herein with
10	* the permission of UNIX System Laboratories, Inc.
11	*
12	* Redistribution and use in source and binary forms, with or without
13	* modification, are permitted provided that the following conditions
14	* are met:
15	* 1. Redistributions of source code must retain the above copyright
16	* notice, this list of conditions and the following disclaimer.
17	* 2. Redistributions in binary form must reproduce the above copyright
18	* notice, this list of conditions and the following disclaimer in the
19	* documentation and/or other materials provided with the distribution.
20	* 3. Neither the name of the University nor the names of its contributors
21	* may be used to endorse or promote products derived from this software
22	* without specific prior written permission.
23	*
24	* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
25	* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
26	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
27	* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
28	* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
29	* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
30	* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
31	* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
32	* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
33	* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34	* SUCH DAMAGE.
35	*
36	* @(#)kern_synch.c 8.9 (Berkeley) 5/19/95
37	*/
38
39	#include <sys/cdefs.h>
40	__FBSDID("$FreeBSD$");
41
42	#include "opt_ktrace.h"
43	#include "opt_sched.h"
44
45	#include <sys/param.h>
46	#include <sys/systm.h>
47	#include <sys/blockcount.h>
48	#include <sys/condvar.h>
49	#include <sys/kdb.h>
50	#include <sys/kernel.h>
51	#include <sys/ktr.h>
52	#include <sys/lock.h>
53	#include <sys/mutex.h>
54	#include <sys/proc.h>
55	#include <sys/resourcevar.h>
56	#include <sys/sched.h>
57	#include <sys/sdt.h>
58	#include <sys/signalvar.h>
59	#include <sys/sleepqueue.h>
60	#include <sys/smp.h>
61	#include <sys/sx.h>
62	#include <sys/sysctl.h>
63	#include <sys/sysproto.h>
64	#include <sys/vmmeter.h>
65	#ifdef KTRACE
66	#include <sys/uio.h>
67	#include <sys/ktrace.h>
68	#endif
69	#ifdef EPOCH_TRACE
70	#include <sys/epoch.h>
71	#endif
72
73	#include <machine/cpu.h>
74
75	static void synch_setup(void *dummy);
76	SYSINIT(synch_setup, SI_SUB_KICK_SCHEDULER, SI_ORDER_FIRST, synch_setup,
77	NULL);
78
79	int hogticks;
80	static const char pause_wchan[MAXCPU];
81
82	static struct callout loadav_callout;
83
84	struct loadavg averunnable =
85	{ {0, 0, 0}, FSCALE }; /* load average, of runnable procs */
86	/*
87	* Constants for averages over 1, 5, and 15 minutes
88	* when sampling at 5 second intervals.
89	*/
90	static fixpt_t cexp[3] = {
91	0.9200444146293232 * FSCALE, /* exp(-1/12) */
92	0.9834714538216174 * FSCALE, /* exp(-1/60) */
93	0.9944598480048967 * FSCALE, /* exp(-1/180) */
94	};
95
96	/* kernel uses `FSCALE', userland (SHOULD) use kern.fscale */
97	SYSCTL_INT(_kern, OID_AUTO, fscale, CTLFLAG_RD, SYSCTL_NULL_INT_PTR, FSCALE,
98	"Fixed-point scale factor used for calculating load average values");
99
100	static void loadav(void *arg);
101
102	SDT_PROVIDER_DECLARE(sched);
103	SDT_PROBE_DEFINE(sched, , , preempt);
104
105	static void
106	sleepinit(void *unused)
107	{
108
109	hogticks = (hz / 10) * 2; /* Default only. */
110	init_sleepqueues();
111	}
112
113	/*
114	* vmem tries to lock the sleepq mutexes when free'ing kva, so make sure
115	* it is available.
116	*/
117	SYSINIT(sleepinit, SI_SUB_KMEM, SI_ORDER_ANY, sleepinit, NULL);
118
119	/*
120	* General sleep call. Suspends the current thread until a wakeup is
121	* performed on the specified identifier. The thread will then be made
122	* runnable with the specified priority. Sleeps at most sbt units of time
123	* (0 means no timeout). If pri includes the PCATCH flag, let signals
124	* interrupt the sleep, otherwise ignore them while sleeping. Returns 0 if
125	* awakened, EWOULDBLOCK if the timeout expires. If PCATCH is set and a
126	* signal becomes pending, ERESTART is returned if the current system
127	* call should be restarted if possible, and EINTR is returned if the system
128	* call should be interrupted by the signal (return EINTR).
129	*
130	* The lock argument is unlocked before the caller is suspended, and
131	* re-locked before _sleep() returns. If priority includes the PDROP
132	* flag the lock is not re-locked before returning.
133	*/
134	int
135	_sleep(const void ident, struct lock_object lock, int priority,
136	const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
137	{
138	struct thread *td;
139	struct lock_class *class;
140	uintptr_t lock_state;
141	int catch, pri, rval, sleepq_flags;
142	WITNESS_SAVE_DECL(lock_witness);
143
144	td = curthread;
145	#ifdef KTRACE
146	if (KTRPOINT(td, KTR_CSW))
147	ktrcsw(1, 0, wmesg);
148	#endif
149	WITNESS_WARN(WARN_GIANTOK \| WARN_SLEEPOK, lock,
150	"Sleeping on \"%s\"", wmesg);
151	KASSERT(sbt != 0 \|\| mtx_owned(&Giant) \|\| lock != NULL,
152	("sleeping without a lock"));
153	KASSERT(ident != NULL, ("_sleep: NULL ident"));
154	KASSERT(TD_IS_RUNNING(td), ("_sleep: curthread not running"));
155	if (priority & PDROP)
156	KASSERT(lock != NULL && lock != &Giant.lock_object,
157	("PDROP requires a non-Giant lock"));
158	if (lock != NULL)
159	class = LOCK_CLASS(lock);
160	else
161	class = NULL;
162
163	if (SCHEDULER_STOPPED_TD(td)) {
164	if (lock != NULL && priority & PDROP)
165	class->lc_unlock(lock);
166	return (0);
167	}
168	catch = priority & PCATCH;
169	pri = priority & PRIMASK;
170
171	KASSERT(!TD_ON_SLEEPQ(td), ("recursive sleep"));
172
173	if ((uintptr_t)ident >= (uintptr_t)&pause_wchan[0] &&
174	(uintptr_t)ident <= (uintptr_t)&pause_wchan[MAXCPU - 1])
175	sleepq_flags = SLEEPQ_PAUSE;
176	else
177	sleepq_flags = SLEEPQ_SLEEP;
178	if (catch)
179	sleepq_flags \|= SLEEPQ_INTERRUPTIBLE;
180
181	sleepq_lock(ident);
182	CTR5(KTR_PROC, "sleep: thread %ld (pid %ld, %s) on %s (%p)",
183	td->td_tid, td->td_proc->p_pid, td->td_name, wmesg, ident);
184
185	if (lock == &Giant.lock_object)
186	mtx_assert(&Giant, MA_OWNED);
187	DROP_GIANT();
188	if (lock != NULL && lock != &Giant.lock_object &&
189	!(class->lc_flags & LC_SLEEPABLE)) {
190	WITNESS_SAVE(lock, lock_witness);
191	lock_state = class->lc_unlock(lock);
192	} else
193	/* GCC needs to follow the Yellow Brick Road */
194	lock_state = -1;
195
196	/*
197	* We put ourselves on the sleep queue and start our timeout
198	* before calling thread_suspend_check, as we could stop there,
199	* and a wakeup or a SIGCONT (or both) could occur while we were
200	* stopped without resuming us. Thus, we must be ready for sleep
201	* when cursig() is called. If the wakeup happens while we're
202	* stopped, then td will no longer be on a sleep queue upon
203	* return from cursig().
204	*/
205	sleepq_add(ident, lock, wmesg, sleepq_flags, 0);
206	if (sbt != 0)
207	sleepq_set_timeout_sbt(ident, sbt, pr, flags);
208	if (lock != NULL && class->lc_flags & LC_SLEEPABLE) {
209	sleepq_release(ident);
210	WITNESS_SAVE(lock, lock_witness);
211	lock_state = class->lc_unlock(lock);
212	sleepq_lock(ident);
213	}
214	if (sbt != 0 && catch)
215	rval = sleepq_timedwait_sig(ident, pri);
216	else if (sbt != 0)
217	rval = sleepq_timedwait(ident, pri);
218	else if (catch)
219	rval = sleepq_wait_sig(ident, pri);
220	else {
221	sleepq_wait(ident, pri);
222	rval = 0;
223	}
224	#ifdef KTRACE
225	if (KTRPOINT(td, KTR_CSW))
226	ktrcsw(0, 0, wmesg);
227	#endif
228	PICKUP_GIANT();
229	if (lock != NULL && lock != &Giant.lock_object && !(priority & PDROP)) {
230	class->lc_lock(lock, lock_state);
231	WITNESS_RESTORE(lock, lock_witness);
232	}
233	return (rval);
234	}
235
236	int
237	msleep_spin_sbt(const void ident, struct mtx mtx, const char *wmesg,
238	sbintime_t sbt, sbintime_t pr, int flags)
239	{
240	struct thread *td;
241	int rval;
242	WITNESS_SAVE_DECL(mtx);
243
244	td = curthread;
245	KASSERT(mtx != NULL, ("sleeping without a mutex"));
246	KASSERT(ident != NULL, ("msleep_spin_sbt: NULL ident"));
247	KASSERT(TD_IS_RUNNING(td), ("msleep_spin_sbt: curthread not running"));
248
249	if (SCHEDULER_STOPPED_TD(td))
250	return (0);
251
252	sleepq_lock(ident);
253	CTR5(KTR_PROC, "msleep_spin: thread %ld (pid %ld, %s) on %s (%p)",
254	td->td_tid, td->td_proc->p_pid, td->td_name, wmesg, ident);
255
256	DROP_GIANT();
257	mtx_assert(mtx, MA_OWNED \| MA_NOTRECURSED);
258	WITNESS_SAVE(&mtx->lock_object, mtx);
259	mtx_unlock_spin(mtx);
260
261	/*
262	* We put ourselves on the sleep queue and start our timeout.
263	*/
264	sleepq_add(ident, &mtx->lock_object, wmesg, SLEEPQ_SLEEP, 0);
265	if (sbt != 0)
266	sleepq_set_timeout_sbt(ident, sbt, pr, flags);
267
268	/*
269	* Can't call ktrace with any spin locks held so it can lock the
270	* ktrace_mtx lock, and WITNESS_WARN considers it an error to hold
271	* any spin lock. Thus, we have to drop the sleepq spin lock while
272	* we handle those requests. This is safe since we have placed our
273	* thread on the sleep queue already.
274	*/
275	#ifdef KTRACE
276	if (KTRPOINT(td, KTR_CSW)) {
277	sleepq_release(ident);
278	ktrcsw(1, 0, wmesg);
279	sleepq_lock(ident);
280	}
281	#endif
282	#ifdef WITNESS
283	sleepq_release(ident);
284	WITNESS_WARN(WARN_GIANTOK \| WARN_SLEEPOK, NULL, "Sleeping on \"%s\"",
285	wmesg);
286	sleepq_lock(ident);
287	#endif
288	if (sbt != 0)
289	rval = sleepq_timedwait(ident, 0);
290	else {
291	sleepq_wait(ident, 0);
292	rval = 0;
293	}
294	#ifdef KTRACE
295	if (KTRPOINT(td, KTR_CSW))
296	ktrcsw(0, 0, wmesg);
297	#endif
298	PICKUP_GIANT();
299	mtx_lock_spin(mtx);
300	WITNESS_RESTORE(&mtx->lock_object, mtx);
301	return (rval);
302	}
303
304	/*
305	* pause_sbt() delays the calling thread by the given signed binary
306	* time. During cold bootup, pause_sbt() uses the DELAY() function
307	* instead of the _sleep() function to do the waiting. The "sbt"
308	* argument must be greater than or equal to zero. A "sbt" value of
309	* zero is equivalent to a "sbt" value of one tick.
310	*/
311	int
312	pause_sbt(const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
313	{
314	KASSERT(sbt >= 0, ("pause_sbt: timeout must be >= 0"));
315
316	/* silently convert invalid timeouts */
317	if (sbt == 0)
318	sbt = tick_sbt;
319
320	if ((cold && curthread == &thread0) \|\| kdb_active \|\|
321	SCHEDULER_STOPPED()) {
322	/*
323	* We delay one second at a time to avoid overflowing the
324	* system specific DELAY() function(s):
325	*/
326	while (sbt >= SBT_1S) {
327	DELAY(1000000);
328	sbt -= SBT_1S;
329	}
330	/* Do the delay remainder, if any */
331	sbt = howmany(sbt, SBT_1US);
332	if (sbt > 0)
333	DELAY(sbt);
334	return (EWOULDBLOCK);
335	}
336	return (_sleep(&pause_wchan[curcpu], NULL,
337	(flags & C_CATCH) ? PCATCH : 0, wmesg, sbt, pr, flags));
338	}
339
340	/*
341	* Make all threads sleeping on the specified identifier runnable.
342	*/
343	void
344	wakeup(const void *ident)
345	{
346	int wakeup_swapper;
347
348	sleepq_lock(ident);
349	wakeup_swapper = sleepq_broadcast(ident, SLEEPQ_SLEEP, 0, 0);
350	sleepq_release(ident);
351	if (wakeup_swapper) {
352	KASSERT(ident != &proc0,
353	("wakeup and wakeup_swapper and proc0"));
354	kick_proc0();
355	}
356	}
357
358	/*
359	* Make a thread sleeping on the specified identifier runnable.
360	* May wake more than one thread if a target thread is currently
361	* swapped out.
362	*/
363	void
364	wakeup_one(const void *ident)
365	{
366	int wakeup_swapper;
367
368	sleepq_lock(ident);
369	wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP, 0, 0);
370	sleepq_release(ident);
371	if (wakeup_swapper)
372	kick_proc0();
373	}
374
375	void
376	wakeup_any(const void *ident)
377	{
378	int wakeup_swapper;
379
380	sleepq_lock(ident);
381	wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP \| SLEEPQ_UNFAIR,
382	0, 0);
383	sleepq_release(ident);
384	if (wakeup_swapper)
385	kick_proc0();
386	}
387
388	/*
389	* Signal sleeping waiters after the counter has reached zero.
390	*/
391	void
392	_blockcount_wakeup(blockcount_t *bc, u_int old)
393	{
394
395	KASSERT(_BLOCKCOUNT_WAITERS(old),
396	("%s: no waiters on %p", __func__, bc));
397
398	if (atomic_cmpset_int(&bc->__count, _BLOCKCOUNT_WAITERS_FLAG, 0))
399	wakeup(bc);
400	}
401
402	/*
403	* Wait for a wakeup or a signal. This does not guarantee that the count is
404	* still zero on return. Callers wanting a precise answer should use
405	* blockcount_wait() with an interlock.
406	*
407	* If there is no work to wait for, return 0. If the sleep was interrupted by a
408	* signal, return EINTR or ERESTART, and return EAGAIN otherwise.
409	*/
410	int
411	_blockcount_sleep(blockcount_t bc, struct lock_object lock, const char *wmesg,
412	int prio)
413	{
414	void *wchan;
415	uintptr_t lock_state;
416	u_int old;
417	int ret;
418	bool catch, drop;
419
420	KASSERT(lock != &Giant.lock_object,
421	("%s: cannot use Giant as the interlock", __func__));
422
423	catch = (prio & PCATCH) != 0;
424	drop = (prio & PDROP) != 0;
425	prio &= PRIMASK;
426
427	/*
428	* Synchronize with the fence in blockcount_release(). If we end up
429	* waiting, the sleepqueue lock acquisition will provide the required
430	* side effects.
431	*
432	* If there is no work to wait for, but waiters are present, try to put
433	* ourselves to sleep to avoid jumping ahead.
434	*/
435	if (atomic_load_acq_int(&bc->__count) == 0) {
436	if (lock != NULL && drop)
437	LOCK_CLASS(lock)->lc_unlock(lock);
438	return (0);
439	}
440	lock_state = 0;
441	wchan = bc;
442	sleepq_lock(wchan);
443	DROP_GIANT();
444	if (lock != NULL)
445	lock_state = LOCK_CLASS(lock)->lc_unlock(lock);
446	old = blockcount_read(bc);
447	ret = 0;
448	do {
449	if (_BLOCKCOUNT_COUNT(old) == 0) {
450	sleepq_release(wchan);
451	goto out;
452	}
453	if (_BLOCKCOUNT_WAITERS(old))
454	break;
455	} while (!atomic_fcmpset_int(&bc->__count, &old,
456	old \| _BLOCKCOUNT_WAITERS_FLAG));
457	sleepq_add(wchan, NULL, wmesg, catch ? SLEEPQ_INTERRUPTIBLE : 0, 0);
458	if (catch)
459	ret = sleepq_wait_sig(wchan, prio);
460	else
461	sleepq_wait(wchan, prio);
462	if (ret == 0)
463	ret = EAGAIN;
464
465	out:
466	PICKUP_GIANT();
467	if (lock != NULL && !drop)
468	LOCK_CLASS(lock)->lc_lock(lock, lock_state);
469
470	return (ret);
471	}
472
473	static void
474	kdb_switch(void)
475	{
476	thread_unlock(curthread);
477	kdb_backtrace();
478	kdb_reenter();
479	panic("%s: did not reenter debugger", __func__);
480	}
481
482	/*
483	* The machine independent parts of context switching.
484	*
485	* The thread lock is required on entry and is no longer held on return.
486	*/
487	void
488	mi_switch(int flags)
489	{
490	uint64_t runtime, new_switchtime;
491	struct thread *td;
492
493	td = curthread; /* XXX */
494	THREAD_LOCK_ASSERT(td, MA_OWNED \| MA_NOTRECURSED);
495	KASSERT(!TD_ON_RUNQ(td), ("mi_switch: called by old code"));
496	#ifdef INVARIANTS
497	if (!TD_ON_LOCK(td) && !TD_IS_RUNNING(td))
498	mtx_assert(&Giant, MA_NOTOWNED);
499	#endif
500	KASSERT(td->td_critnest == 1 \|\| KERNEL_PANICKED(),
501	("mi_switch: switch in a critical section"));
502	KASSERT((flags & (SW_INVOL \| SW_VOL)) != 0,
503	("mi_switch: switch must be voluntary or involuntary"));
504
505	/*
506	* Don't perform context switches from the debugger.
507	*/
508	if (kdb_active)
509	kdb_switch();
510	if (SCHEDULER_STOPPED_TD(td))
511	return;
512	if (flags & SW_VOL) {
513	td->td_ru.ru_nvcsw++;
514	td->td_swvoltick = ticks;
515	} else {
516	td->td_ru.ru_nivcsw++;
517	td->td_swinvoltick = ticks;
518	}
519	#ifdef SCHED_STATS
520	SCHED_STAT_INC(sched_switch_stats[flags & SW_TYPE_MASK]);
521	#endif
522	/*
523	* Compute the amount of time during which the current
524	* thread was running, and add that to its total so far.
525	*/
526	new_switchtime = cpu_ticks();
527	runtime = new_switchtime - PCPU_GET(switchtime);
528	td->td_runtime += runtime;
529	td->td_incruntime += runtime;
530	PCPU_SET(switchtime, new_switchtime);
531	td->td_generation++; /* bump preempt-detect counter */
532	VM_CNT_INC(v_swtch);
533	PCPU_SET(switchticks, ticks);
534	CTR4(KTR_PROC, "mi_switch: old thread %ld (td_sched %p, pid %ld, %s)",
535	td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name);
536	#ifdef KDTRACE_HOOKS
537	if (SDT_PROBES_ENABLED() &&
538	((flags & SW_PREEMPT) != 0 \|\| ((flags & SW_INVOL) != 0 &&
539	(flags & SW_TYPE_MASK) == SWT_NEEDRESCHED)))
540	SDT_PROBE0(sched, , , preempt);
541	#endif
542	sched_switch(td, flags);
543	CTR4(KTR_PROC, "mi_switch: new thread %ld (td_sched %p, pid %ld, %s)",
544	td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name);
545
546	/*
547	* If the last thread was exiting, finish cleaning it up.
548	*/
549	if ((td = PCPU_GET(deadthread))) {
550	PCPU_SET(deadthread, NULL);
551	thread_stash(td);
552	}
553	spinlock_exit();
554	}
555
556	/*
557	* Change thread state to be runnable, placing it on the run queue if
558	* it is in memory. If it is swapped out, return true so our caller
559	* will know to awaken the swapper.
560	*
561	* Requires the thread lock on entry, drops on exit.
562	*/
563	int
564	setrunnable(struct thread *td, int srqflags)
565	{
566	int swapin;
567
568	THREAD_LOCK_ASSERT(td, MA_OWNED);
569	KASSERT(td->td_proc->p_state != PRS_ZOMBIE,
570	("setrunnable: pid %d is a zombie", td->td_proc->p_pid));
571
572	swapin = 0;
573	switch (td->td_state) {
574	case TDS_RUNNING:
575	case TDS_RUNQ:
576	break;
577	case TDS_CAN_RUN:
578	KASSERT((td->td_flags & TDF_INMEM) != 0,
579	("setrunnable: td %p not in mem, flags 0x%X inhibit 0x%X",
580	td, td->td_flags, td->td_inhibitors));
581	/* unlocks thread lock according to flags */
582	sched_wakeup(td, srqflags);
583	return (0);
584	case TDS_INHIBITED:
585	/*
586	* If we are only inhibited because we are swapped out
587	* arrange to swap in this process.
588	*/
589	if (td->td_inhibitors == TDI_SWAPPED &&
590	(td->td_flags & TDF_SWAPINREQ) == 0) {
591	td->td_flags \|= TDF_SWAPINREQ;
592	swapin = 1;
593	}
594	break;
595	default:
596	panic("setrunnable: state 0x%x", td->td_state);
597	}
598	if ((srqflags & (SRQ_HOLD \| SRQ_HOLDTD)) == 0)
599	thread_unlock(td);
600
601	return (swapin);
602	}
603
604	/*
605	* Compute a tenex style load average of a quantity on
606	* 1, 5 and 15 minute intervals.
607	*/
608	static void
609	loadav(void *arg)
610	{
611	int i, nrun;
612	struct loadavg *avg;
613
614	nrun = sched_load();
615	avg = &averunnable;
616
617	for (i = 0; i < 3; i++)
618	avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
619	nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
620
621	/*
622	* Schedule the next update to occur after 5 seconds, but add a
623	* random variation to avoid synchronisation with processes that
624	* run at regular intervals.
625	*/
626	callout_reset_sbt(&loadav_callout,
627	SBT_1US * (4000000 + (int)(random() % 2000001)), SBT_1US,
628	loadav, NULL, C_DIRECT_EXEC \| C_PREL(32));
629	}
630
631	/* ARGSUSED */
632	static void
633	synch_setup(void *dummy)
634	{
635	callout_init(&loadav_callout, 1);
636
637	/* Kick off timeout driven events by calling first time. */
638	loadav(NULL);
639	}
640
641	int
642	should_yield(void)
643	{
644
645	return ((u_int)ticks - (u_int)curthread->td_swvoltick >= hogticks);
646	}
647
648	void
649	maybe_yield(void)
650	{
651
652	if (should_yield())
653	kern_yield(PRI_USER);
654	}
655
656	void
657	kern_yield(int prio)
658	{
659	struct thread *td;
660
661	td = curthread;
662	DROP_GIANT();
663	thread_lock(td);
664	if (prio == PRI_USER)
665	prio = td->td_user_pri;
666	if (prio >= 0)
667	sched_prio(td, prio);
668	mi_switch(SW_VOL \| SWT_RELINQUISH);
669	PICKUP_GIANT();
670	}
671
672	/*
673	* General purpose yield system call.
674	*/
675	int
676	sys_yield(struct thread td, struct yield_args uap)
677	{
678
679	thread_lock(td);
680	if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE)
681	sched_prio(td, PRI_MAX_TIMESHARE);
682	mi_switch(SW_VOL \| SWT_RELINQUISH);
683	td->td_retval[0] = 0;
684	return (0);
685	}