FreeBSD/Linux Kernel Cross Reference
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)) {
                   KASSERT(!(class->lc_flags & LC_SPINLOCK),
                       ("spin locks can only use msleep_spin"));
                   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 | SLEEPQ_DROP, 0, 0);
           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 |
               SLEEPQ_DROP, 0, 0);
           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);
   }

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