The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_synch.c

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    1 /*-
    2  * Copyright (c) 1982, 1986, 1990, 1991, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  * (c) UNIX System Laboratories, Inc.
    5  * All or some portions of this file are derived from material licensed
    6  * to the University of California by American Telephone and Telegraph
    7  * Co. or Unix System Laboratories, Inc. and are reproduced herein with
    8  * the permission of UNIX System Laboratories, Inc.
    9  *
   10  * Redistribution and use in source and binary forms, with or without
   11  * modification, are permitted provided that the following conditions
   12  * are met:
   13  * 1. Redistributions of source code must retain the above copyright
   14  *    notice, this list of conditions and the following disclaimer.
   15  * 2. Redistributions in binary form must reproduce the above copyright
   16  *    notice, this list of conditions and the following disclaimer in the
   17  *    documentation and/or other materials provided with the distribution.
   18  * 4. Neither the name of the University nor the names of its contributors
   19  *    may be used to endorse or promote products derived from this software
   20  *    without specific prior written permission.
   21  *
   22  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   23  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   24  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   25  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   26  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   27  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   28  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   29  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   31  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   32  * SUCH DAMAGE.
   33  *
   34  *      @(#)kern_synch.c        8.9 (Berkeley) 5/19/95
   35  */
   36 
   37 #include <sys/cdefs.h>
   38 __FBSDID("$FreeBSD: releng/10.2/sys/kern/kern_synch.c 284021 2015-06-05 08:36:25Z kib $");
   39 
   40 #include "opt_kdtrace.h"
   41 #include "opt_ktrace.h"
   42 #include "opt_sched.h"
   43 
   44 #include <sys/param.h>
   45 #include <sys/systm.h>
   46 #include <sys/condvar.h>
   47 #include <sys/kdb.h>
   48 #include <sys/kernel.h>
   49 #include <sys/ktr.h>
   50 #include <sys/lock.h>
   51 #include <sys/mutex.h>
   52 #include <sys/proc.h>
   53 #include <sys/resourcevar.h>
   54 #include <sys/sched.h>
   55 #include <sys/sdt.h>
   56 #include <sys/signalvar.h>
   57 #include <sys/sleepqueue.h>
   58 #include <sys/smp.h>
   59 #include <sys/sx.h>
   60 #include <sys/sysctl.h>
   61 #include <sys/sysproto.h>
   62 #include <sys/vmmeter.h>
   63 #ifdef KTRACE
   64 #include <sys/uio.h>
   65 #include <sys/ktrace.h>
   66 #endif
   67 
   68 #include <machine/cpu.h>
   69 
   70 #ifdef XEN
   71 #include <vm/vm.h>
   72 #include <vm/vm_param.h>
   73 #include <vm/pmap.h>
   74 #endif
   75 
   76 #define KTDSTATE(td)                                                    \
   77         (((td)->td_inhibitors & TDI_SLEEPING) != 0 ? "sleep"  :         \
   78         ((td)->td_inhibitors & TDI_SUSPENDED) != 0 ? "suspended" :      \
   79         ((td)->td_inhibitors & TDI_SWAPPED) != 0 ? "swapped" :          \
   80         ((td)->td_inhibitors & TDI_LOCK) != 0 ? "blocked" :             \
   81         ((td)->td_inhibitors & TDI_IWAIT) != 0 ? "iwait" : "yielding")
   82 
   83 static void synch_setup(void *dummy);
   84 SYSINIT(synch_setup, SI_SUB_KICK_SCHEDULER, SI_ORDER_FIRST, synch_setup,
   85     NULL);
   86 
   87 int     hogticks;
   88 static uint8_t pause_wchan[MAXCPU];
   89 
   90 static struct callout loadav_callout;
   91 
   92 struct loadavg averunnable =
   93         { {0, 0, 0}, FSCALE };  /* load average, of runnable procs */
   94 /*
   95  * Constants for averages over 1, 5, and 15 minutes
   96  * when sampling at 5 second intervals.
   97  */
   98 static fixpt_t cexp[3] = {
   99         0.9200444146293232 * FSCALE,    /* exp(-1/12) */
  100         0.9834714538216174 * FSCALE,    /* exp(-1/60) */
  101         0.9944598480048967 * FSCALE,    /* exp(-1/180) */
  102 };
  103 
  104 /* kernel uses `FSCALE', userland (SHOULD) use kern.fscale */
  105 SYSCTL_INT(_kern, OID_AUTO, fscale, CTLFLAG_RD, SYSCTL_NULL_INT_PTR, FSCALE, "");
  106 
  107 static void     loadav(void *arg);
  108 
  109 SDT_PROVIDER_DECLARE(sched);
  110 SDT_PROBE_DEFINE(sched, , , preempt);
  111 
  112 /*
  113  * These probes reference Solaris features that are not implemented in FreeBSD.
  114  * Create the probes anyway for compatibility with existing D scripts; they'll
  115  * just never fire.
  116  */
  117 SDT_PROBE_DEFINE(sched, , , cpucaps__sleep);
  118 SDT_PROBE_DEFINE(sched, , , cpucaps__wakeup);
  119 SDT_PROBE_DEFINE(sched, , , schedctl__nopreempt);
  120 SDT_PROBE_DEFINE(sched, , , schedctl__preempt);
  121 SDT_PROBE_DEFINE(sched, , , schedctl__yield);
  122 
  123 static void
  124 sleepinit(void *unused)
  125 {
  126 
  127         hogticks = (hz / 10) * 2;       /* Default only. */
  128         init_sleepqueues();
  129 }
  130 
  131 /*
  132  * vmem tries to lock the sleepq mutexes when free'ing kva, so make sure
  133  * it is available.
  134  */
  135 SYSINIT(sleepinit, SI_SUB_KMEM, SI_ORDER_ANY, sleepinit, 0);
  136 
  137 /*
  138  * General sleep call.  Suspends the current thread until a wakeup is
  139  * performed on the specified identifier.  The thread will then be made
  140  * runnable with the specified priority.  Sleeps at most sbt units of time
  141  * (0 means no timeout).  If pri includes the PCATCH flag, let signals
  142  * interrupt the sleep, otherwise ignore them while sleeping.  Returns 0 if
  143  * awakened, EWOULDBLOCK if the timeout expires.  If PCATCH is set and a
  144  * signal becomes pending, ERESTART is returned if the current system
  145  * call should be restarted if possible, and EINTR is returned if the system
  146  * call should be interrupted by the signal (return EINTR).
  147  *
  148  * The lock argument is unlocked before the caller is suspended, and
  149  * re-locked before _sleep() returns.  If priority includes the PDROP
  150  * flag the lock is not re-locked before returning.
  151  */
  152 int
  153 _sleep(void *ident, struct lock_object *lock, int priority,
  154     const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
  155 {
  156         struct thread *td;
  157         struct proc *p;
  158         struct lock_class *class;
  159         uintptr_t lock_state;
  160         int catch, pri, rval, sleepq_flags;
  161         WITNESS_SAVE_DECL(lock_witness);
  162 
  163         td = curthread;
  164         p = td->td_proc;
  165 #ifdef KTRACE
  166         if (KTRPOINT(td, KTR_CSW))
  167                 ktrcsw(1, 0, wmesg);
  168 #endif
  169         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
  170             "Sleeping on \"%s\"", wmesg);
  171         KASSERT(sbt != 0 || mtx_owned(&Giant) || lock != NULL,
  172             ("sleeping without a lock"));
  173         KASSERT(p != NULL, ("msleep1"));
  174         KASSERT(ident != NULL && TD_IS_RUNNING(td), ("msleep"));
  175         if (priority & PDROP)
  176                 KASSERT(lock != NULL && lock != &Giant.lock_object,
  177                     ("PDROP requires a non-Giant lock"));
  178         if (lock != NULL)
  179                 class = LOCK_CLASS(lock);
  180         else
  181                 class = NULL;
  182 
  183         if (cold || SCHEDULER_STOPPED()) {
  184                 /*
  185                  * During autoconfiguration, just return;
  186                  * don't run any other threads or panic below,
  187                  * in case this is the idle thread and already asleep.
  188                  * XXX: this used to do "s = splhigh(); splx(safepri);
  189                  * splx(s);" to give interrupts a chance, but there is
  190                  * no way to give interrupts a chance now.
  191                  */
  192                 if (lock != NULL && priority & PDROP)
  193                         class->lc_unlock(lock);
  194                 return (0);
  195         }
  196         catch = priority & PCATCH;
  197         pri = priority & PRIMASK;
  198 
  199         /*
  200          * If we are already on a sleep queue, then remove us from that
  201          * sleep queue first.  We have to do this to handle recursive
  202          * sleeps.
  203          */
  204         if (TD_ON_SLEEPQ(td))
  205                 sleepq_remove(td, td->td_wchan);
  206 
  207         if ((uint8_t *)ident >= &pause_wchan[0] &&
  208             (uint8_t *)ident <= &pause_wchan[MAXCPU - 1])
  209                 sleepq_flags = SLEEPQ_PAUSE;
  210         else
  211                 sleepq_flags = SLEEPQ_SLEEP;
  212         if (catch)
  213                 sleepq_flags |= SLEEPQ_INTERRUPTIBLE;
  214 
  215         sleepq_lock(ident);
  216         CTR5(KTR_PROC, "sleep: thread %ld (pid %ld, %s) on %s (%p)",
  217             td->td_tid, p->p_pid, td->td_name, wmesg, ident);
  218 
  219         if (lock == &Giant.lock_object)
  220                 mtx_assert(&Giant, MA_OWNED);
  221         DROP_GIANT();
  222         if (lock != NULL && lock != &Giant.lock_object &&
  223             !(class->lc_flags & LC_SLEEPABLE)) {
  224                 WITNESS_SAVE(lock, lock_witness);
  225                 lock_state = class->lc_unlock(lock);
  226         } else
  227                 /* GCC needs to follow the Yellow Brick Road */
  228                 lock_state = -1;
  229 
  230         /*
  231          * We put ourselves on the sleep queue and start our timeout
  232          * before calling thread_suspend_check, as we could stop there,
  233          * and a wakeup or a SIGCONT (or both) could occur while we were
  234          * stopped without resuming us.  Thus, we must be ready for sleep
  235          * when cursig() is called.  If the wakeup happens while we're
  236          * stopped, then td will no longer be on a sleep queue upon
  237          * return from cursig().
  238          */
  239         sleepq_add(ident, lock, wmesg, sleepq_flags, 0);
  240         if (sbt != 0)
  241                 sleepq_set_timeout_sbt(ident, sbt, pr, flags);
  242         if (lock != NULL && class->lc_flags & LC_SLEEPABLE) {
  243                 sleepq_release(ident);
  244                 WITNESS_SAVE(lock, lock_witness);
  245                 lock_state = class->lc_unlock(lock);
  246                 sleepq_lock(ident);
  247         }
  248         if (sbt != 0 && catch)
  249                 rval = sleepq_timedwait_sig(ident, pri);
  250         else if (sbt != 0)
  251                 rval = sleepq_timedwait(ident, pri);
  252         else if (catch)
  253                 rval = sleepq_wait_sig(ident, pri);
  254         else {
  255                 sleepq_wait(ident, pri);
  256                 rval = 0;
  257         }
  258 #ifdef KTRACE
  259         if (KTRPOINT(td, KTR_CSW))
  260                 ktrcsw(0, 0, wmesg);
  261 #endif
  262         PICKUP_GIANT();
  263         if (lock != NULL && lock != &Giant.lock_object && !(priority & PDROP)) {
  264                 class->lc_lock(lock, lock_state);
  265                 WITNESS_RESTORE(lock, lock_witness);
  266         }
  267         return (rval);
  268 }
  269 
  270 int
  271 msleep_spin_sbt(void *ident, struct mtx *mtx, const char *wmesg,
  272     sbintime_t sbt, sbintime_t pr, int flags)
  273 {
  274         struct thread *td;
  275         struct proc *p;
  276         int rval;
  277         WITNESS_SAVE_DECL(mtx);
  278 
  279         td = curthread;
  280         p = td->td_proc;
  281         KASSERT(mtx != NULL, ("sleeping without a mutex"));
  282         KASSERT(p != NULL, ("msleep1"));
  283         KASSERT(ident != NULL && TD_IS_RUNNING(td), ("msleep"));
  284 
  285         if (cold || SCHEDULER_STOPPED()) {
  286                 /*
  287                  * During autoconfiguration, just return;
  288                  * don't run any other threads or panic below,
  289                  * in case this is the idle thread and already asleep.
  290                  * XXX: this used to do "s = splhigh(); splx(safepri);
  291                  * splx(s);" to give interrupts a chance, but there is
  292                  * no way to give interrupts a chance now.
  293                  */
  294                 return (0);
  295         }
  296 
  297         sleepq_lock(ident);
  298         CTR5(KTR_PROC, "msleep_spin: thread %ld (pid %ld, %s) on %s (%p)",
  299             td->td_tid, p->p_pid, td->td_name, wmesg, ident);
  300 
  301         DROP_GIANT();
  302         mtx_assert(mtx, MA_OWNED | MA_NOTRECURSED);
  303         WITNESS_SAVE(&mtx->lock_object, mtx);
  304         mtx_unlock_spin(mtx);
  305 
  306         /*
  307          * We put ourselves on the sleep queue and start our timeout.
  308          */
  309         sleepq_add(ident, &mtx->lock_object, wmesg, SLEEPQ_SLEEP, 0);
  310         if (sbt != 0)
  311                 sleepq_set_timeout_sbt(ident, sbt, pr, flags);
  312 
  313         /*
  314          * Can't call ktrace with any spin locks held so it can lock the
  315          * ktrace_mtx lock, and WITNESS_WARN considers it an error to hold
  316          * any spin lock.  Thus, we have to drop the sleepq spin lock while
  317          * we handle those requests.  This is safe since we have placed our
  318          * thread on the sleep queue already.
  319          */
  320 #ifdef KTRACE
  321         if (KTRPOINT(td, KTR_CSW)) {
  322                 sleepq_release(ident);
  323                 ktrcsw(1, 0, wmesg);
  324                 sleepq_lock(ident);
  325         }
  326 #endif
  327 #ifdef WITNESS
  328         sleepq_release(ident);
  329         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, "Sleeping on \"%s\"",
  330             wmesg);
  331         sleepq_lock(ident);
  332 #endif
  333         if (sbt != 0)
  334                 rval = sleepq_timedwait(ident, 0);
  335         else {
  336                 sleepq_wait(ident, 0);
  337                 rval = 0;
  338         }
  339 #ifdef KTRACE
  340         if (KTRPOINT(td, KTR_CSW))
  341                 ktrcsw(0, 0, wmesg);
  342 #endif
  343         PICKUP_GIANT();
  344         mtx_lock_spin(mtx);
  345         WITNESS_RESTORE(&mtx->lock_object, mtx);
  346         return (rval);
  347 }
  348 
  349 /*
  350  * pause() delays the calling thread by the given number of system ticks.
  351  * During cold bootup, pause() uses the DELAY() function instead of
  352  * the tsleep() function to do the waiting. The "timo" argument must be
  353  * greater than or equal to zero. A "timo" value of zero is equivalent
  354  * to a "timo" value of one.
  355  */
  356 int
  357 pause_sbt(const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags)
  358 {
  359         KASSERT(sbt >= 0, ("pause: timeout must be >= 0"));
  360 
  361         /* silently convert invalid timeouts */
  362         if (sbt == 0)
  363                 sbt = tick_sbt;
  364 
  365         if (cold || kdb_active) {
  366                 /*
  367                  * We delay one second at a time to avoid overflowing the
  368                  * system specific DELAY() function(s):
  369                  */
  370                 while (sbt >= SBT_1S) {
  371                         DELAY(1000000);
  372                         sbt -= SBT_1S;
  373                 }
  374                 /* Do the delay remainder, if any */
  375                 sbt = (sbt + SBT_1US - 1) / SBT_1US;
  376                 if (sbt > 0)
  377                         DELAY(sbt);
  378                 return (0);
  379         }
  380         return (_sleep(&pause_wchan[curcpu], NULL, 0, wmesg, sbt, pr, flags));
  381 }
  382 
  383 /*
  384  * Make all threads sleeping on the specified identifier runnable.
  385  */
  386 void
  387 wakeup(void *ident)
  388 {
  389         int wakeup_swapper;
  390 
  391         sleepq_lock(ident);
  392         wakeup_swapper = sleepq_broadcast(ident, SLEEPQ_SLEEP, 0, 0);
  393         sleepq_release(ident);
  394         if (wakeup_swapper) {
  395                 KASSERT(ident != &proc0,
  396                     ("wakeup and wakeup_swapper and proc0"));
  397                 kick_proc0();
  398         }
  399 }
  400 
  401 /*
  402  * Make a thread sleeping on the specified identifier runnable.
  403  * May wake more than one thread if a target thread is currently
  404  * swapped out.
  405  */
  406 void
  407 wakeup_one(void *ident)
  408 {
  409         int wakeup_swapper;
  410 
  411         sleepq_lock(ident);
  412         wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP, 0, 0);
  413         sleepq_release(ident);
  414         if (wakeup_swapper)
  415                 kick_proc0();
  416 }
  417 
  418 static void
  419 kdb_switch(void)
  420 {
  421         thread_unlock(curthread);
  422         kdb_backtrace();
  423         kdb_reenter();
  424         panic("%s: did not reenter debugger", __func__);
  425 }
  426 
  427 /*
  428  * The machine independent parts of context switching.
  429  */
  430 void
  431 mi_switch(int flags, struct thread *newtd)
  432 {
  433         uint64_t runtime, new_switchtime;
  434         struct thread *td;
  435 
  436         td = curthread;                 /* XXX */
  437         THREAD_LOCK_ASSERT(td, MA_OWNED | MA_NOTRECURSED);
  438         KASSERT(!TD_ON_RUNQ(td), ("mi_switch: called by old code"));
  439 #ifdef INVARIANTS
  440         if (!TD_ON_LOCK(td) && !TD_IS_RUNNING(td))
  441                 mtx_assert(&Giant, MA_NOTOWNED);
  442 #endif
  443         KASSERT(td->td_critnest == 1 || panicstr,
  444             ("mi_switch: switch in a critical section"));
  445         KASSERT((flags & (SW_INVOL | SW_VOL)) != 0,
  446             ("mi_switch: switch must be voluntary or involuntary"));
  447         KASSERT(newtd != curthread, ("mi_switch: preempting back to ourself"));
  448 
  449         /*
  450          * Don't perform context switches from the debugger.
  451          */
  452         if (kdb_active)
  453                 kdb_switch();
  454         if (SCHEDULER_STOPPED())
  455                 return;
  456         if (flags & SW_VOL) {
  457                 td->td_ru.ru_nvcsw++;
  458                 td->td_swvoltick = ticks;
  459         } else
  460                 td->td_ru.ru_nivcsw++;
  461 #ifdef SCHED_STATS
  462         SCHED_STAT_INC(sched_switch_stats[flags & SW_TYPE_MASK]);
  463 #endif
  464         /*
  465          * Compute the amount of time during which the current
  466          * thread was running, and add that to its total so far.
  467          */
  468         new_switchtime = cpu_ticks();
  469         runtime = new_switchtime - PCPU_GET(switchtime);
  470         td->td_runtime += runtime;
  471         td->td_incruntime += runtime;
  472         PCPU_SET(switchtime, new_switchtime);
  473         td->td_generation++;    /* bump preempt-detect counter */
  474         PCPU_INC(cnt.v_swtch);
  475         PCPU_SET(switchticks, ticks);
  476         CTR4(KTR_PROC, "mi_switch: old thread %ld (td_sched %p, pid %ld, %s)",
  477             td->td_tid, td->td_sched, td->td_proc->p_pid, td->td_name);
  478 #if (KTR_COMPILE & KTR_SCHED) != 0
  479         if (TD_IS_IDLETHREAD(td))
  480                 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "idle",
  481                     "prio:%d", td->td_priority);
  482         else
  483                 KTR_STATE3(KTR_SCHED, "thread", sched_tdname(td), KTDSTATE(td),
  484                     "prio:%d", td->td_priority, "wmesg:\"%s\"", td->td_wmesg,
  485                     "lockname:\"%s\"", td->td_lockname);
  486 #endif
  487         SDT_PROBE0(sched, , , preempt);
  488 #ifdef XEN
  489         PT_UPDATES_FLUSH();
  490 #endif
  491         sched_switch(td, newtd, flags);
  492         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "running",
  493             "prio:%d", td->td_priority);
  494 
  495         CTR4(KTR_PROC, "mi_switch: new thread %ld (td_sched %p, pid %ld, %s)",
  496             td->td_tid, td->td_sched, td->td_proc->p_pid, td->td_name);
  497 
  498         /* 
  499          * If the last thread was exiting, finish cleaning it up.
  500          */
  501         if ((td = PCPU_GET(deadthread))) {
  502                 PCPU_SET(deadthread, NULL);
  503                 thread_stash(td);
  504         }
  505 }
  506 
  507 /*
  508  * Change thread state to be runnable, placing it on the run queue if
  509  * it is in memory.  If it is swapped out, return true so our caller
  510  * will know to awaken the swapper.
  511  */
  512 int
  513 setrunnable(struct thread *td)
  514 {
  515 
  516         THREAD_LOCK_ASSERT(td, MA_OWNED);
  517         KASSERT(td->td_proc->p_state != PRS_ZOMBIE,
  518             ("setrunnable: pid %d is a zombie", td->td_proc->p_pid));
  519         switch (td->td_state) {
  520         case TDS_RUNNING:
  521         case TDS_RUNQ:
  522                 return (0);
  523         case TDS_INHIBITED:
  524                 /*
  525                  * If we are only inhibited because we are swapped out
  526                  * then arange to swap in this process. Otherwise just return.
  527                  */
  528                 if (td->td_inhibitors != TDI_SWAPPED)
  529                         return (0);
  530                 /* FALLTHROUGH */
  531         case TDS_CAN_RUN:
  532                 break;
  533         default:
  534                 printf("state is 0x%x", td->td_state);
  535                 panic("setrunnable(2)");
  536         }
  537         if ((td->td_flags & TDF_INMEM) == 0) {
  538                 if ((td->td_flags & TDF_SWAPINREQ) == 0) {
  539                         td->td_flags |= TDF_SWAPINREQ;
  540                         return (1);
  541                 }
  542         } else
  543                 sched_wakeup(td);
  544         return (0);
  545 }
  546 
  547 /*
  548  * Compute a tenex style load average of a quantity on
  549  * 1, 5 and 15 minute intervals.
  550  */
  551 static void
  552 loadav(void *arg)
  553 {
  554         int i, nrun;
  555         struct loadavg *avg;
  556 
  557         nrun = sched_load();
  558         avg = &averunnable;
  559 
  560         for (i = 0; i < 3; i++)
  561                 avg->ldavg[i] = (cexp[i] * avg->ldavg[i] +
  562                     nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT;
  563 
  564         /*
  565          * Schedule the next update to occur after 5 seconds, but add a
  566          * random variation to avoid synchronisation with processes that
  567          * run at regular intervals.
  568          */
  569         callout_reset_sbt(&loadav_callout,
  570             SBT_1US * (4000000 + (int)(random() % 2000001)), SBT_1US,
  571             loadav, NULL, C_DIRECT_EXEC | C_PREL(32));
  572 }
  573 
  574 /* ARGSUSED */
  575 static void
  576 synch_setup(void *dummy)
  577 {
  578         callout_init(&loadav_callout, CALLOUT_MPSAFE);
  579 
  580         /* Kick off timeout driven events by calling first time. */
  581         loadav(NULL);
  582 }
  583 
  584 int
  585 should_yield(void)
  586 {
  587 
  588         return ((u_int)ticks - (u_int)curthread->td_swvoltick >= hogticks);
  589 }
  590 
  591 void
  592 maybe_yield(void)
  593 {
  594 
  595         if (should_yield())
  596                 kern_yield(PRI_USER);
  597 }
  598 
  599 void
  600 kern_yield(int prio)
  601 {
  602         struct thread *td;
  603 
  604         td = curthread;
  605         DROP_GIANT();
  606         thread_lock(td);
  607         if (prio == PRI_USER)
  608                 prio = td->td_user_pri;
  609         if (prio >= 0)
  610                 sched_prio(td, prio);
  611         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
  612         thread_unlock(td);
  613         PICKUP_GIANT();
  614 }
  615 
  616 /*
  617  * General purpose yield system call.
  618  */
  619 int
  620 sys_yield(struct thread *td, struct yield_args *uap)
  621 {
  622 
  623         thread_lock(td);
  624         if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE)
  625                 sched_prio(td, PRI_MAX_TIMESHARE);
  626         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
  627         thread_unlock(td);
  628         td->td_retval[0] = 0;
  629         return (0);
  630 }

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