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
75static void synch_setup(void *dummy);
76SYSINIT(synch_setup, SI_SUB_KICK_SCHEDULER, SI_ORDER_FIRST, synch_setup,
77    NULL);
78
79int	hogticks;
80static const char pause_wchan[MAXCPU];
81
82static struct callout loadav_callout;
83
84struct 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 */
90static 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 */
97SYSCTL_INT(_kern, OID_AUTO, fscale, CTLFLAG_RD, SYSCTL_NULL_INT_PTR, FSCALE,
98    "Fixed-point scale factor used for calculating load average values");
99
100static void	loadav(void *arg);
101
102SDT_PROVIDER_DECLARE(sched);
103SDT_PROBE_DEFINE(sched, , , preempt);
104
105static void
106sleepinit(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 */
117SYSINIT(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 */
134int
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
236int
237msleep_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 */
311int
312pause_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 */
343void
344wakeup(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 */
363void
364wakeup_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
375void
376wakeup_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 */
391void
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 */
410int
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
465out:
466	PICKUP_GIANT();
467	if (lock != NULL && !drop)
468		LOCK_CLASS(lock)->lc_lock(lock, lock_state);
469
470	return (ret);
471}
472
473static void
474kdb_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 */
487void
488mi_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 */
563int
564setrunnable(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 */
608static void
609loadav(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 */
632static void
633synch_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
641int
642should_yield(void)
643{
644
645	return ((u_int)ticks - (u_int)curthread->td_swvoltick >= hogticks);
646}
647
648void
649maybe_yield(void)
650{
651
652	if (should_yield())
653		kern_yield(PRI_USER);
654}
655
656void
657kern_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 */
675int
676sys_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}
686