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/condvar.h> 48 #include <sys/kdb.h> 49 #include <sys/kernel.h> 50 #include <sys/ktr.h> 51 #include <sys/lock.h> 52 #include <sys/mutex.h> 53 #include <sys/proc.h> 54 #include <sys/resourcevar.h> 55 #include <sys/refcount.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 70 #include <machine/cpu.h> 71 72 static void synch_setup(void *dummy); 73 SYSINIT(synch_setup, SI_SUB_KICK_SCHEDULER, SI_ORDER_FIRST, synch_setup, 74 NULL); 75 76 int hogticks; 77 static uint8_t pause_wchan[MAXCPU]; 78 79 static struct callout loadav_callout; 80 81 struct loadavg averunnable = 82 { {0, 0, 0}, FSCALE }; /* load average, of runnable procs */ 83 /* 84 * Constants for averages over 1, 5, and 15 minutes 85 * when sampling at 5 second intervals. 86 */ 87 static fixpt_t cexp[3] = { 88 0.9200444146293232 * FSCALE, /* exp(-1/12) */ 89 0.9834714538216174 * FSCALE, /* exp(-1/60) */ 90 0.9944598480048967 * FSCALE, /* exp(-1/180) */ 91 }; 92 93 /* kernel uses `FSCALE', userland (SHOULD) use kern.fscale */ 94 SYSCTL_INT(_kern, OID_AUTO, fscale, CTLFLAG_RD, SYSCTL_NULL_INT_PTR, FSCALE, ""); 95 96 static void loadav(void *arg); 97 98 SDT_PROVIDER_DECLARE(sched); 99 SDT_PROBE_DEFINE(sched, , , preempt); 100 101 static void 102 sleepinit(void *unused) 103 { 104 105 hogticks = (hz / 10) * 2; /* Default only. */ 106 init_sleepqueues(); 107 } 108 109 /* 110 * vmem tries to lock the sleepq mutexes when free'ing kva, so make sure 111 * it is available. 112 */ 113 SYSINIT(sleepinit, SI_SUB_KMEM, SI_ORDER_ANY, sleepinit, NULL); 114 115 /* 116 * General sleep call. Suspends the current thread until a wakeup is 117 * performed on the specified identifier. The thread will then be made 118 * runnable with the specified priority. Sleeps at most sbt units of time 119 * (0 means no timeout). If pri includes the PCATCH flag, let signals 120 * interrupt the sleep, otherwise ignore them while sleeping. Returns 0 if 121 * awakened, EWOULDBLOCK if the timeout expires. If PCATCH is set and a 122 * signal becomes pending, ERESTART is returned if the current system 123 * call should be restarted if possible, and EINTR is returned if the system 124 * call should be interrupted by the signal (return EINTR). 125 * 126 * The lock argument is unlocked before the caller is suspended, and 127 * re-locked before _sleep() returns. If priority includes the PDROP 128 * flag the lock is not re-locked before returning. 129 */ 130 int 131 _sleep(void *ident, struct lock_object *lock, int priority, 132 const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags) 133 { 134 struct thread *td; 135 struct lock_class *class; 136 uintptr_t lock_state; 137 int catch, pri, rval, sleepq_flags; 138 WITNESS_SAVE_DECL(lock_witness); 139 140 td = curthread; 141 #ifdef KTRACE 142 if (KTRPOINT(td, KTR_CSW)) 143 ktrcsw(1, 0, wmesg); 144 #endif 145 WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock, 146 "Sleeping on \"%s\"", wmesg); 147 KASSERT(sbt != 0 || mtx_owned(&Giant) || lock != NULL, 148 ("sleeping without a lock")); 149 KASSERT(ident != NULL, ("_sleep: NULL ident")); 150 KASSERT(TD_IS_RUNNING(td), ("_sleep: curthread not running")); 151 KASSERT(td->td_epochnest == 0, ("sleeping in an epoch section")); 152 if (priority & PDROP) 153 KASSERT(lock != NULL && lock != &Giant.lock_object, 154 ("PDROP requires a non-Giant lock")); 155 if (lock != NULL) 156 class = LOCK_CLASS(lock); 157 else 158 class = NULL; 159 160 if (SCHEDULER_STOPPED_TD(td)) { 161 if (lock != NULL && priority & PDROP) 162 class->lc_unlock(lock); 163 return (0); 164 } 165 catch = priority & PCATCH; 166 pri = priority & PRIMASK; 167 168 KASSERT(!TD_ON_SLEEPQ(td), ("recursive sleep")); 169 170 if ((uint8_t *)ident >= &pause_wchan[0] && 171 (uint8_t *)ident <= &pause_wchan[MAXCPU - 1]) 172 sleepq_flags = SLEEPQ_PAUSE; 173 else 174 sleepq_flags = SLEEPQ_SLEEP; 175 if (catch) 176 sleepq_flags |= SLEEPQ_INTERRUPTIBLE; 177 178 sleepq_lock(ident); 179 CTR5(KTR_PROC, "sleep: thread %ld (pid %ld, %s) on %s (%p)", 180 td->td_tid, td->td_proc->p_pid, td->td_name, wmesg, ident); 181 182 if (lock == &Giant.lock_object) 183 mtx_assert(&Giant, MA_OWNED); 184 DROP_GIANT(); 185 if (lock != NULL && lock != &Giant.lock_object && 186 !(class->lc_flags & LC_SLEEPABLE)) { 187 WITNESS_SAVE(lock, lock_witness); 188 lock_state = class->lc_unlock(lock); 189 } else 190 /* GCC needs to follow the Yellow Brick Road */ 191 lock_state = -1; 192 193 /* 194 * We put ourselves on the sleep queue and start our timeout 195 * before calling thread_suspend_check, as we could stop there, 196 * and a wakeup or a SIGCONT (or both) could occur while we were 197 * stopped without resuming us. Thus, we must be ready for sleep 198 * when cursig() is called. If the wakeup happens while we're 199 * stopped, then td will no longer be on a sleep queue upon 200 * return from cursig(). 201 */ 202 sleepq_add(ident, lock, wmesg, sleepq_flags, 0); 203 if (sbt != 0) 204 sleepq_set_timeout_sbt(ident, sbt, pr, flags); 205 if (lock != NULL && class->lc_flags & LC_SLEEPABLE) { 206 sleepq_release(ident); 207 WITNESS_SAVE(lock, lock_witness); 208 lock_state = class->lc_unlock(lock); 209 sleepq_lock(ident); 210 } 211 if (sbt != 0 && catch) 212 rval = sleepq_timedwait_sig(ident, pri); 213 else if (sbt != 0) 214 rval = sleepq_timedwait(ident, pri); 215 else if (catch) 216 rval = sleepq_wait_sig(ident, pri); 217 else { 218 sleepq_wait(ident, pri); 219 rval = 0; 220 } 221 #ifdef KTRACE 222 if (KTRPOINT(td, KTR_CSW)) 223 ktrcsw(0, 0, wmesg); 224 #endif 225 PICKUP_GIANT(); 226 if (lock != NULL && lock != &Giant.lock_object && !(priority & PDROP)) { 227 class->lc_lock(lock, lock_state); 228 WITNESS_RESTORE(lock, lock_witness); 229 } 230 return (rval); 231 } 232 233 int 234 msleep_spin_sbt(void *ident, struct mtx *mtx, const char *wmesg, 235 sbintime_t sbt, sbintime_t pr, int flags) 236 { 237 struct thread *td; 238 int rval; 239 WITNESS_SAVE_DECL(mtx); 240 241 td = curthread; 242 KASSERT(mtx != NULL, ("sleeping without a mutex")); 243 KASSERT(ident != NULL, ("msleep_spin_sbt: NULL ident")); 244 KASSERT(TD_IS_RUNNING(td), ("msleep_spin_sbt: curthread not running")); 245 246 if (SCHEDULER_STOPPED_TD(td)) 247 return (0); 248 249 sleepq_lock(ident); 250 CTR5(KTR_PROC, "msleep_spin: thread %ld (pid %ld, %s) on %s (%p)", 251 td->td_tid, td->td_proc->p_pid, td->td_name, wmesg, ident); 252 253 DROP_GIANT(); 254 mtx_assert(mtx, MA_OWNED | MA_NOTRECURSED); 255 WITNESS_SAVE(&mtx->lock_object, mtx); 256 mtx_unlock_spin(mtx); 257 258 /* 259 * We put ourselves on the sleep queue and start our timeout. 260 */ 261 sleepq_add(ident, &mtx->lock_object, wmesg, SLEEPQ_SLEEP, 0); 262 if (sbt != 0) 263 sleepq_set_timeout_sbt(ident, sbt, pr, flags); 264 265 /* 266 * Can't call ktrace with any spin locks held so it can lock the 267 * ktrace_mtx lock, and WITNESS_WARN considers it an error to hold 268 * any spin lock. Thus, we have to drop the sleepq spin lock while 269 * we handle those requests. This is safe since we have placed our 270 * thread on the sleep queue already. 271 */ 272 #ifdef KTRACE 273 if (KTRPOINT(td, KTR_CSW)) { 274 sleepq_release(ident); 275 ktrcsw(1, 0, wmesg); 276 sleepq_lock(ident); 277 } 278 #endif 279 #ifdef WITNESS 280 sleepq_release(ident); 281 WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, "Sleeping on \"%s\"", 282 wmesg); 283 sleepq_lock(ident); 284 #endif 285 if (sbt != 0) 286 rval = sleepq_timedwait(ident, 0); 287 else { 288 sleepq_wait(ident, 0); 289 rval = 0; 290 } 291 #ifdef KTRACE 292 if (KTRPOINT(td, KTR_CSW)) 293 ktrcsw(0, 0, wmesg); 294 #endif 295 PICKUP_GIANT(); 296 mtx_lock_spin(mtx); 297 WITNESS_RESTORE(&mtx->lock_object, mtx); 298 return (rval); 299 } 300 301 /* 302 * pause_sbt() delays the calling thread by the given signed binary 303 * time. During cold bootup, pause_sbt() uses the DELAY() function 304 * instead of the _sleep() function to do the waiting. The "sbt" 305 * argument must be greater than or equal to zero. A "sbt" value of 306 * zero is equivalent to a "sbt" value of one tick. 307 */ 308 int 309 pause_sbt(const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags) 310 { 311 KASSERT(sbt >= 0, ("pause_sbt: timeout must be >= 0")); 312 313 /* silently convert invalid timeouts */ 314 if (sbt == 0) 315 sbt = tick_sbt; 316 317 if ((cold && curthread == &thread0) || kdb_active || 318 SCHEDULER_STOPPED()) { 319 /* 320 * We delay one second at a time to avoid overflowing the 321 * system specific DELAY() function(s): 322 */ 323 while (sbt >= SBT_1S) { 324 DELAY(1000000); 325 sbt -= SBT_1S; 326 } 327 /* Do the delay remainder, if any */ 328 sbt = howmany(sbt, SBT_1US); 329 if (sbt > 0) 330 DELAY(sbt); 331 return (EWOULDBLOCK); 332 } 333 return (_sleep(&pause_wchan[curcpu], NULL, 334 (flags & C_CATCH) ? PCATCH : 0, wmesg, sbt, pr, flags)); 335 } 336 337 /* 338 * Potentially release the last reference for refcount. Check for 339 * unlikely conditions and signal the caller as to whether it was 340 * the final ref. 341 */ 342 bool 343 refcount_release_last(volatile u_int *count, u_int n, u_int old) 344 { 345 u_int waiter; 346 347 waiter = old & REFCOUNT_WAITER; 348 old = REFCOUNT_COUNT(old); 349 if (__predict_false(n > old || REFCOUNT_SATURATED(old))) { 350 /* 351 * Avoid multiple destructor invocations if underflow occurred. 352 * This is not perfect since the memory backing the containing 353 * object may already have been reallocated. 354 */ 355 _refcount_update_saturated(count); 356 return (false); 357 } 358 359 /* 360 * Attempt to atomically clear the waiter bit. Wakeup waiters 361 * if we are successful. 362 */ 363 if (waiter != 0 && atomic_cmpset_int(count, REFCOUNT_WAITER, 0)) 364 wakeup(__DEVOLATILE(u_int *, count)); 365 366 /* 367 * Last reference. Signal the user to call the destructor. 368 * 369 * Ensure that the destructor sees all updates. The fence_rel 370 * at the start of refcount_releasen synchronizes with this fence. 371 */ 372 atomic_thread_fence_acq(); 373 return (true); 374 } 375 376 /* 377 * Wait for a refcount wakeup. This does not guarantee that the ref is still 378 * zero on return and may be subject to transient wakeups. Callers wanting 379 * a precise answer should use refcount_wait(). 380 */ 381 void 382 refcount_sleep(volatile u_int *count, const char *wmesg, int pri) 383 { 384 void *wchan; 385 u_int old; 386 387 if (REFCOUNT_COUNT(*count) == 0) 388 return; 389 wchan = __DEVOLATILE(void *, count); 390 sleepq_lock(wchan); 391 old = *count; 392 for (;;) { 393 if (REFCOUNT_COUNT(old) == 0) { 394 sleepq_release(wchan); 395 return; 396 } 397 if (old & REFCOUNT_WAITER) 398 break; 399 if (atomic_fcmpset_int(count, &old, old | REFCOUNT_WAITER)) 400 break; 401 } 402 sleepq_add(wchan, NULL, wmesg, 0, 0); 403 sleepq_wait(wchan, pri); 404 } 405 406 /* 407 * Make all threads sleeping on the specified identifier runnable. 408 */ 409 void 410 wakeup(void *ident) 411 { 412 int wakeup_swapper; 413 414 sleepq_lock(ident); 415 wakeup_swapper = sleepq_broadcast(ident, SLEEPQ_SLEEP, 0, 0); 416 sleepq_release(ident); 417 if (wakeup_swapper) { 418 KASSERT(ident != &proc0, 419 ("wakeup and wakeup_swapper and proc0")); 420 kick_proc0(); 421 } 422 } 423 424 /* 425 * Make a thread sleeping on the specified identifier runnable. 426 * May wake more than one thread if a target thread is currently 427 * swapped out. 428 */ 429 void 430 wakeup_one(void *ident) 431 { 432 int wakeup_swapper; 433 434 sleepq_lock(ident); 435 wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP, 0, 0); 436 sleepq_release(ident); 437 if (wakeup_swapper) 438 kick_proc0(); 439 } 440 441 void 442 wakeup_any(void *ident) 443 { 444 int wakeup_swapper; 445 446 sleepq_lock(ident); 447 wakeup_swapper = sleepq_signal(ident, SLEEPQ_SLEEP | SLEEPQ_UNFAIR, 448 0, 0); 449 sleepq_release(ident); 450 if (wakeup_swapper) 451 kick_proc0(); 452 } 453 454 static void 455 kdb_switch(void) 456 { 457 thread_unlock(curthread); 458 kdb_backtrace(); 459 kdb_reenter(); 460 panic("%s: did not reenter debugger", __func__); 461 } 462 463 /* 464 * The machine independent parts of context switching. 465 */ 466 void 467 mi_switch(int flags, struct thread *newtd) 468 { 469 uint64_t runtime, new_switchtime; 470 struct thread *td; 471 472 td = curthread; /* XXX */ 473 THREAD_LOCK_ASSERT(td, MA_OWNED | MA_NOTRECURSED); 474 KASSERT(!TD_ON_RUNQ(td), ("mi_switch: called by old code")); 475 #ifdef INVARIANTS 476 if (!TD_ON_LOCK(td) && !TD_IS_RUNNING(td)) 477 mtx_assert(&Giant, MA_NOTOWNED); 478 #endif 479 KASSERT(td->td_critnest == 1 || panicstr, 480 ("mi_switch: switch in a critical section")); 481 KASSERT((flags & (SW_INVOL | SW_VOL)) != 0, 482 ("mi_switch: switch must be voluntary or involuntary")); 483 KASSERT(newtd != curthread, ("mi_switch: preempting back to ourself")); 484 485 /* 486 * Don't perform context switches from the debugger. 487 */ 488 if (kdb_active) 489 kdb_switch(); 490 if (SCHEDULER_STOPPED_TD(td)) 491 return; 492 if (flags & SW_VOL) { 493 td->td_ru.ru_nvcsw++; 494 td->td_swvoltick = ticks; 495 } else { 496 td->td_ru.ru_nivcsw++; 497 td->td_swinvoltick = ticks; 498 } 499 #ifdef SCHED_STATS 500 SCHED_STAT_INC(sched_switch_stats[flags & SW_TYPE_MASK]); 501 #endif 502 /* 503 * Compute the amount of time during which the current 504 * thread was running, and add that to its total so far. 505 */ 506 new_switchtime = cpu_ticks(); 507 runtime = new_switchtime - PCPU_GET(switchtime); 508 td->td_runtime += runtime; 509 td->td_incruntime += runtime; 510 PCPU_SET(switchtime, new_switchtime); 511 td->td_generation++; /* bump preempt-detect counter */ 512 VM_CNT_INC(v_swtch); 513 PCPU_SET(switchticks, ticks); 514 CTR4(KTR_PROC, "mi_switch: old thread %ld (td_sched %p, pid %ld, %s)", 515 td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name); 516 #ifdef KDTRACE_HOOKS 517 if (SDT_PROBES_ENABLED() && 518 ((flags & SW_PREEMPT) != 0 || ((flags & SW_INVOL) != 0 && 519 (flags & SW_TYPE_MASK) == SWT_NEEDRESCHED))) 520 SDT_PROBE0(sched, , , preempt); 521 #endif 522 sched_switch(td, newtd, flags); 523 CTR4(KTR_PROC, "mi_switch: new thread %ld (td_sched %p, pid %ld, %s)", 524 td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name); 525 526 /* 527 * If the last thread was exiting, finish cleaning it up. 528 */ 529 if ((td = PCPU_GET(deadthread))) { 530 PCPU_SET(deadthread, NULL); 531 thread_stash(td); 532 } 533 } 534 535 /* 536 * Change thread state to be runnable, placing it on the run queue if 537 * it is in memory. If it is swapped out, return true so our caller 538 * will know to awaken the swapper. 539 */ 540 int 541 setrunnable(struct thread *td) 542 { 543 544 THREAD_LOCK_ASSERT(td, MA_OWNED); 545 KASSERT(td->td_proc->p_state != PRS_ZOMBIE, 546 ("setrunnable: pid %d is a zombie", td->td_proc->p_pid)); 547 switch (td->td_state) { 548 case TDS_RUNNING: 549 case TDS_RUNQ: 550 return (0); 551 case TDS_INHIBITED: 552 /* 553 * If we are only inhibited because we are swapped out 554 * then arange to swap in this process. Otherwise just return. 555 */ 556 if (td->td_inhibitors != TDI_SWAPPED) 557 return (0); 558 /* FALLTHROUGH */ 559 case TDS_CAN_RUN: 560 break; 561 default: 562 printf("state is 0x%x", td->td_state); 563 panic("setrunnable(2)"); 564 } 565 if ((td->td_flags & TDF_INMEM) == 0) { 566 if ((td->td_flags & TDF_SWAPINREQ) == 0) { 567 td->td_flags |= TDF_SWAPINREQ; 568 return (1); 569 } 570 } else 571 sched_wakeup(td); 572 return (0); 573 } 574 575 /* 576 * Compute a tenex style load average of a quantity on 577 * 1, 5 and 15 minute intervals. 578 */ 579 static void 580 loadav(void *arg) 581 { 582 int i, nrun; 583 struct loadavg *avg; 584 585 nrun = sched_load(); 586 avg = &averunnable; 587 588 for (i = 0; i < 3; i++) 589 avg->ldavg[i] = (cexp[i] * avg->ldavg[i] + 590 nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT; 591 592 /* 593 * Schedule the next update to occur after 5 seconds, but add a 594 * random variation to avoid synchronisation with processes that 595 * run at regular intervals. 596 */ 597 callout_reset_sbt(&loadav_callout, 598 SBT_1US * (4000000 + (int)(random() % 2000001)), SBT_1US, 599 loadav, NULL, C_DIRECT_EXEC | C_PREL(32)); 600 } 601 602 /* ARGSUSED */ 603 static void 604 synch_setup(void *dummy) 605 { 606 callout_init(&loadav_callout, 1); 607 608 /* Kick off timeout driven events by calling first time. */ 609 loadav(NULL); 610 } 611 612 int 613 should_yield(void) 614 { 615 616 return ((u_int)ticks - (u_int)curthread->td_swvoltick >= hogticks); 617 } 618 619 void 620 maybe_yield(void) 621 { 622 623 if (should_yield()) 624 kern_yield(PRI_USER); 625 } 626 627 void 628 kern_yield(int prio) 629 { 630 struct thread *td; 631 632 td = curthread; 633 DROP_GIANT(); 634 thread_lock(td); 635 if (prio == PRI_USER) 636 prio = td->td_user_pri; 637 if (prio >= 0) 638 sched_prio(td, prio); 639 mi_switch(SW_VOL | SWT_RELINQUISH, NULL); 640 thread_unlock(td); 641 PICKUP_GIANT(); 642 } 643 644 /* 645 * General purpose yield system call. 646 */ 647 int 648 sys_yield(struct thread *td, struct yield_args *uap) 649 { 650 651 thread_lock(td); 652 if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE) 653 sched_prio(td, PRI_MAX_TIMESHARE); 654 mi_switch(SW_VOL | SWT_RELINQUISH, NULL); 655 thread_unlock(td); 656 td->td_retval[0] = 0; 657 return (0); 658 } 659