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_switch.c
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1 /*- 2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD 3 * 4 * Copyright (c) 2001 Jake Burkholder <jake@FreeBSD.org> 5 * All rights reserved. 6 * 7 * Redistribution and use in source and binary forms, with or without 8 * modification, are permitted provided that the following conditions 9 * are met: 10 * 1. Redistributions of source code must retain the above copyright 11 * notice, this list of conditions and the following disclaimer. 12 * 2. Redistributions in binary form must reproduce the above copyright 13 * notice, this list of conditions and the following disclaimer in the 14 * documentation and/or other materials provided with the distribution. 15 * 16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 * SUCH DAMAGE. 27 */ 28 29 #include <sys/cdefs.h> 30 __FBSDID("$FreeBSD$"); 31 32 #include "opt_sched.h" 33 34 #include <sys/param.h> 35 #include <sys/systm.h> 36 #include <sys/kdb.h> 37 #include <sys/kernel.h> 38 #include <sys/ktr.h> 39 #include <sys/lock.h> 40 #include <sys/mutex.h> 41 #include <sys/proc.h> 42 #include <sys/queue.h> 43 #include <sys/sched.h> 44 #include <sys/smp.h> 45 #include <sys/sysctl.h> 46 47 #include <machine/cpu.h> 48 49 /* Uncomment this to enable logging of critical_enter/exit. */ 50 #if 0 51 #define KTR_CRITICAL KTR_SCHED 52 #else 53 #define KTR_CRITICAL 0 54 #endif 55 56 #ifdef FULL_PREEMPTION 57 #ifndef PREEMPTION 58 #error "The FULL_PREEMPTION option requires the PREEMPTION option" 59 #endif 60 #endif 61 62 CTASSERT((RQB_BPW * RQB_LEN) == RQ_NQS); 63 64 /* 65 * kern.sched.preemption allows user space to determine if preemption support 66 * is compiled in or not. It is not currently a boot or runtime flag that 67 * can be changed. 68 */ 69 #ifdef PREEMPTION 70 static int kern_sched_preemption = 1; 71 #else 72 static int kern_sched_preemption = 0; 73 #endif 74 SYSCTL_INT(_kern_sched, OID_AUTO, preemption, CTLFLAG_RD, 75 &kern_sched_preemption, 0, "Kernel preemption enabled"); 76 77 /* 78 * Support for scheduler stats exported via kern.sched.stats. All stats may 79 * be reset with kern.sched.stats.reset = 1. Stats may be defined elsewhere 80 * with SCHED_STAT_DEFINE(). 81 */ 82 #ifdef SCHED_STATS 83 SYSCTL_NODE(_kern_sched, OID_AUTO, stats, CTLFLAG_RW | CTLFLAG_MPSAFE, 0, 84 "switch stats"); 85 86 /* Switch reasons from mi_switch(). */ 87 DPCPU_DEFINE(long, sched_switch_stats[SWT_COUNT]); 88 SCHED_STAT_DEFINE_VAR(uncategorized, 89 &DPCPU_NAME(sched_switch_stats[SWT_NONE]), ""); 90 SCHED_STAT_DEFINE_VAR(preempt, 91 &DPCPU_NAME(sched_switch_stats[SWT_PREEMPT]), ""); 92 SCHED_STAT_DEFINE_VAR(owepreempt, 93 &DPCPU_NAME(sched_switch_stats[SWT_OWEPREEMPT]), ""); 94 SCHED_STAT_DEFINE_VAR(turnstile, 95 &DPCPU_NAME(sched_switch_stats[SWT_TURNSTILE]), ""); 96 SCHED_STAT_DEFINE_VAR(sleepq, 97 &DPCPU_NAME(sched_switch_stats[SWT_SLEEPQ]), ""); 98 SCHED_STAT_DEFINE_VAR(sleepqtimo, 99 &DPCPU_NAME(sched_switch_stats[SWT_SLEEPQTIMO]), ""); 100 SCHED_STAT_DEFINE_VAR(relinquish, 101 &DPCPU_NAME(sched_switch_stats[SWT_RELINQUISH]), ""); 102 SCHED_STAT_DEFINE_VAR(needresched, 103 &DPCPU_NAME(sched_switch_stats[SWT_NEEDRESCHED]), ""); 104 SCHED_STAT_DEFINE_VAR(idle, 105 &DPCPU_NAME(sched_switch_stats[SWT_IDLE]), ""); 106 SCHED_STAT_DEFINE_VAR(iwait, 107 &DPCPU_NAME(sched_switch_stats[SWT_IWAIT]), ""); 108 SCHED_STAT_DEFINE_VAR(suspend, 109 &DPCPU_NAME(sched_switch_stats[SWT_SUSPEND]), ""); 110 SCHED_STAT_DEFINE_VAR(remotepreempt, 111 &DPCPU_NAME(sched_switch_stats[SWT_REMOTEPREEMPT]), ""); 112 SCHED_STAT_DEFINE_VAR(remotewakeidle, 113 &DPCPU_NAME(sched_switch_stats[SWT_REMOTEWAKEIDLE]), ""); 114 115 static int 116 sysctl_stats_reset(SYSCTL_HANDLER_ARGS) 117 { 118 struct sysctl_oid *p; 119 uintptr_t counter; 120 int error; 121 int val; 122 int i; 123 124 val = 0; 125 error = sysctl_handle_int(oidp, &val, 0, req); 126 if (error != 0 || req->newptr == NULL) 127 return (error); 128 if (val == 0) 129 return (0); 130 /* 131 * Traverse the list of children of _kern_sched_stats and reset each 132 * to 0. Skip the reset entry. 133 */ 134 SLIST_FOREACH(p, oidp->oid_parent, oid_link) { 135 if (p == oidp || p->oid_arg1 == NULL) 136 continue; 137 counter = (uintptr_t)p->oid_arg1; 138 CPU_FOREACH(i) { 139 *(long *)(dpcpu_off[i] + counter) = 0; 140 } 141 } 142 return (0); 143 } 144 145 SYSCTL_PROC(_kern_sched_stats, OID_AUTO, reset, 146 CTLTYPE_INT | CTLFLAG_WR | CTLFLAG_NEEDGIANT, NULL, 0, 147 sysctl_stats_reset, "I", 148 "Reset scheduler statistics"); 149 #endif 150 151 /************************************************************************ 152 * Functions that manipulate runnability from a thread perspective. * 153 ************************************************************************/ 154 /* 155 * Select the thread that will be run next. 156 */ 157 158 static __noinline struct thread * 159 choosethread_panic(struct thread *td) 160 { 161 162 /* 163 * If we are in panic, only allow system threads, 164 * plus the one we are running in, to be run. 165 */ 166 retry: 167 if (((td->td_proc->p_flag & P_SYSTEM) == 0 && 168 (td->td_flags & TDF_INPANIC) == 0)) { 169 /* note that it is no longer on the run queue */ 170 TD_SET_CAN_RUN(td); 171 td = sched_choose(); 172 goto retry; 173 } 174 175 TD_SET_RUNNING(td); 176 return (td); 177 } 178 179 struct thread * 180 choosethread(void) 181 { 182 struct thread *td; 183 184 td = sched_choose(); 185 186 if (KERNEL_PANICKED()) 187 return (choosethread_panic(td)); 188 189 TD_SET_RUNNING(td); 190 return (td); 191 } 192 193 /* 194 * Kernel thread preemption implementation. Critical sections mark 195 * regions of code in which preemptions are not allowed. 196 * 197 * It might seem a good idea to inline critical_enter() but, in order 198 * to prevent instructions reordering by the compiler, a __compiler_membar() 199 * would have to be used here (the same as sched_pin()). The performance 200 * penalty imposed by the membar could, then, produce slower code than 201 * the function call itself, for most cases. 202 */ 203 void 204 critical_enter_KBI(void) 205 { 206 #ifdef KTR 207 struct thread *td = curthread; 208 #endif 209 critical_enter(); 210 CTR4(KTR_CRITICAL, "critical_enter by thread %p (%ld, %s) to %d", td, 211 (long)td->td_proc->p_pid, td->td_name, td->td_critnest); 212 } 213 214 void __noinline 215 critical_exit_preempt(void) 216 { 217 struct thread *td; 218 int flags; 219 220 /* 221 * If td_critnest is 0, it is possible that we are going to get 222 * preempted again before reaching the code below. This happens 223 * rarely and is harmless. However, this means td_owepreempt may 224 * now be unset. 225 */ 226 td = curthread; 227 if (td->td_critnest != 0) 228 return; 229 if (kdb_active) 230 return; 231 232 /* 233 * Microoptimization: we committed to switch, 234 * disable preemption in interrupt handlers 235 * while spinning for the thread lock. 236 */ 237 td->td_critnest = 1; 238 thread_lock(td); 239 td->td_critnest--; 240 flags = SW_INVOL | SW_PREEMPT; 241 if (TD_IS_IDLETHREAD(td)) 242 flags |= SWT_IDLE; 243 else 244 flags |= SWT_OWEPREEMPT; 245 mi_switch(flags); 246 } 247 248 void 249 critical_exit_KBI(void) 250 { 251 #ifdef KTR 252 struct thread *td = curthread; 253 #endif 254 critical_exit(); 255 CTR4(KTR_CRITICAL, "critical_exit by thread %p (%ld, %s) to %d", td, 256 (long)td->td_proc->p_pid, td->td_name, td->td_critnest); 257 } 258 259 /************************************************************************ 260 * SYSTEM RUN QUEUE manipulations and tests * 261 ************************************************************************/ 262 /* 263 * Initialize a run structure. 264 */ 265 void 266 runq_init(struct runq *rq) 267 { 268 int i; 269 270 bzero(rq, sizeof *rq); 271 for (i = 0; i < RQ_NQS; i++) 272 TAILQ_INIT(&rq->rq_queues[i]); 273 } 274 275 /* 276 * Clear the status bit of the queue corresponding to priority level pri, 277 * indicating that it is empty. 278 */ 279 static __inline void 280 runq_clrbit(struct runq *rq, int pri) 281 { 282 struct rqbits *rqb; 283 284 rqb = &rq->rq_status; 285 CTR4(KTR_RUNQ, "runq_clrbit: bits=%#x %#x bit=%#x word=%d", 286 rqb->rqb_bits[RQB_WORD(pri)], 287 rqb->rqb_bits[RQB_WORD(pri)] & ~RQB_BIT(pri), 288 RQB_BIT(pri), RQB_WORD(pri)); 289 rqb->rqb_bits[RQB_WORD(pri)] &= ~RQB_BIT(pri); 290 } 291 292 /* 293 * Find the index of the first non-empty run queue. This is done by 294 * scanning the status bits, a set bit indicates a non-empty queue. 295 */ 296 static __inline int 297 runq_findbit(struct runq *rq) 298 { 299 struct rqbits *rqb; 300 int pri; 301 int i; 302 303 rqb = &rq->rq_status; 304 for (i = 0; i < RQB_LEN; i++) 305 if (rqb->rqb_bits[i]) { 306 pri = RQB_FFS(rqb->rqb_bits[i]) + (i << RQB_L2BPW); 307 CTR3(KTR_RUNQ, "runq_findbit: bits=%#x i=%d pri=%d", 308 rqb->rqb_bits[i], i, pri); 309 return (pri); 310 } 311 312 return (-1); 313 } 314 315 static __inline int 316 runq_findbit_from(struct runq *rq, u_char pri) 317 { 318 struct rqbits *rqb; 319 rqb_word_t mask; 320 int i; 321 322 /* 323 * Set the mask for the first word so we ignore priorities before 'pri'. 324 */ 325 mask = (rqb_word_t)-1 << (pri & (RQB_BPW - 1)); 326 rqb = &rq->rq_status; 327 again: 328 for (i = RQB_WORD(pri); i < RQB_LEN; mask = -1, i++) { 329 mask = rqb->rqb_bits[i] & mask; 330 if (mask == 0) 331 continue; 332 pri = RQB_FFS(mask) + (i << RQB_L2BPW); 333 CTR3(KTR_RUNQ, "runq_findbit_from: bits=%#x i=%d pri=%d", 334 mask, i, pri); 335 return (pri); 336 } 337 if (pri == 0) 338 return (-1); 339 /* 340 * Wrap back around to the beginning of the list just once so we 341 * scan the whole thing. 342 */ 343 pri = 0; 344 goto again; 345 } 346 347 /* 348 * Set the status bit of the queue corresponding to priority level pri, 349 * indicating that it is non-empty. 350 */ 351 static __inline void 352 runq_setbit(struct runq *rq, int pri) 353 { 354 struct rqbits *rqb; 355 356 rqb = &rq->rq_status; 357 CTR4(KTR_RUNQ, "runq_setbit: bits=%#x %#x bit=%#x word=%d", 358 rqb->rqb_bits[RQB_WORD(pri)], 359 rqb->rqb_bits[RQB_WORD(pri)] | RQB_BIT(pri), 360 RQB_BIT(pri), RQB_WORD(pri)); 361 rqb->rqb_bits[RQB_WORD(pri)] |= RQB_BIT(pri); 362 } 363 364 /* 365 * Add the thread to the queue specified by its priority, and set the 366 * corresponding status bit. 367 */ 368 void 369 runq_add(struct runq *rq, struct thread *td, int flags) 370 { 371 struct rqhead *rqh; 372 int pri; 373 374 pri = td->td_priority / RQ_PPQ; 375 td->td_rqindex = pri; 376 runq_setbit(rq, pri); 377 rqh = &rq->rq_queues[pri]; 378 CTR4(KTR_RUNQ, "runq_add: td=%p pri=%d %d rqh=%p", 379 td, td->td_priority, pri, rqh); 380 if (flags & SRQ_PREEMPTED) { 381 TAILQ_INSERT_HEAD(rqh, td, td_runq); 382 } else { 383 TAILQ_INSERT_TAIL(rqh, td, td_runq); 384 } 385 } 386 387 void 388 runq_add_pri(struct runq *rq, struct thread *td, u_char pri, int flags) 389 { 390 struct rqhead *rqh; 391 392 KASSERT(pri < RQ_NQS, ("runq_add_pri: %d out of range", pri)); 393 td->td_rqindex = pri; 394 runq_setbit(rq, pri); 395 rqh = &rq->rq_queues[pri]; 396 CTR4(KTR_RUNQ, "runq_add_pri: td=%p pri=%d idx=%d rqh=%p", 397 td, td->td_priority, pri, rqh); 398 if (flags & SRQ_PREEMPTED) { 399 TAILQ_INSERT_HEAD(rqh, td, td_runq); 400 } else { 401 TAILQ_INSERT_TAIL(rqh, td, td_runq); 402 } 403 } 404 /* 405 * Return true if there are runnable processes of any priority on the run 406 * queue, false otherwise. Has no side effects, does not modify the run 407 * queue structure. 408 */ 409 int 410 runq_check(struct runq *rq) 411 { 412 struct rqbits *rqb; 413 int i; 414 415 rqb = &rq->rq_status; 416 for (i = 0; i < RQB_LEN; i++) 417 if (rqb->rqb_bits[i]) { 418 CTR2(KTR_RUNQ, "runq_check: bits=%#x i=%d", 419 rqb->rqb_bits[i], i); 420 return (1); 421 } 422 CTR0(KTR_RUNQ, "runq_check: empty"); 423 424 return (0); 425 } 426 427 /* 428 * Find the highest priority process on the run queue. 429 */ 430 struct thread * 431 runq_choose_fuzz(struct runq *rq, int fuzz) 432 { 433 struct rqhead *rqh; 434 struct thread *td; 435 int pri; 436 437 while ((pri = runq_findbit(rq)) != -1) { 438 rqh = &rq->rq_queues[pri]; 439 /* fuzz == 1 is normal.. 0 or less are ignored */ 440 if (fuzz > 1) { 441 /* 442 * In the first couple of entries, check if 443 * there is one for our CPU as a preference. 444 */ 445 int count = fuzz; 446 int cpu = PCPU_GET(cpuid); 447 struct thread *td2; 448 td2 = td = TAILQ_FIRST(rqh); 449 450 while (count-- && td2) { 451 if (td2->td_lastcpu == cpu) { 452 td = td2; 453 break; 454 } 455 td2 = TAILQ_NEXT(td2, td_runq); 456 } 457 } else 458 td = TAILQ_FIRST(rqh); 459 KASSERT(td != NULL, ("runq_choose_fuzz: no proc on busy queue")); 460 CTR3(KTR_RUNQ, 461 "runq_choose_fuzz: pri=%d thread=%p rqh=%p", pri, td, rqh); 462 return (td); 463 } 464 CTR1(KTR_RUNQ, "runq_choose_fuzz: idleproc pri=%d", pri); 465 466 return (NULL); 467 } 468 469 /* 470 * Find the highest priority process on the run queue. 471 */ 472 struct thread * 473 runq_choose(struct runq *rq) 474 { 475 struct rqhead *rqh; 476 struct thread *td; 477 int pri; 478 479 while ((pri = runq_findbit(rq)) != -1) { 480 rqh = &rq->rq_queues[pri]; 481 td = TAILQ_FIRST(rqh); 482 KASSERT(td != NULL, ("runq_choose: no thread on busy queue")); 483 CTR3(KTR_RUNQ, 484 "runq_choose: pri=%d thread=%p rqh=%p", pri, td, rqh); 485 return (td); 486 } 487 CTR1(KTR_RUNQ, "runq_choose: idlethread pri=%d", pri); 488 489 return (NULL); 490 } 491 492 struct thread * 493 runq_choose_from(struct runq *rq, u_char idx) 494 { 495 struct rqhead *rqh; 496 struct thread *td; 497 int pri; 498 499 if ((pri = runq_findbit_from(rq, idx)) != -1) { 500 rqh = &rq->rq_queues[pri]; 501 td = TAILQ_FIRST(rqh); 502 KASSERT(td != NULL, ("runq_choose: no thread on busy queue")); 503 CTR4(KTR_RUNQ, 504 "runq_choose_from: pri=%d thread=%p idx=%d rqh=%p", 505 pri, td, td->td_rqindex, rqh); 506 return (td); 507 } 508 CTR1(KTR_RUNQ, "runq_choose_from: idlethread pri=%d", pri); 509 510 return (NULL); 511 } 512 /* 513 * Remove the thread from the queue specified by its priority, and clear the 514 * corresponding status bit if the queue becomes empty. 515 * Caller must set state afterwards. 516 */ 517 void 518 runq_remove(struct runq *rq, struct thread *td) 519 { 520 521 runq_remove_idx(rq, td, NULL); 522 } 523 524 void 525 runq_remove_idx(struct runq *rq, struct thread *td, u_char *idx) 526 { 527 struct rqhead *rqh; 528 u_char pri; 529 530 KASSERT(td->td_flags & TDF_INMEM, 531 ("runq_remove_idx: thread swapped out")); 532 pri = td->td_rqindex; 533 KASSERT(pri < RQ_NQS, ("runq_remove_idx: Invalid index %d\n", pri)); 534 rqh = &rq->rq_queues[pri]; 535 CTR4(KTR_RUNQ, "runq_remove_idx: td=%p, pri=%d %d rqh=%p", 536 td, td->td_priority, pri, rqh); 537 TAILQ_REMOVE(rqh, td, td_runq); 538 if (TAILQ_EMPTY(rqh)) { 539 CTR0(KTR_RUNQ, "runq_remove_idx: empty"); 540 runq_clrbit(rq, pri); 541 if (idx != NULL && *idx == pri) 542 *idx = (pri + 1) % RQ_NQS; 543 } 544 }
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