source: mainline/src/proc/scheduler.c@ a6f8899

lfn serial ticket/834-toolchain-update topic/msim-upgrade topic/simplify-dev-export
Last change on this file since a6f8899 was a8f9a82, checked in by Jakub Jermar <jakub@…>, 20 years ago

Add comment to scheduler.c explaining the reason behind saving CPU priority of preempted thread.

Remove two unnecessary cpu_priority_high() calls.
  • Property mode set to 100644
File size: 12.5 KB
Line 
1/*
2 * Copyright (C) 2001-2004 Jakub Jermar
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 *
9 * - Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * - Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
14 * - The name of the author may not be used to endorse or promote products
15 * derived from this software without specific prior written permission.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
18 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
19 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
20 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
21 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
22 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
26 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 */
28
29#include <proc/scheduler.h>
30#include <proc/thread.h>
31#include <proc/task.h>
32#include <cpu.h>
33#include <mm/vm.h>
34#include <config.h>
35#include <context.h>
36#include <func.h>
37#include <arch.h>
38#include <arch/asm.h>
39#include <list.h>
40#include <panic.h>
41#include <typedefs.h>
42#include <mm/page.h>
43#include <synch/spinlock.h>
44#include <arch/faddr.h>
45#include <arch/atomic.h>
46
47volatile int nrdy;
48
49
50/** Take actions before new thread runs
51 *
52 * Perform actions that need to be
53 * taken before the newly selected
54 * tread is passed control.
55 *
56 */
57void before_thread_runs(void)
58{
59 before_thread_runs_arch();
60 fpu_context_restore(&(THREAD->saved_fpu_context));
61}
62
63
64/** Initialize scheduler
65 *
66 * Initialize kernel scheduler.
67 *
68 */
69void scheduler_init(void)
70{
71}
72
73
74/** Get thread to be scheduled
75 *
76 * Get the optimal thread to be scheduled
77 * according to thread accounting and scheduler
78 * policy.
79 *
80 * @return Thread to be scheduled.
81 *
82 */
83struct thread *find_best_thread(void)
84{
85 thread_t *t;
86 runq_t *r;
87 int i, n;
88
89loop:
90 cpu_priority_high();
91
92 spinlock_lock(&CPU->lock);
93 n = CPU->nrdy;
94 spinlock_unlock(&CPU->lock);
95
96 cpu_priority_low();
97
98 if (n == 0) {
99 #ifdef __SMP__
100 /*
101 * If the load balancing thread is not running, wake it up and
102 * set CPU-private flag that the kcpulb has been started.
103 */
104 if (test_and_set(&CPU->kcpulbstarted) == 0) {
105 waitq_wakeup(&CPU->kcpulb_wq, 0);
106 goto loop;
107 }
108 #endif /* __SMP__ */
109
110 /*
111 * For there was nothing to run, the CPU goes to sleep
112 * until a hardware interrupt or an IPI comes.
113 * This improves energy saving and hyperthreading.
114 * On the other hand, several hardware interrupts can be ignored.
115 */
116 cpu_sleep();
117 goto loop;
118 }
119
120 cpu_priority_high();
121
122 i = 0;
123retry:
124 for (; i<RQ_COUNT; i++) {
125 r = &CPU->rq[i];
126 spinlock_lock(&r->lock);
127 if (r->n == 0) {
128 /*
129 * If this queue is empty, try a lower-priority queue.
130 */
131 spinlock_unlock(&r->lock);
132 continue;
133 }
134
135 /* avoid deadlock with relink_rq() */
136 if (!spinlock_trylock(&CPU->lock)) {
137 /*
138 * Unlock r and try again.
139 */
140 spinlock_unlock(&r->lock);
141 goto retry;
142 }
143 CPU->nrdy--;
144 spinlock_unlock(&CPU->lock);
145
146 atomic_dec(&nrdy);
147 r->n--;
148
149 /*
150 * Take the first thread from the queue.
151 */
152 t = list_get_instance(r->rq_head.next, thread_t, rq_link);
153 list_remove(&t->rq_link);
154
155 spinlock_unlock(&r->lock);
156
157 spinlock_lock(&t->lock);
158 t->cpu = CPU;
159
160 t->ticks = us2ticks((i+1)*10000);
161 t->pri = i; /* eventually correct rq index */
162
163 /*
164 * Clear the X_STOLEN flag so that t can be migrated when load balancing needs emerge.
165 */
166 t->flags &= ~X_STOLEN;
167 spinlock_unlock(&t->lock);
168
169 return t;
170 }
171 goto loop;
172
173}
174
175
176/** Prevent rq starvation
177 *
178 * Prevent low priority threads from starving in rq's.
179 *
180 * When the function decides to relink rq's, it reconnects
181 * respective pointers so that in result threads with 'pri'
182 * greater or equal 'start' are moved to a higher-priority queue.
183 *
184 * @param start Threshold priority.
185 *
186 */
187void relink_rq(int start)
188{
189 link_t head;
190 runq_t *r;
191 int i, n;
192
193 list_initialize(&head);
194 spinlock_lock(&CPU->lock);
195 if (CPU->needs_relink > NEEDS_RELINK_MAX) {
196 for (i = start; i<RQ_COUNT-1; i++) {
197 /* remember and empty rq[i + 1] */
198 r = &CPU->rq[i + 1];
199 spinlock_lock(&r->lock);
200 list_concat(&head, &r->rq_head);
201 n = r->n;
202 r->n = 0;
203 spinlock_unlock(&r->lock);
204
205 /* append rq[i + 1] to rq[i] */
206 r = &CPU->rq[i];
207 spinlock_lock(&r->lock);
208 list_concat(&r->rq_head, &head);
209 r->n += n;
210 spinlock_unlock(&r->lock);
211 }
212 CPU->needs_relink = 0;
213 }
214 spinlock_unlock(&CPU->lock);
215
216}
217
218
219/** The scheduler
220 *
221 * The thread scheduling procedure.
222 *
223 */
224void scheduler(void)
225{
226 volatile pri_t pri;
227
228 pri = cpu_priority_high();
229
230 if (haltstate)
231 halt();
232
233 if (THREAD) {
234 spinlock_lock(&THREAD->lock);
235 fpu_context_save(&(THREAD->saved_fpu_context));
236 if (!context_save(&THREAD->saved_context)) {
237 /*
238 * This is the place where threads leave scheduler();
239 */
240 before_thread_runs();
241 spinlock_unlock(&THREAD->lock);
242 cpu_priority_restore(THREAD->saved_context.pri);
243 return;
244 }
245
246 /*
247 * CPU priority of preempted thread is recorded here
248 * to facilitate scheduler() invocations from
249 * cpu_priority_high()'ed code (e.g. waitq_sleep_timeout()).
250 */
251 THREAD->saved_context.pri = pri;
252 }
253
254 /*
255 * We may not keep the old stack.
256 * Reason: If we kept the old stack and got blocked, for instance, in
257 * find_best_thread(), the old thread could get rescheduled by another
258 * CPU and overwrite the part of its own stack that was also used by
259 * the scheduler on this CPU.
260 *
261 * Moreover, we have to bypass the compiler-generated POP sequence
262 * which is fooled by SP being set to the very top of the stack.
263 * Therefore the scheduler() function continues in
264 * scheduler_separated_stack().
265 */
266 context_save(&CPU->saved_context);
267 context_set(&CPU->saved_context, FADDR(scheduler_separated_stack), CPU->stack, CPU_STACK_SIZE);
268 context_restore(&CPU->saved_context);
269 /* not reached */
270}
271
272
273/** Scheduler stack switch wrapper
274 *
275 * Second part of the scheduler() function
276 * using new stack. Handling the actual context
277 * switch to a new thread.
278 *
279 */
280void scheduler_separated_stack(void)
281{
282 int priority;
283
284 if (THREAD) {
285 switch (THREAD->state) {
286 case Running:
287 THREAD->state = Ready;
288 spinlock_unlock(&THREAD->lock);
289 thread_ready(THREAD);
290 break;
291
292 case Exiting:
293 frame_free((__address) THREAD->kstack);
294 if (THREAD->ustack) {
295 frame_free((__address) THREAD->ustack);
296 }
297
298 /*
299 * Detach from the containing task.
300 */
301 spinlock_lock(&TASK->lock);
302 list_remove(&THREAD->th_link);
303 spinlock_unlock(&TASK->lock);
304
305 spinlock_unlock(&THREAD->lock);
306
307 spinlock_lock(&threads_lock);
308 list_remove(&THREAD->threads_link);
309 spinlock_unlock(&threads_lock);
310
311 spinlock_lock(&CPU->lock);
312 if(CPU->fpu_owner==THREAD) CPU->fpu_owner=NULL;
313 spinlock_unlock(&CPU->lock);
314
315 free(THREAD);
316
317 break;
318
319 case Sleeping:
320 /*
321 * Prefer the thread after it's woken up.
322 */
323 THREAD->pri = -1;
324
325 /*
326 * We need to release wq->lock which we locked in waitq_sleep().
327 * Address of wq->lock is kept in THREAD->sleep_queue.
328 */
329 spinlock_unlock(&THREAD->sleep_queue->lock);
330
331 /*
332 * Check for possible requests for out-of-context invocation.
333 */
334 if (THREAD->call_me) {
335 THREAD->call_me(THREAD->call_me_with);
336 THREAD->call_me = NULL;
337 THREAD->call_me_with = NULL;
338 }
339
340 spinlock_unlock(&THREAD->lock);
341
342 break;
343
344 default:
345 /*
346 * Entering state is unexpected.
347 */
348 panic("tid%d: unexpected state %s\n", THREAD->tid, thread_states[THREAD->state]);
349 break;
350 }
351 THREAD = NULL;
352 }
353
354 THREAD = find_best_thread();
355
356 spinlock_lock(&THREAD->lock);
357 priority = THREAD->pri;
358 spinlock_unlock(&THREAD->lock);
359
360 relink_rq(priority);
361
362 spinlock_lock(&THREAD->lock);
363
364 /*
365 * If both the old and the new task are the same, lots of work is avoided.
366 */
367 if (TASK != THREAD->task) {
368 vm_t *m1 = NULL;
369 vm_t *m2;
370
371 if (TASK) {
372 spinlock_lock(&TASK->lock);
373 m1 = TASK->vm;
374 spinlock_unlock(&TASK->lock);
375 }
376
377 spinlock_lock(&THREAD->task->lock);
378 m2 = THREAD->task->vm;
379 spinlock_unlock(&THREAD->task->lock);
380
381 /*
382 * Note that it is possible for two tasks to share one vm mapping.
383 */
384 if (m1 != m2) {
385 /*
386 * Both tasks and vm mappings are different.
387 * Replace the old one with the new one.
388 */
389 vm_install(m2);
390 }
391 TASK = THREAD->task;
392 }
393
394 THREAD->state = Running;
395
396 #ifdef SCHEDULER_VERBOSE
397 printf("cpu%d: tid %d (pri=%d,ticks=%d,nrdy=%d)\n", CPU->id, THREAD->tid, THREAD->pri, THREAD->ticks, CPU->nrdy);
398 #endif
399
400 context_restore(&THREAD->saved_context);
401 /* not reached */
402}
403
404
405#ifdef __SMP__
406/** Load balancing thread
407 *
408 * SMP load balancing thread, supervising thread supplies
409 * for the CPU it's wired to.
410 *
411 * @param arg Generic thread argument (unused).
412 *
413 */
414void kcpulb(void *arg)
415{
416 thread_t *t;
417 int count, i, j, k = 0;
418 pri_t pri;
419
420loop:
421 /*
422 * Sleep until there's some work to do.
423 */
424 waitq_sleep(&CPU->kcpulb_wq);
425
426not_satisfied:
427 /*
428 * Calculate the number of threads that will be migrated/stolen from
429 * other CPU's. Note that situation can have changed between two
430 * passes. Each time get the most up to date counts.
431 */
432 pri = cpu_priority_high();
433 spinlock_lock(&CPU->lock);
434 count = nrdy / config.cpu_active;
435 count -= CPU->nrdy;
436 spinlock_unlock(&CPU->lock);
437 cpu_priority_restore(pri);
438
439 if (count <= 0)
440 goto satisfied;
441
442 /*
443 * Searching least priority queues on all CPU's first and most priority queues on all CPU's last.
444 */
445 for (j=RQ_COUNT-1; j >= 0; j--) {
446 for (i=0; i < config.cpu_active; i++) {
447 link_t *l;
448 runq_t *r;
449 cpu_t *cpu;
450
451 cpu = &cpus[(i + k) % config.cpu_active];
452
453 /*
454 * Not interested in ourselves.
455 * Doesn't require interrupt disabling for kcpulb is X_WIRED.
456 */
457 if (CPU == cpu)
458 continue;
459
460restart: pri = cpu_priority_high();
461 r = &cpu->rq[j];
462 spinlock_lock(&r->lock);
463 if (r->n == 0) {
464 spinlock_unlock(&r->lock);
465 cpu_priority_restore(pri);
466 continue;
467 }
468
469 t = NULL;
470 l = r->rq_head.prev; /* search rq from the back */
471 while (l != &r->rq_head) {
472 t = list_get_instance(l, thread_t, rq_link);
473 /*
474 * We don't want to steal CPU-wired threads neither threads already stolen.
475 * The latter prevents threads from migrating between CPU's without ever being run.
476 * We don't want to steal threads whose FPU context is still in CPU.
477 */
478 spinlock_lock(&t->lock);
479 if ( (!(t->flags & (X_WIRED | X_STOLEN))) && (!(t->fpu_context_engaged)) ) {
480
481 /*
482 * Remove t from r.
483 */
484
485 spinlock_unlock(&t->lock);
486
487 /*
488 * Here we have to avoid deadlock with relink_rq(),
489 * because it locks cpu and r in a different order than we do.
490 */
491 if (!spinlock_trylock(&cpu->lock)) {
492 /* Release all locks and try again. */
493 spinlock_unlock(&r->lock);
494 cpu_priority_restore(pri);
495 goto restart;
496 }
497 cpu->nrdy--;
498 spinlock_unlock(&cpu->lock);
499
500 atomic_dec(&nrdy);
501
502 r->n--;
503 list_remove(&t->rq_link);
504
505 break;
506 }
507 spinlock_unlock(&t->lock);
508 l = l->prev;
509 t = NULL;
510 }
511 spinlock_unlock(&r->lock);
512
513 if (t) {
514 /*
515 * Ready t on local CPU
516 */
517 spinlock_lock(&t->lock);
518 #ifdef KCPULB_VERBOSE
519 printf("kcpulb%d: TID %d -> cpu%d, nrdy=%d, avg=%d\n", CPU->id, t->tid, CPU->id, CPU->nrdy, nrdy / config.cpu_active);
520 #endif
521 t->flags |= X_STOLEN;
522 spinlock_unlock(&t->lock);
523
524 thread_ready(t);
525
526 cpu_priority_restore(pri);
527
528 if (--count == 0)
529 goto satisfied;
530
531 /*
532 * We are not satisfied yet, focus on another CPU next time.
533 */
534 k++;
535
536 continue;
537 }
538 cpu_priority_restore(pri);
539 }
540 }
541
542 if (CPU->nrdy) {
543 /*
544 * Be a little bit light-weight and let migrated threads run.
545 */
546 scheduler();
547 }
548 else {
549 /*
550 * We failed to migrate a single thread.
551 * Something more sophisticated should be done.
552 */
553 scheduler();
554 }
555
556 goto not_satisfied;
557
558satisfied:
559 /*
560 * Tell find_best_thread() to wake us up later again.
561 */
562 CPU->kcpulbstarted = 0;
563 goto loop;
564}
565
566#endif /* __SMP__ */
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