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1// SPDX-License-Identifier: GPL-2.0+
2/*
3 * RCU CPU stall warnings for normal RCU grace periods
4 *
5 * Copyright IBM Corporation, 2019
6 *
7 * Author: Paul E. McKenney <paulmck@linux.ibm.com>
8 */
9
10//////////////////////////////////////////////////////////////////////////////
11//
12// Controlling CPU stall warnings, including delay calculation.
13
14/* panic() on RCU Stall sysctl. */
15int sysctl_panic_on_rcu_stall __read_mostly;
16
17#ifdef CONFIG_PROVE_RCU
18#define RCU_STALL_DELAY_DELTA (5 * HZ)
19#else
20#define RCU_STALL_DELAY_DELTA 0
21#endif
22
23/* Limit-check stall timeouts specified at boottime and runtime. */
24int rcu_jiffies_till_stall_check(void)
25{
26 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
27
28 /*
29 * Limit check must be consistent with the Kconfig limits
30 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
31 */
32 if (till_stall_check < 3) {
33 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
34 till_stall_check = 3;
35 } else if (till_stall_check > 300) {
36 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
37 till_stall_check = 300;
38 }
39 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
40}
41EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
42
43/* Don't do RCU CPU stall warnings during long sysrq printouts. */
44void rcu_sysrq_start(void)
45{
46 if (!rcu_cpu_stall_suppress)
47 rcu_cpu_stall_suppress = 2;
48}
49
50void rcu_sysrq_end(void)
51{
52 if (rcu_cpu_stall_suppress == 2)
53 rcu_cpu_stall_suppress = 0;
54}
55
56/* Don't print RCU CPU stall warnings during a kernel panic. */
57static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
58{
59 rcu_cpu_stall_suppress = 1;
60 return NOTIFY_DONE;
61}
62
63static struct notifier_block rcu_panic_block = {
64 .notifier_call = rcu_panic,
65};
66
67static int __init check_cpu_stall_init(void)
68{
69 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
70 return 0;
71}
72early_initcall(check_cpu_stall_init);
73
74/* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
75static void panic_on_rcu_stall(void)
76{
77 if (sysctl_panic_on_rcu_stall)
78 panic("RCU Stall\n");
79}
80
81/**
82 * rcu_cpu_stall_reset - prevent further stall warnings in current grace period
83 *
84 * Set the stall-warning timeout way off into the future, thus preventing
85 * any RCU CPU stall-warning messages from appearing in the current set of
86 * RCU grace periods.
87 *
88 * The caller must disable hard irqs.
89 */
90void rcu_cpu_stall_reset(void)
91{
92 WRITE_ONCE(rcu_state.jiffies_stall, jiffies + ULONG_MAX / 2);
93}
94
95//////////////////////////////////////////////////////////////////////////////
96//
97// Interaction with RCU grace periods
98
99/* Start of new grace period, so record stall time (and forcing times). */
100static void record_gp_stall_check_time(void)
101{
102 unsigned long j = jiffies;
103 unsigned long j1;
104
105 rcu_state.gp_start = j;
106 j1 = rcu_jiffies_till_stall_check();
107 /* Record ->gp_start before ->jiffies_stall. */
108 smp_store_release(&rcu_state.jiffies_stall, j + j1); /* ^^^ */
109 rcu_state.jiffies_resched = j + j1 / 2;
110 rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
111}
112
113/* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
114static void zero_cpu_stall_ticks(struct rcu_data *rdp)
115{
116 rdp->ticks_this_gp = 0;
117 rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
118 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
119}
120
121/*
122 * If too much time has passed in the current grace period, and if
123 * so configured, go kick the relevant kthreads.
124 */
125static void rcu_stall_kick_kthreads(void)
126{
127 unsigned long j;
128
129 if (!rcu_kick_kthreads)
130 return;
131 j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
132 if (time_after(jiffies, j) && rcu_state.gp_kthread &&
133 (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
134 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
135 rcu_state.name);
136 rcu_ftrace_dump(DUMP_ALL);
137 wake_up_process(rcu_state.gp_kthread);
138 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
139 }
140}
141
142/*
143 * Handler for the irq_work request posted about halfway into the RCU CPU
144 * stall timeout, and used to detect excessive irq disabling. Set state
145 * appropriately, but just complain if there is unexpected state on entry.
146 */
147static void rcu_iw_handler(struct irq_work *iwp)
148{
149 struct rcu_data *rdp;
150 struct rcu_node *rnp;
151
152 rdp = container_of(iwp, struct rcu_data, rcu_iw);
153 rnp = rdp->mynode;
154 raw_spin_lock_rcu_node(rnp);
155 if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
156 rdp->rcu_iw_gp_seq = rnp->gp_seq;
157 rdp->rcu_iw_pending = false;
158 }
159 raw_spin_unlock_rcu_node(rnp);
160}
161
162//////////////////////////////////////////////////////////////////////////////
163//
164// Printing RCU CPU stall warnings
165
166#ifdef CONFIG_PREEMPTION
167
168/*
169 * Dump detailed information for all tasks blocking the current RCU
170 * grace period on the specified rcu_node structure.
171 */
172static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
173{
174 unsigned long flags;
175 struct task_struct *t;
176
177 raw_spin_lock_irqsave_rcu_node(rnp, flags);
178 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
179 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
180 return;
181 }
182 t = list_entry(rnp->gp_tasks->prev,
183 struct task_struct, rcu_node_entry);
184 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
185 /*
186 * We could be printing a lot while holding a spinlock.
187 * Avoid triggering hard lockup.
188 */
189 touch_nmi_watchdog();
190 sched_show_task(t);
191 }
192 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
193}
194
195/*
196 * Scan the current list of tasks blocked within RCU read-side critical
197 * sections, printing out the tid of each.
198 */
199static int rcu_print_task_stall(struct rcu_node *rnp)
200{
201 struct task_struct *t;
202 int ndetected = 0;
203
204 if (!rcu_preempt_blocked_readers_cgp(rnp))
205 return 0;
206 pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
207 rnp->level, rnp->grplo, rnp->grphi);
208 t = list_entry(rnp->gp_tasks->prev,
209 struct task_struct, rcu_node_entry);
210 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
211 pr_cont(" P%d", t->pid);
212 ndetected++;
213 }
214 pr_cont("\n");
215 return ndetected;
216}
217
218#else /* #ifdef CONFIG_PREEMPTION */
219
220/*
221 * Because preemptible RCU does not exist, we never have to check for
222 * tasks blocked within RCU read-side critical sections.
223 */
224static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
225{
226}
227
228/*
229 * Because preemptible RCU does not exist, we never have to check for
230 * tasks blocked within RCU read-side critical sections.
231 */
232static int rcu_print_task_stall(struct rcu_node *rnp)
233{
234 return 0;
235}
236#endif /* #else #ifdef CONFIG_PREEMPTION */
237
238/*
239 * Dump stacks of all tasks running on stalled CPUs. First try using
240 * NMIs, but fall back to manual remote stack tracing on architectures
241 * that don't support NMI-based stack dumps. The NMI-triggered stack
242 * traces are more accurate because they are printed by the target CPU.
243 */
244static void rcu_dump_cpu_stacks(void)
245{
246 int cpu;
247 unsigned long flags;
248 struct rcu_node *rnp;
249
250 rcu_for_each_leaf_node(rnp) {
251 raw_spin_lock_irqsave_rcu_node(rnp, flags);
252 for_each_leaf_node_possible_cpu(rnp, cpu)
253 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
254 if (!trigger_single_cpu_backtrace(cpu))
255 dump_cpu_task(cpu);
256 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
257 }
258}
259
260#ifdef CONFIG_RCU_FAST_NO_HZ
261
262static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
263{
264 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
265
266 sprintf(cp, "last_accelerate: %04lx/%04lx, Nonlazy posted: %c%c%c",
267 rdp->last_accelerate & 0xffff, jiffies & 0xffff,
268 ".l"[rdp->all_lazy],
269 ".L"[!rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)],
270 ".D"[!!rdp->tick_nohz_enabled_snap]);
271}
272
273#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
274
275static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
276{
277 *cp = '\0';
278}
279
280#endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
281
282/*
283 * Print out diagnostic information for the specified stalled CPU.
284 *
285 * If the specified CPU is aware of the current RCU grace period, then
286 * print the number of scheduling clock interrupts the CPU has taken
287 * during the time that it has been aware. Otherwise, print the number
288 * of RCU grace periods that this CPU is ignorant of, for example, "1"
289 * if the CPU was aware of the previous grace period.
290 *
291 * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
292 */
293static void print_cpu_stall_info(int cpu)
294{
295 unsigned long delta;
296 char fast_no_hz[72];
297 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
298 char *ticks_title;
299 unsigned long ticks_value;
300
301 /*
302 * We could be printing a lot while holding a spinlock. Avoid
303 * triggering hard lockup.
304 */
305 touch_nmi_watchdog();
306
307 ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
308 if (ticks_value) {
309 ticks_title = "GPs behind";
310 } else {
311 ticks_title = "ticks this GP";
312 ticks_value = rdp->ticks_this_gp;
313 }
314 print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
315 delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
316 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s\n",
317 cpu,
318 "O."[!!cpu_online(cpu)],
319 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
320 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
321 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
322 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
323 "!."[!delta],
324 ticks_value, ticks_title,
325 rcu_dynticks_snap(rdp) & 0xfff,
326 rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
327 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
328 READ_ONCE(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
329 fast_no_hz);
330}
331
332/* Complain about starvation of grace-period kthread. */
333static void rcu_check_gp_kthread_starvation(void)
334{
335 struct task_struct *gpk = rcu_state.gp_kthread;
336 unsigned long j;
337
338 j = jiffies - READ_ONCE(rcu_state.gp_activity);
339 if (j > 2 * HZ) {
340 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx ->cpu=%d\n",
341 rcu_state.name, j,
342 (long)rcu_seq_current(&rcu_state.gp_seq),
343 READ_ONCE(rcu_state.gp_flags),
344 gp_state_getname(rcu_state.gp_state), rcu_state.gp_state,
345 gpk ? gpk->state : ~0, gpk ? task_cpu(gpk) : -1);
346 if (gpk) {
347 pr_err("RCU grace-period kthread stack dump:\n");
348 sched_show_task(gpk);
349 wake_up_process(gpk);
350 }
351 }
352}
353
354static void print_other_cpu_stall(unsigned long gp_seq)
355{
356 int cpu;
357 unsigned long flags;
358 unsigned long gpa;
359 unsigned long j;
360 int ndetected = 0;
361 struct rcu_node *rnp;
362 long totqlen = 0;
363
364 /* Kick and suppress, if so configured. */
365 rcu_stall_kick_kthreads();
366 if (rcu_cpu_stall_suppress)
367 return;
368
369 /*
370 * OK, time to rat on our buddy...
371 * See Documentation/RCU/stallwarn.txt for info on how to debug
372 * RCU CPU stall warnings.
373 */
374 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
375 rcu_for_each_leaf_node(rnp) {
376 raw_spin_lock_irqsave_rcu_node(rnp, flags);
377 ndetected += rcu_print_task_stall(rnp);
378 if (rnp->qsmask != 0) {
379 for_each_leaf_node_possible_cpu(rnp, cpu)
380 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
381 print_cpu_stall_info(cpu);
382 ndetected++;
383 }
384 }
385 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
386 }
387
388 for_each_possible_cpu(cpu)
389 totqlen += rcu_get_n_cbs_cpu(cpu);
390 pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
391 smp_processor_id(), (long)(jiffies - rcu_state.gp_start),
392 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
393 if (ndetected) {
394 rcu_dump_cpu_stacks();
395
396 /* Complain about tasks blocking the grace period. */
397 rcu_for_each_leaf_node(rnp)
398 rcu_print_detail_task_stall_rnp(rnp);
399 } else {
400 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
401 pr_err("INFO: Stall ended before state dump start\n");
402 } else {
403 j = jiffies;
404 gpa = READ_ONCE(rcu_state.gp_activity);
405 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
406 rcu_state.name, j - gpa, j, gpa,
407 READ_ONCE(jiffies_till_next_fqs),
408 rcu_get_root()->qsmask);
409 /* In this case, the current CPU might be at fault. */
410 sched_show_task(current);
411 }
412 }
413 /* Rewrite if needed in case of slow consoles. */
414 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
415 WRITE_ONCE(rcu_state.jiffies_stall,
416 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
417
418 rcu_check_gp_kthread_starvation();
419
420 panic_on_rcu_stall();
421
422 rcu_force_quiescent_state(); /* Kick them all. */
423}
424
425static void print_cpu_stall(void)
426{
427 int cpu;
428 unsigned long flags;
429 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
430 struct rcu_node *rnp = rcu_get_root();
431 long totqlen = 0;
432
433 /* Kick and suppress, if so configured. */
434 rcu_stall_kick_kthreads();
435 if (rcu_cpu_stall_suppress)
436 return;
437
438 /*
439 * OK, time to rat on ourselves...
440 * See Documentation/RCU/stallwarn.txt for info on how to debug
441 * RCU CPU stall warnings.
442 */
443 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
444 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
445 print_cpu_stall_info(smp_processor_id());
446 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
447 for_each_possible_cpu(cpu)
448 totqlen += rcu_get_n_cbs_cpu(cpu);
449 pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n",
450 jiffies - rcu_state.gp_start,
451 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
452
453 rcu_check_gp_kthread_starvation();
454
455 rcu_dump_cpu_stacks();
456
457 raw_spin_lock_irqsave_rcu_node(rnp, flags);
458 /* Rewrite if needed in case of slow consoles. */
459 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
460 WRITE_ONCE(rcu_state.jiffies_stall,
461 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
462 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
463
464 panic_on_rcu_stall();
465
466 /*
467 * Attempt to revive the RCU machinery by forcing a context switch.
468 *
469 * A context switch would normally allow the RCU state machine to make
470 * progress and it could be we're stuck in kernel space without context
471 * switches for an entirely unreasonable amount of time.
472 */
473 set_tsk_need_resched(current);
474 set_preempt_need_resched();
475}
476
477static void check_cpu_stall(struct rcu_data *rdp)
478{
479 unsigned long gs1;
480 unsigned long gs2;
481 unsigned long gps;
482 unsigned long j;
483 unsigned long jn;
484 unsigned long js;
485 struct rcu_node *rnp;
486
487 if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
488 !rcu_gp_in_progress())
489 return;
490 rcu_stall_kick_kthreads();
491 j = jiffies;
492
493 /*
494 * Lots of memory barriers to reject false positives.
495 *
496 * The idea is to pick up rcu_state.gp_seq, then
497 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
498 * another copy of rcu_state.gp_seq. These values are updated in
499 * the opposite order with memory barriers (or equivalent) during
500 * grace-period initialization and cleanup. Now, a false positive
501 * can occur if we get an new value of rcu_state.gp_start and a old
502 * value of rcu_state.jiffies_stall. But given the memory barriers,
503 * the only way that this can happen is if one grace period ends
504 * and another starts between these two fetches. This is detected
505 * by comparing the second fetch of rcu_state.gp_seq with the
506 * previous fetch from rcu_state.gp_seq.
507 *
508 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
509 * and rcu_state.gp_start suffice to forestall false positives.
510 */
511 gs1 = READ_ONCE(rcu_state.gp_seq);
512 smp_rmb(); /* Pick up ->gp_seq first... */
513 js = READ_ONCE(rcu_state.jiffies_stall);
514 smp_rmb(); /* ...then ->jiffies_stall before the rest... */
515 gps = READ_ONCE(rcu_state.gp_start);
516 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
517 gs2 = READ_ONCE(rcu_state.gp_seq);
518 if (gs1 != gs2 ||
519 ULONG_CMP_LT(j, js) ||
520 ULONG_CMP_GE(gps, js))
521 return; /* No stall or GP completed since entering function. */
522 rnp = rdp->mynode;
523 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
524 if (rcu_gp_in_progress() &&
525 (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
526 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
527
528 /* We haven't checked in, so go dump stack. */
529 print_cpu_stall();
530 if (rcu_cpu_stall_ftrace_dump)
531 rcu_ftrace_dump(DUMP_ALL);
532
533 } else if (rcu_gp_in_progress() &&
534 ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
535 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
536
537 /* They had a few time units to dump stack, so complain. */
538 print_other_cpu_stall(gs2);
539 if (rcu_cpu_stall_ftrace_dump)
540 rcu_ftrace_dump(DUMP_ALL);
541 }
542}
543
544//////////////////////////////////////////////////////////////////////////////
545//
546// RCU forward-progress mechanisms, including of callback invocation.
547
548
549/*
550 * Show the state of the grace-period kthreads.
551 */
552void show_rcu_gp_kthreads(void)
553{
554 int cpu;
555 unsigned long j;
556 unsigned long ja;
557 unsigned long jr;
558 unsigned long jw;
559 struct rcu_data *rdp;
560 struct rcu_node *rnp;
561
562 j = jiffies;
563 ja = j - READ_ONCE(rcu_state.gp_activity);
564 jr = j - READ_ONCE(rcu_state.gp_req_activity);
565 jw = j - READ_ONCE(rcu_state.gp_wake_time);
566 pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
567 rcu_state.name, gp_state_getname(rcu_state.gp_state),
568 rcu_state.gp_state,
569 rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL,
570 ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq),
571 (long)READ_ONCE(rcu_state.gp_seq),
572 (long)READ_ONCE(rcu_get_root()->gp_seq_needed),
573 READ_ONCE(rcu_state.gp_flags));
574 rcu_for_each_node_breadth_first(rnp) {
575 if (ULONG_CMP_GE(rcu_state.gp_seq, rnp->gp_seq_needed))
576 continue;
577 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld\n",
578 rnp->grplo, rnp->grphi, (long)rnp->gp_seq,
579 (long)rnp->gp_seq_needed);
580 if (!rcu_is_leaf_node(rnp))
581 continue;
582 for_each_leaf_node_possible_cpu(rnp, cpu) {
583 rdp = per_cpu_ptr(&rcu_data, cpu);
584 if (rdp->gpwrap ||
585 ULONG_CMP_GE(rcu_state.gp_seq,
586 rdp->gp_seq_needed))
587 continue;
588 pr_info("\tcpu %d ->gp_seq_needed %ld\n",
589 cpu, (long)rdp->gp_seq_needed);
590 }
591 }
592 for_each_possible_cpu(cpu) {
593 rdp = per_cpu_ptr(&rcu_data, cpu);
594 if (rcu_segcblist_is_offloaded(&rdp->cblist))
595 show_rcu_nocb_state(rdp);
596 }
597 /* sched_show_task(rcu_state.gp_kthread); */
598}
599EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
600
601/*
602 * This function checks for grace-period requests that fail to motivate
603 * RCU to come out of its idle mode.
604 */
605static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
606 const unsigned long gpssdelay)
607{
608 unsigned long flags;
609 unsigned long j;
610 struct rcu_node *rnp_root = rcu_get_root();
611 static atomic_t warned = ATOMIC_INIT(0);
612
613 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
614 ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed))
615 return;
616 j = jiffies; /* Expensive access, and in common case don't get here. */
617 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
618 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
619 atomic_read(&warned))
620 return;
621
622 raw_spin_lock_irqsave_rcu_node(rnp, flags);
623 j = jiffies;
624 if (rcu_gp_in_progress() ||
625 ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
626 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
627 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
628 atomic_read(&warned)) {
629 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
630 return;
631 }
632 /* Hold onto the leaf lock to make others see warned==1. */
633
634 if (rnp_root != rnp)
635 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
636 j = jiffies;
637 if (rcu_gp_in_progress() ||
638 ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
639 time_before(j, rcu_state.gp_req_activity + gpssdelay) ||
640 time_before(j, rcu_state.gp_activity + gpssdelay) ||
641 atomic_xchg(&warned, 1)) {
642 if (rnp_root != rnp)
643 /* irqs remain disabled. */
644 raw_spin_unlock_rcu_node(rnp_root);
645 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
646 return;
647 }
648 WARN_ON(1);
649 if (rnp_root != rnp)
650 raw_spin_unlock_rcu_node(rnp_root);
651 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
652 show_rcu_gp_kthreads();
653}
654
655/*
656 * Do a forward-progress check for rcutorture. This is normally invoked
657 * due to an OOM event. The argument "j" gives the time period during
658 * which rcutorture would like progress to have been made.
659 */
660void rcu_fwd_progress_check(unsigned long j)
661{
662 unsigned long cbs;
663 int cpu;
664 unsigned long max_cbs = 0;
665 int max_cpu = -1;
666 struct rcu_data *rdp;
667
668 if (rcu_gp_in_progress()) {
669 pr_info("%s: GP age %lu jiffies\n",
670 __func__, jiffies - rcu_state.gp_start);
671 show_rcu_gp_kthreads();
672 } else {
673 pr_info("%s: Last GP end %lu jiffies ago\n",
674 __func__, jiffies - rcu_state.gp_end);
675 preempt_disable();
676 rdp = this_cpu_ptr(&rcu_data);
677 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
678 preempt_enable();
679 }
680 for_each_possible_cpu(cpu) {
681 cbs = rcu_get_n_cbs_cpu(cpu);
682 if (!cbs)
683 continue;
684 if (max_cpu < 0)
685 pr_info("%s: callbacks", __func__);
686 pr_cont(" %d: %lu", cpu, cbs);
687 if (cbs <= max_cbs)
688 continue;
689 max_cbs = cbs;
690 max_cpu = cpu;
691 }
692 if (max_cpu >= 0)
693 pr_cont("\n");
694}
695EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
696
697/* Commandeer a sysrq key to dump RCU's tree. */
698static bool sysrq_rcu;
699module_param(sysrq_rcu, bool, 0444);
700
701/* Dump grace-period-request information due to commandeered sysrq. */
702static void sysrq_show_rcu(int key)
703{
704 show_rcu_gp_kthreads();
705}
706
707static struct sysrq_key_op sysrq_rcudump_op = {
708 .handler = sysrq_show_rcu,
709 .help_msg = "show-rcu(y)",
710 .action_msg = "Show RCU tree",
711 .enable_mask = SYSRQ_ENABLE_DUMP,
712};
713
714static int __init rcu_sysrq_init(void)
715{
716 if (sysrq_rcu)
717 return register_sysrq_key('y', &sysrq_rcudump_op);
718 return 0;
719}
720early_initcall(rcu_sysrq_init);
1// SPDX-License-Identifier: GPL-2.0+
2/*
3 * RCU CPU stall warnings for normal RCU grace periods
4 *
5 * Copyright IBM Corporation, 2019
6 *
7 * Author: Paul E. McKenney <paulmck@linux.ibm.com>
8 */
9
10#include <linux/kvm_para.h>
11
12//////////////////////////////////////////////////////////////////////////////
13//
14// Controlling CPU stall warnings, including delay calculation.
15
16/* panic() on RCU Stall sysctl. */
17int sysctl_panic_on_rcu_stall __read_mostly;
18int sysctl_max_rcu_stall_to_panic __read_mostly;
19
20#ifdef CONFIG_PROVE_RCU
21#define RCU_STALL_DELAY_DELTA (5 * HZ)
22#else
23#define RCU_STALL_DELAY_DELTA 0
24#endif
25#define RCU_STALL_MIGHT_DIV 8
26#define RCU_STALL_MIGHT_MIN (2 * HZ)
27
28int rcu_exp_jiffies_till_stall_check(void)
29{
30 int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
31 int exp_stall_delay_delta = 0;
32 int till_stall_check;
33
34 // Zero says to use rcu_cpu_stall_timeout, but in milliseconds.
35 if (!cpu_stall_timeout)
36 cpu_stall_timeout = jiffies_to_msecs(rcu_jiffies_till_stall_check());
37
38 // Limit check must be consistent with the Kconfig limits for
39 // CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
40 // The minimum clamped value is "2UL", because at least one full
41 // tick has to be guaranteed.
42 till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 21UL * HZ);
43
44 if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
45 WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));
46
47#ifdef CONFIG_PROVE_RCU
48 /* Add extra ~25% out of till_stall_check. */
49 exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
50#endif
51
52 return till_stall_check + exp_stall_delay_delta;
53}
54EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
55
56/* Limit-check stall timeouts specified at boottime and runtime. */
57int rcu_jiffies_till_stall_check(void)
58{
59 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
60
61 /*
62 * Limit check must be consistent with the Kconfig limits
63 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
64 */
65 if (till_stall_check < 3) {
66 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
67 till_stall_check = 3;
68 } else if (till_stall_check > 300) {
69 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
70 till_stall_check = 300;
71 }
72 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
73}
74EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
75
76/**
77 * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
78 *
79 * Returns @true if the current grace period is sufficiently old that
80 * it is reasonable to assume that it might be stalled. This can be
81 * useful when deciding whether to allocate memory to enable RCU-mediated
82 * freeing on the one hand or just invoking synchronize_rcu() on the other.
83 * The latter is preferable when the grace period is stalled.
84 *
85 * Note that sampling of the .gp_start and .gp_seq fields must be done
86 * carefully to avoid false positives at the beginnings and ends of
87 * grace periods.
88 */
89bool rcu_gp_might_be_stalled(void)
90{
91 unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
92 unsigned long j = jiffies;
93
94 if (d < RCU_STALL_MIGHT_MIN)
95 d = RCU_STALL_MIGHT_MIN;
96 smp_mb(); // jiffies before .gp_seq to avoid false positives.
97 if (!rcu_gp_in_progress())
98 return false;
99 // Long delays at this point avoids false positive, but a delay
100 // of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
101 smp_mb(); // .gp_seq before second .gp_start
102 // And ditto here.
103 return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
104}
105
106/* Don't do RCU CPU stall warnings during long sysrq printouts. */
107void rcu_sysrq_start(void)
108{
109 if (!rcu_cpu_stall_suppress)
110 rcu_cpu_stall_suppress = 2;
111}
112
113void rcu_sysrq_end(void)
114{
115 if (rcu_cpu_stall_suppress == 2)
116 rcu_cpu_stall_suppress = 0;
117}
118
119/* Don't print RCU CPU stall warnings during a kernel panic. */
120static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
121{
122 rcu_cpu_stall_suppress = 1;
123 return NOTIFY_DONE;
124}
125
126static struct notifier_block rcu_panic_block = {
127 .notifier_call = rcu_panic,
128};
129
130static int __init check_cpu_stall_init(void)
131{
132 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
133 return 0;
134}
135early_initcall(check_cpu_stall_init);
136
137/* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
138static void panic_on_rcu_stall(void)
139{
140 static int cpu_stall;
141
142 if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
143 return;
144
145 if (sysctl_panic_on_rcu_stall)
146 panic("RCU Stall\n");
147}
148
149/**
150 * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
151 *
152 * The caller must disable hard irqs.
153 */
154void rcu_cpu_stall_reset(void)
155{
156 WRITE_ONCE(rcu_state.jiffies_stall,
157 jiffies + rcu_jiffies_till_stall_check());
158}
159
160//////////////////////////////////////////////////////////////////////////////
161//
162// Interaction with RCU grace periods
163
164/* Start of new grace period, so record stall time (and forcing times). */
165static void record_gp_stall_check_time(void)
166{
167 unsigned long j = jiffies;
168 unsigned long j1;
169
170 WRITE_ONCE(rcu_state.gp_start, j);
171 j1 = rcu_jiffies_till_stall_check();
172 smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
173 WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
174 rcu_state.jiffies_resched = j + j1 / 2;
175 rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
176}
177
178/* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
179static void zero_cpu_stall_ticks(struct rcu_data *rdp)
180{
181 rdp->ticks_this_gp = 0;
182 rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
183 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
184}
185
186/*
187 * If too much time has passed in the current grace period, and if
188 * so configured, go kick the relevant kthreads.
189 */
190static void rcu_stall_kick_kthreads(void)
191{
192 unsigned long j;
193
194 if (!READ_ONCE(rcu_kick_kthreads))
195 return;
196 j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
197 if (time_after(jiffies, j) && rcu_state.gp_kthread &&
198 (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
199 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
200 rcu_state.name);
201 rcu_ftrace_dump(DUMP_ALL);
202 wake_up_process(rcu_state.gp_kthread);
203 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
204 }
205}
206
207/*
208 * Handler for the irq_work request posted about halfway into the RCU CPU
209 * stall timeout, and used to detect excessive irq disabling. Set state
210 * appropriately, but just complain if there is unexpected state on entry.
211 */
212static void rcu_iw_handler(struct irq_work *iwp)
213{
214 struct rcu_data *rdp;
215 struct rcu_node *rnp;
216
217 rdp = container_of(iwp, struct rcu_data, rcu_iw);
218 rnp = rdp->mynode;
219 raw_spin_lock_rcu_node(rnp);
220 if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
221 rdp->rcu_iw_gp_seq = rnp->gp_seq;
222 rdp->rcu_iw_pending = false;
223 }
224 raw_spin_unlock_rcu_node(rnp);
225}
226
227//////////////////////////////////////////////////////////////////////////////
228//
229// Printing RCU CPU stall warnings
230
231#ifdef CONFIG_PREEMPT_RCU
232
233/*
234 * Dump detailed information for all tasks blocking the current RCU
235 * grace period on the specified rcu_node structure.
236 */
237static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
238{
239 unsigned long flags;
240 struct task_struct *t;
241
242 raw_spin_lock_irqsave_rcu_node(rnp, flags);
243 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
244 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
245 return;
246 }
247 t = list_entry(rnp->gp_tasks->prev,
248 struct task_struct, rcu_node_entry);
249 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
250 /*
251 * We could be printing a lot while holding a spinlock.
252 * Avoid triggering hard lockup.
253 */
254 touch_nmi_watchdog();
255 sched_show_task(t);
256 }
257 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
258}
259
260// Communicate task state back to the RCU CPU stall warning request.
261struct rcu_stall_chk_rdr {
262 int nesting;
263 union rcu_special rs;
264 bool on_blkd_list;
265};
266
267/*
268 * Report out the state of a not-running task that is stalling the
269 * current RCU grace period.
270 */
271static int check_slow_task(struct task_struct *t, void *arg)
272{
273 struct rcu_stall_chk_rdr *rscrp = arg;
274
275 if (task_curr(t))
276 return -EBUSY; // It is running, so decline to inspect it.
277 rscrp->nesting = t->rcu_read_lock_nesting;
278 rscrp->rs = t->rcu_read_unlock_special;
279 rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
280 return 0;
281}
282
283/*
284 * Scan the current list of tasks blocked within RCU read-side critical
285 * sections, printing out the tid of each of the first few of them.
286 */
287static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
288 __releases(rnp->lock)
289{
290 int i = 0;
291 int ndetected = 0;
292 struct rcu_stall_chk_rdr rscr;
293 struct task_struct *t;
294 struct task_struct *ts[8];
295
296 lockdep_assert_irqs_disabled();
297 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
298 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
299 return 0;
300 }
301 pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
302 rnp->level, rnp->grplo, rnp->grphi);
303 t = list_entry(rnp->gp_tasks->prev,
304 struct task_struct, rcu_node_entry);
305 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
306 get_task_struct(t);
307 ts[i++] = t;
308 if (i >= ARRAY_SIZE(ts))
309 break;
310 }
311 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
312 while (i) {
313 t = ts[--i];
314 if (task_call_func(t, check_slow_task, &rscr))
315 pr_cont(" P%d", t->pid);
316 else
317 pr_cont(" P%d/%d:%c%c%c%c",
318 t->pid, rscr.nesting,
319 ".b"[rscr.rs.b.blocked],
320 ".q"[rscr.rs.b.need_qs],
321 ".e"[rscr.rs.b.exp_hint],
322 ".l"[rscr.on_blkd_list]);
323 lockdep_assert_irqs_disabled();
324 put_task_struct(t);
325 ndetected++;
326 }
327 pr_cont("\n");
328 return ndetected;
329}
330
331#else /* #ifdef CONFIG_PREEMPT_RCU */
332
333/*
334 * Because preemptible RCU does not exist, we never have to check for
335 * tasks blocked within RCU read-side critical sections.
336 */
337static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
338{
339}
340
341/*
342 * Because preemptible RCU does not exist, we never have to check for
343 * tasks blocked within RCU read-side critical sections.
344 */
345static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
346 __releases(rnp->lock)
347{
348 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
349 return 0;
350}
351#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
352
353/*
354 * Dump stacks of all tasks running on stalled CPUs. First try using
355 * NMIs, but fall back to manual remote stack tracing on architectures
356 * that don't support NMI-based stack dumps. The NMI-triggered stack
357 * traces are more accurate because they are printed by the target CPU.
358 */
359static void rcu_dump_cpu_stacks(void)
360{
361 int cpu;
362 unsigned long flags;
363 struct rcu_node *rnp;
364
365 rcu_for_each_leaf_node(rnp) {
366 raw_spin_lock_irqsave_rcu_node(rnp, flags);
367 for_each_leaf_node_possible_cpu(rnp, cpu)
368 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
369 if (cpu_is_offline(cpu))
370 pr_err("Offline CPU %d blocking current GP.\n", cpu);
371 else
372 dump_cpu_task(cpu);
373 }
374 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
375 }
376}
377
378static const char * const gp_state_names[] = {
379 [RCU_GP_IDLE] = "RCU_GP_IDLE",
380 [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
381 [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
382 [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
383 [RCU_GP_INIT] = "RCU_GP_INIT",
384 [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
385 [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
386 [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
387 [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
388};
389
390/*
391 * Convert a ->gp_state value to a character string.
392 */
393static const char *gp_state_getname(short gs)
394{
395 if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
396 return "???";
397 return gp_state_names[gs];
398}
399
400/* Is the RCU grace-period kthread being starved of CPU time? */
401static bool rcu_is_gp_kthread_starving(unsigned long *jp)
402{
403 unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
404
405 if (jp)
406 *jp = j;
407 return j > 2 * HZ;
408}
409
410static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
411{
412 int cpu;
413 struct task_struct *rcuc;
414 unsigned long j;
415
416 rcuc = rdp->rcu_cpu_kthread_task;
417 if (!rcuc)
418 return false;
419
420 cpu = task_cpu(rcuc);
421 if (cpu_is_offline(cpu) || idle_cpu(cpu))
422 return false;
423
424 j = jiffies - READ_ONCE(rdp->rcuc_activity);
425
426 if (jp)
427 *jp = j;
428 return j > 2 * HZ;
429}
430
431/*
432 * Print out diagnostic information for the specified stalled CPU.
433 *
434 * If the specified CPU is aware of the current RCU grace period, then
435 * print the number of scheduling clock interrupts the CPU has taken
436 * during the time that it has been aware. Otherwise, print the number
437 * of RCU grace periods that this CPU is ignorant of, for example, "1"
438 * if the CPU was aware of the previous grace period.
439 *
440 * Also print out idle info.
441 */
442static void print_cpu_stall_info(int cpu)
443{
444 unsigned long delta;
445 bool falsepositive;
446 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
447 char *ticks_title;
448 unsigned long ticks_value;
449 bool rcuc_starved;
450 unsigned long j;
451 char buf[32];
452
453 /*
454 * We could be printing a lot while holding a spinlock. Avoid
455 * triggering hard lockup.
456 */
457 touch_nmi_watchdog();
458
459 ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
460 if (ticks_value) {
461 ticks_title = "GPs behind";
462 } else {
463 ticks_title = "ticks this GP";
464 ticks_value = rdp->ticks_this_gp;
465 }
466 delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
467 falsepositive = rcu_is_gp_kthread_starving(NULL) &&
468 rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
469 rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
470 if (rcuc_starved)
471 sprintf(buf, " rcuc=%ld jiffies(starved)", j);
472 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
473 cpu,
474 "O."[!!cpu_online(cpu)],
475 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
476 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
477 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
478 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
479 "!."[!delta],
480 ticks_value, ticks_title,
481 rcu_dynticks_snap(cpu) & 0xffff,
482 ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),
483 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
484 data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
485 rcuc_starved ? buf : "",
486 falsepositive ? " (false positive?)" : "");
487}
488
489/* Complain about starvation of grace-period kthread. */
490static void rcu_check_gp_kthread_starvation(void)
491{
492 int cpu;
493 struct task_struct *gpk = rcu_state.gp_kthread;
494 unsigned long j;
495
496 if (rcu_is_gp_kthread_starving(&j)) {
497 cpu = gpk ? task_cpu(gpk) : -1;
498 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
499 rcu_state.name, j,
500 (long)rcu_seq_current(&rcu_state.gp_seq),
501 data_race(READ_ONCE(rcu_state.gp_flags)),
502 gp_state_getname(rcu_state.gp_state),
503 data_race(READ_ONCE(rcu_state.gp_state)),
504 gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
505 if (gpk) {
506 pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
507 pr_err("RCU grace-period kthread stack dump:\n");
508 sched_show_task(gpk);
509 if (cpu >= 0) {
510 if (cpu_is_offline(cpu)) {
511 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
512 } else {
513 pr_err("Stack dump where RCU GP kthread last ran:\n");
514 dump_cpu_task(cpu);
515 }
516 }
517 wake_up_process(gpk);
518 }
519 }
520}
521
522/* Complain about missing wakeups from expired fqs wait timer */
523static void rcu_check_gp_kthread_expired_fqs_timer(void)
524{
525 struct task_struct *gpk = rcu_state.gp_kthread;
526 short gp_state;
527 unsigned long jiffies_fqs;
528 int cpu;
529
530 /*
531 * Order reads of .gp_state and .jiffies_force_qs.
532 * Matching smp_wmb() is present in rcu_gp_fqs_loop().
533 */
534 gp_state = smp_load_acquire(&rcu_state.gp_state);
535 jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
536
537 if (gp_state == RCU_GP_WAIT_FQS &&
538 time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
539 gpk && !READ_ONCE(gpk->on_rq)) {
540 cpu = task_cpu(gpk);
541 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
542 rcu_state.name, (jiffies - jiffies_fqs),
543 (long)rcu_seq_current(&rcu_state.gp_seq),
544 data_race(rcu_state.gp_flags),
545 gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
546 data_race(READ_ONCE(gpk->__state)));
547 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
548 cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
549 }
550}
551
552static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
553{
554 int cpu;
555 unsigned long flags;
556 unsigned long gpa;
557 unsigned long j;
558 int ndetected = 0;
559 struct rcu_node *rnp;
560 long totqlen = 0;
561
562 lockdep_assert_irqs_disabled();
563
564 /* Kick and suppress, if so configured. */
565 rcu_stall_kick_kthreads();
566 if (rcu_stall_is_suppressed())
567 return;
568
569 /*
570 * OK, time to rat on our buddy...
571 * See Documentation/RCU/stallwarn.rst for info on how to debug
572 * RCU CPU stall warnings.
573 */
574 trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
575 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
576 rcu_for_each_leaf_node(rnp) {
577 raw_spin_lock_irqsave_rcu_node(rnp, flags);
578 if (rnp->qsmask != 0) {
579 for_each_leaf_node_possible_cpu(rnp, cpu)
580 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
581 print_cpu_stall_info(cpu);
582 ndetected++;
583 }
584 }
585 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
586 lockdep_assert_irqs_disabled();
587 }
588
589 for_each_possible_cpu(cpu)
590 totqlen += rcu_get_n_cbs_cpu(cpu);
591 pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
592 smp_processor_id(), (long)(jiffies - gps),
593 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
594 if (ndetected) {
595 rcu_dump_cpu_stacks();
596
597 /* Complain about tasks blocking the grace period. */
598 rcu_for_each_leaf_node(rnp)
599 rcu_print_detail_task_stall_rnp(rnp);
600 } else {
601 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
602 pr_err("INFO: Stall ended before state dump start\n");
603 } else {
604 j = jiffies;
605 gpa = data_race(READ_ONCE(rcu_state.gp_activity));
606 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
607 rcu_state.name, j - gpa, j, gpa,
608 data_race(READ_ONCE(jiffies_till_next_fqs)),
609 data_race(READ_ONCE(rcu_get_root()->qsmask)));
610 }
611 }
612 /* Rewrite if needed in case of slow consoles. */
613 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
614 WRITE_ONCE(rcu_state.jiffies_stall,
615 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
616
617 rcu_check_gp_kthread_expired_fqs_timer();
618 rcu_check_gp_kthread_starvation();
619
620 panic_on_rcu_stall();
621
622 rcu_force_quiescent_state(); /* Kick them all. */
623}
624
625static void print_cpu_stall(unsigned long gps)
626{
627 int cpu;
628 unsigned long flags;
629 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
630 struct rcu_node *rnp = rcu_get_root();
631 long totqlen = 0;
632
633 lockdep_assert_irqs_disabled();
634
635 /* Kick and suppress, if so configured. */
636 rcu_stall_kick_kthreads();
637 if (rcu_stall_is_suppressed())
638 return;
639
640 /*
641 * OK, time to rat on ourselves...
642 * See Documentation/RCU/stallwarn.rst for info on how to debug
643 * RCU CPU stall warnings.
644 */
645 trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
646 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
647 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
648 print_cpu_stall_info(smp_processor_id());
649 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
650 for_each_possible_cpu(cpu)
651 totqlen += rcu_get_n_cbs_cpu(cpu);
652 pr_cont("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
653 jiffies - gps,
654 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
655
656 rcu_check_gp_kthread_expired_fqs_timer();
657 rcu_check_gp_kthread_starvation();
658
659 rcu_dump_cpu_stacks();
660
661 raw_spin_lock_irqsave_rcu_node(rnp, flags);
662 /* Rewrite if needed in case of slow consoles. */
663 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
664 WRITE_ONCE(rcu_state.jiffies_stall,
665 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
666 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
667
668 panic_on_rcu_stall();
669
670 /*
671 * Attempt to revive the RCU machinery by forcing a context switch.
672 *
673 * A context switch would normally allow the RCU state machine to make
674 * progress and it could be we're stuck in kernel space without context
675 * switches for an entirely unreasonable amount of time.
676 */
677 set_tsk_need_resched(current);
678 set_preempt_need_resched();
679}
680
681static void check_cpu_stall(struct rcu_data *rdp)
682{
683 bool didstall = false;
684 unsigned long gs1;
685 unsigned long gs2;
686 unsigned long gps;
687 unsigned long j;
688 unsigned long jn;
689 unsigned long js;
690 struct rcu_node *rnp;
691
692 lockdep_assert_irqs_disabled();
693 if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
694 !rcu_gp_in_progress())
695 return;
696 rcu_stall_kick_kthreads();
697 j = jiffies;
698
699 /*
700 * Lots of memory barriers to reject false positives.
701 *
702 * The idea is to pick up rcu_state.gp_seq, then
703 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
704 * another copy of rcu_state.gp_seq. These values are updated in
705 * the opposite order with memory barriers (or equivalent) during
706 * grace-period initialization and cleanup. Now, a false positive
707 * can occur if we get an new value of rcu_state.gp_start and a old
708 * value of rcu_state.jiffies_stall. But given the memory barriers,
709 * the only way that this can happen is if one grace period ends
710 * and another starts between these two fetches. This is detected
711 * by comparing the second fetch of rcu_state.gp_seq with the
712 * previous fetch from rcu_state.gp_seq.
713 *
714 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
715 * and rcu_state.gp_start suffice to forestall false positives.
716 */
717 gs1 = READ_ONCE(rcu_state.gp_seq);
718 smp_rmb(); /* Pick up ->gp_seq first... */
719 js = READ_ONCE(rcu_state.jiffies_stall);
720 smp_rmb(); /* ...then ->jiffies_stall before the rest... */
721 gps = READ_ONCE(rcu_state.gp_start);
722 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
723 gs2 = READ_ONCE(rcu_state.gp_seq);
724 if (gs1 != gs2 ||
725 ULONG_CMP_LT(j, js) ||
726 ULONG_CMP_GE(gps, js))
727 return; /* No stall or GP completed since entering function. */
728 rnp = rdp->mynode;
729 jn = jiffies + ULONG_MAX / 2;
730 if (rcu_gp_in_progress() &&
731 (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
732 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
733
734 /*
735 * If a virtual machine is stopped by the host it can look to
736 * the watchdog like an RCU stall. Check to see if the host
737 * stopped the vm.
738 */
739 if (kvm_check_and_clear_guest_paused())
740 return;
741
742 /* We haven't checked in, so go dump stack. */
743 print_cpu_stall(gps);
744 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
745 rcu_ftrace_dump(DUMP_ALL);
746 didstall = true;
747
748 } else if (rcu_gp_in_progress() &&
749 ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
750 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
751
752 /*
753 * If a virtual machine is stopped by the host it can look to
754 * the watchdog like an RCU stall. Check to see if the host
755 * stopped the vm.
756 */
757 if (kvm_check_and_clear_guest_paused())
758 return;
759
760 /* They had a few time units to dump stack, so complain. */
761 print_other_cpu_stall(gs2, gps);
762 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
763 rcu_ftrace_dump(DUMP_ALL);
764 didstall = true;
765 }
766 if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
767 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
768 WRITE_ONCE(rcu_state.jiffies_stall, jn);
769 }
770}
771
772//////////////////////////////////////////////////////////////////////////////
773//
774// RCU forward-progress mechanisms, including of callback invocation.
775
776
777/*
778 * Check to see if a failure to end RCU priority inversion was due to
779 * a CPU not passing through a quiescent state. When this happens, there
780 * is nothing that RCU priority boosting can do to help, so we shouldn't
781 * count this as an RCU priority boosting failure. A return of true says
782 * RCU priority boosting is to blame, and false says otherwise. If false
783 * is returned, the first of the CPUs to blame is stored through cpup.
784 * If there was no CPU blocking the current grace period, but also nothing
785 * in need of being boosted, *cpup is set to -1. This can happen in case
786 * of vCPU preemption while the last CPU is reporting its quiscent state,
787 * for example.
788 *
789 * If cpup is NULL, then a lockless quick check is carried out, suitable
790 * for high-rate usage. On the other hand, if cpup is non-NULL, each
791 * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
792 */
793bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
794{
795 bool atb = false;
796 int cpu;
797 unsigned long flags;
798 struct rcu_node *rnp;
799
800 rcu_for_each_leaf_node(rnp) {
801 if (!cpup) {
802 if (data_race(READ_ONCE(rnp->qsmask))) {
803 return false;
804 } else {
805 if (READ_ONCE(rnp->gp_tasks))
806 atb = true;
807 continue;
808 }
809 }
810 *cpup = -1;
811 raw_spin_lock_irqsave_rcu_node(rnp, flags);
812 if (rnp->gp_tasks)
813 atb = true;
814 if (!rnp->qsmask) {
815 // No CPUs without quiescent states for this rnp.
816 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
817 continue;
818 }
819 // Find the first holdout CPU.
820 for_each_leaf_node_possible_cpu(rnp, cpu) {
821 if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
822 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
823 *cpup = cpu;
824 return false;
825 }
826 }
827 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
828 }
829 // Can't blame CPUs, so must blame RCU priority boosting.
830 return atb;
831}
832EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
833
834/*
835 * Show the state of the grace-period kthreads.
836 */
837void show_rcu_gp_kthreads(void)
838{
839 unsigned long cbs = 0;
840 int cpu;
841 unsigned long j;
842 unsigned long ja;
843 unsigned long jr;
844 unsigned long js;
845 unsigned long jw;
846 struct rcu_data *rdp;
847 struct rcu_node *rnp;
848 struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
849
850 j = jiffies;
851 ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
852 jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
853 js = j - data_race(READ_ONCE(rcu_state.gp_start));
854 jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
855 pr_info("%s: wait state: %s(%d) ->state: %#x ->rt_priority %u delta ->gp_start %lu ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_max %lu ->gp_flags %#x\n",
856 rcu_state.name, gp_state_getname(rcu_state.gp_state),
857 data_race(READ_ONCE(rcu_state.gp_state)),
858 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
859 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
860 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
861 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
862 data_race(READ_ONCE(rcu_state.gp_max)),
863 data_race(READ_ONCE(rcu_state.gp_flags)));
864 rcu_for_each_node_breadth_first(rnp) {
865 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
866 !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
867 !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
868 continue;
869 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
870 rnp->grplo, rnp->grphi,
871 (long)data_race(READ_ONCE(rnp->gp_seq)),
872 (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
873 data_race(READ_ONCE(rnp->qsmask)),
874 ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
875 ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
876 ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
877 ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
878 data_race(READ_ONCE(rnp->n_boosts)));
879 if (!rcu_is_leaf_node(rnp))
880 continue;
881 for_each_leaf_node_possible_cpu(rnp, cpu) {
882 rdp = per_cpu_ptr(&rcu_data, cpu);
883 if (READ_ONCE(rdp->gpwrap) ||
884 ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
885 READ_ONCE(rdp->gp_seq_needed)))
886 continue;
887 pr_info("\tcpu %d ->gp_seq_needed %ld\n",
888 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
889 }
890 }
891 for_each_possible_cpu(cpu) {
892 rdp = per_cpu_ptr(&rcu_data, cpu);
893 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
894 if (rcu_segcblist_is_offloaded(&rdp->cblist))
895 show_rcu_nocb_state(rdp);
896 }
897 pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
898 show_rcu_tasks_gp_kthreads();
899}
900EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
901
902/*
903 * This function checks for grace-period requests that fail to motivate
904 * RCU to come out of its idle mode.
905 */
906static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
907 const unsigned long gpssdelay)
908{
909 unsigned long flags;
910 unsigned long j;
911 struct rcu_node *rnp_root = rcu_get_root();
912 static atomic_t warned = ATOMIC_INIT(0);
913
914 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
915 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
916 READ_ONCE(rnp_root->gp_seq_needed)) ||
917 !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
918 return;
919 j = jiffies; /* Expensive access, and in common case don't get here. */
920 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
921 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
922 atomic_read(&warned))
923 return;
924
925 raw_spin_lock_irqsave_rcu_node(rnp, flags);
926 j = jiffies;
927 if (rcu_gp_in_progress() ||
928 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
929 READ_ONCE(rnp_root->gp_seq_needed)) ||
930 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
931 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
932 atomic_read(&warned)) {
933 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
934 return;
935 }
936 /* Hold onto the leaf lock to make others see warned==1. */
937
938 if (rnp_root != rnp)
939 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
940 j = jiffies;
941 if (rcu_gp_in_progress() ||
942 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
943 READ_ONCE(rnp_root->gp_seq_needed)) ||
944 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
945 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
946 atomic_xchg(&warned, 1)) {
947 if (rnp_root != rnp)
948 /* irqs remain disabled. */
949 raw_spin_unlock_rcu_node(rnp_root);
950 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
951 return;
952 }
953 WARN_ON(1);
954 if (rnp_root != rnp)
955 raw_spin_unlock_rcu_node(rnp_root);
956 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
957 show_rcu_gp_kthreads();
958}
959
960/*
961 * Do a forward-progress check for rcutorture. This is normally invoked
962 * due to an OOM event. The argument "j" gives the time period during
963 * which rcutorture would like progress to have been made.
964 */
965void rcu_fwd_progress_check(unsigned long j)
966{
967 unsigned long cbs;
968 int cpu;
969 unsigned long max_cbs = 0;
970 int max_cpu = -1;
971 struct rcu_data *rdp;
972
973 if (rcu_gp_in_progress()) {
974 pr_info("%s: GP age %lu jiffies\n",
975 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
976 show_rcu_gp_kthreads();
977 } else {
978 pr_info("%s: Last GP end %lu jiffies ago\n",
979 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
980 preempt_disable();
981 rdp = this_cpu_ptr(&rcu_data);
982 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
983 preempt_enable();
984 }
985 for_each_possible_cpu(cpu) {
986 cbs = rcu_get_n_cbs_cpu(cpu);
987 if (!cbs)
988 continue;
989 if (max_cpu < 0)
990 pr_info("%s: callbacks", __func__);
991 pr_cont(" %d: %lu", cpu, cbs);
992 if (cbs <= max_cbs)
993 continue;
994 max_cbs = cbs;
995 max_cpu = cpu;
996 }
997 if (max_cpu >= 0)
998 pr_cont("\n");
999}
1000EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
1001
1002/* Commandeer a sysrq key to dump RCU's tree. */
1003static bool sysrq_rcu;
1004module_param(sysrq_rcu, bool, 0444);
1005
1006/* Dump grace-period-request information due to commandeered sysrq. */
1007static void sysrq_show_rcu(int key)
1008{
1009 show_rcu_gp_kthreads();
1010}
1011
1012static const struct sysrq_key_op sysrq_rcudump_op = {
1013 .handler = sysrq_show_rcu,
1014 .help_msg = "show-rcu(y)",
1015 .action_msg = "Show RCU tree",
1016 .enable_mask = SYSRQ_ENABLE_DUMP,
1017};
1018
1019static int __init rcu_sysrq_init(void)
1020{
1021 if (sysrq_rcu)
1022 return register_sysrq_key('y', &sysrq_rcudump_op);
1023 return 0;
1024}
1025early_initcall(rcu_sysrq_init);