1 // SPDX-License-Identifier: GPL-2.0+
3 * RCU CPU stall warnings for normal RCU grace periods
5 * Copyright IBM Corporation, 2019
7 * Author: Paul E. McKenney <paulmck@linux.ibm.com>
10 //////////////////////////////////////////////////////////////////////////////
12 // Controlling CPU stall warnings, including delay calculation.
14 /* panic() on RCU Stall sysctl. */
15 int sysctl_panic_on_rcu_stall __read_mostly;
16 int sysctl_max_rcu_stall_to_panic __read_mostly;
18 #ifdef CONFIG_PROVE_RCU
19 #define RCU_STALL_DELAY_DELTA (5 * HZ)
21 #define RCU_STALL_DELAY_DELTA 0
23 #define RCU_STALL_MIGHT_DIV 8
24 #define RCU_STALL_MIGHT_MIN (2 * HZ)
26 /* Limit-check stall timeouts specified at boottime and runtime. */
27 int rcu_jiffies_till_stall_check(void)
29 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
32 * Limit check must be consistent with the Kconfig limits
33 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
35 if (till_stall_check < 3) {
36 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
38 } else if (till_stall_check > 300) {
39 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
40 till_stall_check = 300;
42 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
44 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
47 * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
49 * Returns @true if the current grace period is sufficiently old that
50 * it is reasonable to assume that it might be stalled. This can be
51 * useful when deciding whether to allocate memory to enable RCU-mediated
52 * freeing on the one hand or just invoking synchronize_rcu() on the other.
53 * The latter is preferable when the grace period is stalled.
55 * Note that sampling of the .gp_start and .gp_seq fields must be done
56 * carefully to avoid false positives at the beginnings and ends of
59 bool rcu_gp_might_be_stalled(void)
61 unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
62 unsigned long j = jiffies;
64 if (d < RCU_STALL_MIGHT_MIN)
65 d = RCU_STALL_MIGHT_MIN;
66 smp_mb(); // jiffies before .gp_seq to avoid false positives.
67 if (!rcu_gp_in_progress())
69 // Long delays at this point avoids false positive, but a delay
70 // of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
71 smp_mb(); // .gp_seq before second .gp_start
73 return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
76 /* Don't do RCU CPU stall warnings during long sysrq printouts. */
77 void rcu_sysrq_start(void)
79 if (!rcu_cpu_stall_suppress)
80 rcu_cpu_stall_suppress = 2;
83 void rcu_sysrq_end(void)
85 if (rcu_cpu_stall_suppress == 2)
86 rcu_cpu_stall_suppress = 0;
89 /* Don't print RCU CPU stall warnings during a kernel panic. */
90 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
92 rcu_cpu_stall_suppress = 1;
96 static struct notifier_block rcu_panic_block = {
97 .notifier_call = rcu_panic,
100 static int __init check_cpu_stall_init(void)
102 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
105 early_initcall(check_cpu_stall_init);
107 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
108 static void panic_on_rcu_stall(void)
110 static int cpu_stall;
112 if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
115 if (sysctl_panic_on_rcu_stall)
116 panic("RCU Stall\n");
120 * rcu_cpu_stall_reset - prevent further stall warnings in current grace period
122 * Set the stall-warning timeout way off into the future, thus preventing
123 * any RCU CPU stall-warning messages from appearing in the current set of
126 * The caller must disable hard irqs.
128 void rcu_cpu_stall_reset(void)
130 WRITE_ONCE(rcu_state.jiffies_stall, jiffies + ULONG_MAX / 2);
133 //////////////////////////////////////////////////////////////////////////////
135 // Interaction with RCU grace periods
137 /* Start of new grace period, so record stall time (and forcing times). */
138 static void record_gp_stall_check_time(void)
140 unsigned long j = jiffies;
143 WRITE_ONCE(rcu_state.gp_start, j);
144 j1 = rcu_jiffies_till_stall_check();
145 smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
146 WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
147 rcu_state.jiffies_resched = j + j1 / 2;
148 rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
151 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
152 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
154 rdp->ticks_this_gp = 0;
155 rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
156 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
160 * If too much time has passed in the current grace period, and if
161 * so configured, go kick the relevant kthreads.
163 static void rcu_stall_kick_kthreads(void)
167 if (!READ_ONCE(rcu_kick_kthreads))
169 j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
170 if (time_after(jiffies, j) && rcu_state.gp_kthread &&
171 (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
172 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
174 rcu_ftrace_dump(DUMP_ALL);
175 wake_up_process(rcu_state.gp_kthread);
176 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
181 * Handler for the irq_work request posted about halfway into the RCU CPU
182 * stall timeout, and used to detect excessive irq disabling. Set state
183 * appropriately, but just complain if there is unexpected state on entry.
185 static void rcu_iw_handler(struct irq_work *iwp)
187 struct rcu_data *rdp;
188 struct rcu_node *rnp;
190 rdp = container_of(iwp, struct rcu_data, rcu_iw);
192 raw_spin_lock_rcu_node(rnp);
193 if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
194 rdp->rcu_iw_gp_seq = rnp->gp_seq;
195 rdp->rcu_iw_pending = false;
197 raw_spin_unlock_rcu_node(rnp);
200 //////////////////////////////////////////////////////////////////////////////
202 // Printing RCU CPU stall warnings
204 #ifdef CONFIG_PREEMPT_RCU
207 * Dump detailed information for all tasks blocking the current RCU
208 * grace period on the specified rcu_node structure.
210 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
213 struct task_struct *t;
215 raw_spin_lock_irqsave_rcu_node(rnp, flags);
216 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
217 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
220 t = list_entry(rnp->gp_tasks->prev,
221 struct task_struct, rcu_node_entry);
222 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
224 * We could be printing a lot while holding a spinlock.
225 * Avoid triggering hard lockup.
227 touch_nmi_watchdog();
230 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
233 // Communicate task state back to the RCU CPU stall warning request.
234 struct rcu_stall_chk_rdr {
236 union rcu_special rs;
241 * Report out the state of a not-running task that is stalling the
242 * current RCU grace period.
244 static bool check_slow_task(struct task_struct *t, void *arg)
246 struct rcu_stall_chk_rdr *rscrp = arg;
249 return false; // It is running, so decline to inspect it.
250 rscrp->nesting = t->rcu_read_lock_nesting;
251 rscrp->rs = t->rcu_read_unlock_special;
252 rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
257 * Scan the current list of tasks blocked within RCU read-side critical
258 * sections, printing out the tid of each of the first few of them.
260 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
261 __releases(rnp->lock)
265 struct rcu_stall_chk_rdr rscr;
266 struct task_struct *t;
267 struct task_struct *ts[8];
269 lockdep_assert_irqs_disabled();
270 if (!rcu_preempt_blocked_readers_cgp(rnp))
272 pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
273 rnp->level, rnp->grplo, rnp->grphi);
274 t = list_entry(rnp->gp_tasks->prev,
275 struct task_struct, rcu_node_entry);
276 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
279 if (i >= ARRAY_SIZE(ts))
282 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
285 if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
286 pr_cont(" P%d", t->pid);
288 pr_cont(" P%d/%d:%c%c%c%c",
289 t->pid, rscr.nesting,
290 ".b"[rscr.rs.b.blocked],
291 ".q"[rscr.rs.b.need_qs],
292 ".e"[rscr.rs.b.exp_hint],
293 ".l"[rscr.on_blkd_list]);
294 lockdep_assert_irqs_disabled();
302 #else /* #ifdef CONFIG_PREEMPT_RCU */
305 * Because preemptible RCU does not exist, we never have to check for
306 * tasks blocked within RCU read-side critical sections.
308 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
313 * Because preemptible RCU does not exist, we never have to check for
314 * tasks blocked within RCU read-side critical sections.
316 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
317 __releases(rnp->lock)
319 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
322 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
325 * Dump stacks of all tasks running on stalled CPUs. First try using
326 * NMIs, but fall back to manual remote stack tracing on architectures
327 * that don't support NMI-based stack dumps. The NMI-triggered stack
328 * traces are more accurate because they are printed by the target CPU.
330 static void rcu_dump_cpu_stacks(void)
334 struct rcu_node *rnp;
336 rcu_for_each_leaf_node(rnp) {
337 raw_spin_lock_irqsave_rcu_node(rnp, flags);
338 for_each_leaf_node_possible_cpu(rnp, cpu)
339 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
340 if (cpu_is_offline(cpu))
341 pr_err("Offline CPU %d blocking current GP.\n", cpu);
342 else if (!trigger_single_cpu_backtrace(cpu))
345 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
349 #ifdef CONFIG_RCU_FAST_NO_HZ
351 static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
353 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
355 sprintf(cp, "last_accelerate: %04lx/%04lx dyntick_enabled: %d",
356 rdp->last_accelerate & 0xffff, jiffies & 0xffff,
357 !!rdp->tick_nohz_enabled_snap);
360 #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
362 static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
367 #endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
369 static const char * const gp_state_names[] = {
370 [RCU_GP_IDLE] = "RCU_GP_IDLE",
371 [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
372 [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
373 [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
374 [RCU_GP_INIT] = "RCU_GP_INIT",
375 [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
376 [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
377 [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
378 [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
382 * Convert a ->gp_state value to a character string.
384 static const char *gp_state_getname(short gs)
386 if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
388 return gp_state_names[gs];
391 /* Is the RCU grace-period kthread being starved of CPU time? */
392 static bool rcu_is_gp_kthread_starving(unsigned long *jp)
394 unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
402 * Print out diagnostic information for the specified stalled CPU.
404 * If the specified CPU is aware of the current RCU grace period, then
405 * print the number of scheduling clock interrupts the CPU has taken
406 * during the time that it has been aware. Otherwise, print the number
407 * of RCU grace periods that this CPU is ignorant of, for example, "1"
408 * if the CPU was aware of the previous grace period.
410 * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
412 static void print_cpu_stall_info(int cpu)
417 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
419 unsigned long ticks_value;
422 * We could be printing a lot while holding a spinlock. Avoid
423 * triggering hard lockup.
425 touch_nmi_watchdog();
427 ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
429 ticks_title = "GPs behind";
431 ticks_title = "ticks this GP";
432 ticks_value = rdp->ticks_this_gp;
434 print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
435 delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
436 falsepositive = rcu_is_gp_kthread_starving(NULL) &&
437 rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp));
438 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s%s\n",
440 "O."[!!cpu_online(cpu)],
441 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
442 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
443 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
444 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
446 ticks_value, ticks_title,
447 rcu_dynticks_snap(rdp) & 0xfff,
448 rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
449 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
450 data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
452 falsepositive ? " (false positive?)" : "");
455 /* Complain about starvation of grace-period kthread. */
456 static void rcu_check_gp_kthread_starvation(void)
459 struct task_struct *gpk = rcu_state.gp_kthread;
462 if (rcu_is_gp_kthread_starving(&j)) {
463 cpu = gpk ? task_cpu(gpk) : -1;
464 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx ->cpu=%d\n",
466 (long)rcu_seq_current(&rcu_state.gp_seq),
467 data_race(rcu_state.gp_flags),
468 gp_state_getname(rcu_state.gp_state), rcu_state.gp_state,
469 gpk ? gpk->state : ~0, cpu);
471 pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
472 pr_err("RCU grace-period kthread stack dump:\n");
473 sched_show_task(gpk);
475 if (cpu_is_offline(cpu)) {
476 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
478 pr_err("Stack dump where RCU GP kthread last ran:\n");
479 if (!trigger_single_cpu_backtrace(cpu))
483 wake_up_process(gpk);
488 /* Complain about missing wakeups from expired fqs wait timer */
489 static void rcu_check_gp_kthread_expired_fqs_timer(void)
491 struct task_struct *gpk = rcu_state.gp_kthread;
493 unsigned long jiffies_fqs;
497 * Order reads of .gp_state and .jiffies_force_qs.
498 * Matching smp_wmb() is present in rcu_gp_fqs_loop().
500 gp_state = smp_load_acquire(&rcu_state.gp_state);
501 jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
503 if (gp_state == RCU_GP_WAIT_FQS &&
504 time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
505 gpk && !READ_ONCE(gpk->on_rq)) {
507 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx\n",
508 rcu_state.name, (jiffies - jiffies_fqs),
509 (long)rcu_seq_current(&rcu_state.gp_seq),
510 data_race(rcu_state.gp_flags),
511 gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
513 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
514 cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
518 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
525 struct rcu_node *rnp;
528 lockdep_assert_irqs_disabled();
530 /* Kick and suppress, if so configured. */
531 rcu_stall_kick_kthreads();
532 if (rcu_stall_is_suppressed())
536 * OK, time to rat on our buddy...
537 * See Documentation/RCU/stallwarn.rst for info on how to debug
538 * RCU CPU stall warnings.
540 trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
541 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
542 rcu_for_each_leaf_node(rnp) {
543 raw_spin_lock_irqsave_rcu_node(rnp, flags);
544 if (rnp->qsmask != 0) {
545 for_each_leaf_node_possible_cpu(rnp, cpu)
546 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
547 print_cpu_stall_info(cpu);
551 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
552 lockdep_assert_irqs_disabled();
555 for_each_possible_cpu(cpu)
556 totqlen += rcu_get_n_cbs_cpu(cpu);
557 pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
558 smp_processor_id(), (long)(jiffies - gps),
559 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
561 rcu_dump_cpu_stacks();
563 /* Complain about tasks blocking the grace period. */
564 rcu_for_each_leaf_node(rnp)
565 rcu_print_detail_task_stall_rnp(rnp);
567 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
568 pr_err("INFO: Stall ended before state dump start\n");
571 gpa = data_race(rcu_state.gp_activity);
572 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
573 rcu_state.name, j - gpa, j, gpa,
574 data_race(jiffies_till_next_fqs),
575 rcu_get_root()->qsmask);
578 /* Rewrite if needed in case of slow consoles. */
579 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
580 WRITE_ONCE(rcu_state.jiffies_stall,
581 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
583 rcu_check_gp_kthread_expired_fqs_timer();
584 rcu_check_gp_kthread_starvation();
586 panic_on_rcu_stall();
588 rcu_force_quiescent_state(); /* Kick them all. */
591 static void print_cpu_stall(unsigned long gps)
595 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
596 struct rcu_node *rnp = rcu_get_root();
599 lockdep_assert_irqs_disabled();
601 /* Kick and suppress, if so configured. */
602 rcu_stall_kick_kthreads();
603 if (rcu_stall_is_suppressed())
607 * OK, time to rat on ourselves...
608 * See Documentation/RCU/stallwarn.rst for info on how to debug
609 * RCU CPU stall warnings.
611 trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
612 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
613 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
614 print_cpu_stall_info(smp_processor_id());
615 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
616 for_each_possible_cpu(cpu)
617 totqlen += rcu_get_n_cbs_cpu(cpu);
618 pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n",
620 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
622 rcu_check_gp_kthread_expired_fqs_timer();
623 rcu_check_gp_kthread_starvation();
625 rcu_dump_cpu_stacks();
627 raw_spin_lock_irqsave_rcu_node(rnp, flags);
628 /* Rewrite if needed in case of slow consoles. */
629 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
630 WRITE_ONCE(rcu_state.jiffies_stall,
631 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
632 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
634 panic_on_rcu_stall();
637 * Attempt to revive the RCU machinery by forcing a context switch.
639 * A context switch would normally allow the RCU state machine to make
640 * progress and it could be we're stuck in kernel space without context
641 * switches for an entirely unreasonable amount of time.
643 set_tsk_need_resched(current);
644 set_preempt_need_resched();
647 static void check_cpu_stall(struct rcu_data *rdp)
655 struct rcu_node *rnp;
657 lockdep_assert_irqs_disabled();
658 if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
659 !rcu_gp_in_progress())
661 rcu_stall_kick_kthreads();
665 * Lots of memory barriers to reject false positives.
667 * The idea is to pick up rcu_state.gp_seq, then
668 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
669 * another copy of rcu_state.gp_seq. These values are updated in
670 * the opposite order with memory barriers (or equivalent) during
671 * grace-period initialization and cleanup. Now, a false positive
672 * can occur if we get an new value of rcu_state.gp_start and a old
673 * value of rcu_state.jiffies_stall. But given the memory barriers,
674 * the only way that this can happen is if one grace period ends
675 * and another starts between these two fetches. This is detected
676 * by comparing the second fetch of rcu_state.gp_seq with the
677 * previous fetch from rcu_state.gp_seq.
679 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
680 * and rcu_state.gp_start suffice to forestall false positives.
682 gs1 = READ_ONCE(rcu_state.gp_seq);
683 smp_rmb(); /* Pick up ->gp_seq first... */
684 js = READ_ONCE(rcu_state.jiffies_stall);
685 smp_rmb(); /* ...then ->jiffies_stall before the rest... */
686 gps = READ_ONCE(rcu_state.gp_start);
687 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
688 gs2 = READ_ONCE(rcu_state.gp_seq);
690 ULONG_CMP_LT(j, js) ||
691 ULONG_CMP_GE(gps, js))
692 return; /* No stall or GP completed since entering function. */
694 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
695 if (rcu_gp_in_progress() &&
696 (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
697 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
699 /* We haven't checked in, so go dump stack. */
700 print_cpu_stall(gps);
701 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
702 rcu_ftrace_dump(DUMP_ALL);
704 } else if (rcu_gp_in_progress() &&
705 ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
706 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
708 /* They had a few time units to dump stack, so complain. */
709 print_other_cpu_stall(gs2, gps);
710 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
711 rcu_ftrace_dump(DUMP_ALL);
715 //////////////////////////////////////////////////////////////////////////////
717 // RCU forward-progress mechanisms, including of callback invocation.
721 * Check to see if a failure to end RCU priority inversion was due to
722 * a CPU not passing through a quiescent state. When this happens, there
723 * is nothing that RCU priority boosting can do to help, so we shouldn't
724 * count this as an RCU priority boosting failure. A return of true says
725 * RCU priority boosting is to blame, and false says otherwise. If false
726 * is returned, the first of the CPUs to blame is stored through cpup.
727 * If there was no CPU blocking the current grace period, but also nothing
728 * in need of being boosted, *cpup is set to -1. This can happen in case
729 * of vCPU preemption while the last CPU is reporting its quiscent state,
732 * If cpup is NULL, then a lockless quick check is carried out, suitable
733 * for high-rate usage. On the other hand, if cpup is non-NULL, each
734 * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
736 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
741 struct rcu_node *rnp;
743 rcu_for_each_leaf_node(rnp) {
745 if (READ_ONCE(rnp->qsmask)) {
748 if (READ_ONCE(rnp->gp_tasks))
754 raw_spin_lock_irqsave_rcu_node(rnp, flags);
758 // No CPUs without quiescent states for this rnp.
759 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
762 // Find the first holdout CPU.
763 for_each_leaf_node_possible_cpu(rnp, cpu) {
764 if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
765 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
770 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
772 // Can't blame CPUs, so must blame RCU priority boosting.
775 EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
778 * Show the state of the grace-period kthreads.
780 void show_rcu_gp_kthreads(void)
782 unsigned long cbs = 0;
789 struct rcu_data *rdp;
790 struct rcu_node *rnp;
791 struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
794 ja = j - data_race(rcu_state.gp_activity);
795 jr = j - data_race(rcu_state.gp_req_activity);
796 js = j - data_race(rcu_state.gp_start);
797 jw = j - data_race(rcu_state.gp_wake_time);
798 pr_info("%s: wait state: %s(%d) ->state: %#lx ->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",
799 rcu_state.name, gp_state_getname(rcu_state.gp_state),
800 rcu_state.gp_state, t ? t->state : 0x1ffffL, t ? t->rt_priority : 0xffU,
801 js, ja, jr, jw, (long)data_race(rcu_state.gp_wake_seq),
802 (long)data_race(rcu_state.gp_seq),
803 (long)data_race(rcu_get_root()->gp_seq_needed),
804 data_race(rcu_state.gp_max),
805 data_race(rcu_state.gp_flags));
806 rcu_for_each_node_breadth_first(rnp) {
807 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
808 !data_race(rnp->qsmask) && !data_race(rnp->boost_tasks) &&
809 !data_race(rnp->exp_tasks) && !data_race(rnp->gp_tasks))
811 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
812 rnp->grplo, rnp->grphi,
813 (long)data_race(rnp->gp_seq), (long)data_race(rnp->gp_seq_needed),
814 data_race(rnp->qsmask),
815 ".b"[!!data_race(rnp->boost_kthread_task)],
816 ".B"[!!data_race(rnp->boost_tasks)],
817 ".E"[!!data_race(rnp->exp_tasks)],
818 ".G"[!!data_race(rnp->gp_tasks)],
819 data_race(rnp->n_boosts));
820 if (!rcu_is_leaf_node(rnp))
822 for_each_leaf_node_possible_cpu(rnp, cpu) {
823 rdp = per_cpu_ptr(&rcu_data, cpu);
824 if (READ_ONCE(rdp->gpwrap) ||
825 ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
826 READ_ONCE(rdp->gp_seq_needed)))
828 pr_info("\tcpu %d ->gp_seq_needed %ld\n",
829 cpu, (long)data_race(rdp->gp_seq_needed));
832 for_each_possible_cpu(cpu) {
833 rdp = per_cpu_ptr(&rcu_data, cpu);
834 cbs += data_race(rdp->n_cbs_invoked);
835 if (rcu_segcblist_is_offloaded(&rdp->cblist))
836 show_rcu_nocb_state(rdp);
838 pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
839 show_rcu_tasks_gp_kthreads();
841 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
844 * This function checks for grace-period requests that fail to motivate
845 * RCU to come out of its idle mode.
847 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
848 const unsigned long gpssdelay)
852 struct rcu_node *rnp_root = rcu_get_root();
853 static atomic_t warned = ATOMIC_INIT(0);
855 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
856 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
857 READ_ONCE(rnp_root->gp_seq_needed)) ||
858 !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
860 j = jiffies; /* Expensive access, and in common case don't get here. */
861 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
862 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
863 atomic_read(&warned))
866 raw_spin_lock_irqsave_rcu_node(rnp, flags);
868 if (rcu_gp_in_progress() ||
869 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
870 READ_ONCE(rnp_root->gp_seq_needed)) ||
871 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
872 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
873 atomic_read(&warned)) {
874 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
877 /* Hold onto the leaf lock to make others see warned==1. */
880 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
882 if (rcu_gp_in_progress() ||
883 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
884 READ_ONCE(rnp_root->gp_seq_needed)) ||
885 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
886 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
887 atomic_xchg(&warned, 1)) {
889 /* irqs remain disabled. */
890 raw_spin_unlock_rcu_node(rnp_root);
891 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
896 raw_spin_unlock_rcu_node(rnp_root);
897 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
898 show_rcu_gp_kthreads();
902 * Do a forward-progress check for rcutorture. This is normally invoked
903 * due to an OOM event. The argument "j" gives the time period during
904 * which rcutorture would like progress to have been made.
906 void rcu_fwd_progress_check(unsigned long j)
910 unsigned long max_cbs = 0;
912 struct rcu_data *rdp;
914 if (rcu_gp_in_progress()) {
915 pr_info("%s: GP age %lu jiffies\n",
916 __func__, jiffies - rcu_state.gp_start);
917 show_rcu_gp_kthreads();
919 pr_info("%s: Last GP end %lu jiffies ago\n",
920 __func__, jiffies - rcu_state.gp_end);
922 rdp = this_cpu_ptr(&rcu_data);
923 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
926 for_each_possible_cpu(cpu) {
927 cbs = rcu_get_n_cbs_cpu(cpu);
931 pr_info("%s: callbacks", __func__);
932 pr_cont(" %d: %lu", cpu, cbs);
941 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
943 /* Commandeer a sysrq key to dump RCU's tree. */
944 static bool sysrq_rcu;
945 module_param(sysrq_rcu, bool, 0444);
947 /* Dump grace-period-request information due to commandeered sysrq. */
948 static void sysrq_show_rcu(int key)
950 show_rcu_gp_kthreads();
953 static const struct sysrq_key_op sysrq_rcudump_op = {
954 .handler = sysrq_show_rcu,
955 .help_msg = "show-rcu(y)",
956 .action_msg = "Show RCU tree",
957 .enable_mask = SYSRQ_ENABLE_DUMP,
960 static int __init rcu_sysrq_init(void)
963 return register_sysrq_key('y', &sysrq_rcudump_op);
966 early_initcall(rcu_sysrq_init);