xref: /openbmc/linux/kernel/rcu/tree_stall.h (revision b830f94f)
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. */
15 int 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. */
24 int 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 }
41 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
42 
43 /* Don't do RCU CPU stall warnings during long sysrq printouts. */
44 void rcu_sysrq_start(void)
45 {
46 	if (!rcu_cpu_stall_suppress)
47 		rcu_cpu_stall_suppress = 2;
48 }
49 
50 void 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. */
57 static 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 
63 static struct notifier_block rcu_panic_block = {
64 	.notifier_call = rcu_panic,
65 };
66 
67 static int __init check_cpu_stall_init(void)
68 {
69 	atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
70 	return 0;
71 }
72 early_initcall(check_cpu_stall_init);
73 
74 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
75 static 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  */
90 void 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). */
100 static 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. */
114 static 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  */
125 static 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  */
147 static 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_PREEMPT
167 
168 /*
169  * Dump detailed information for all tasks blocking the current RCU
170  * grace period on the specified rcu_node structure.
171  */
172 static 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  */
199 static 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_PREEMPT */
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  */
224 static 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  */
232 static int rcu_print_task_stall(struct rcu_node *rnp)
233 {
234 	return 0;
235 }
236 #endif /* #else #ifdef CONFIG_PREEMPT */
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  */
244 static 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 
262 static 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 
275 static 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  */
293 static 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.  */
333 static 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 
354 static 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 
425 static 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 
477 static 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 
531 	} else if (rcu_gp_in_progress() &&
532 		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
533 		   cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
534 
535 		/* They had a few time units to dump stack, so complain. */
536 		print_other_cpu_stall(gs2);
537 	}
538 }
539 
540 //////////////////////////////////////////////////////////////////////////////
541 //
542 // RCU forward-progress mechanisms, including of callback invocation.
543 
544 
545 /*
546  * Show the state of the grace-period kthreads.
547  */
548 void show_rcu_gp_kthreads(void)
549 {
550 	int cpu;
551 	unsigned long j;
552 	unsigned long ja;
553 	unsigned long jr;
554 	unsigned long jw;
555 	struct rcu_data *rdp;
556 	struct rcu_node *rnp;
557 
558 	j = jiffies;
559 	ja = j - READ_ONCE(rcu_state.gp_activity);
560 	jr = j - READ_ONCE(rcu_state.gp_req_activity);
561 	jw = j - READ_ONCE(rcu_state.gp_wake_time);
562 	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",
563 		rcu_state.name, gp_state_getname(rcu_state.gp_state),
564 		rcu_state.gp_state,
565 		rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL,
566 		ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq),
567 		(long)READ_ONCE(rcu_state.gp_seq),
568 		(long)READ_ONCE(rcu_get_root()->gp_seq_needed),
569 		READ_ONCE(rcu_state.gp_flags));
570 	rcu_for_each_node_breadth_first(rnp) {
571 		if (ULONG_CMP_GE(rcu_state.gp_seq, rnp->gp_seq_needed))
572 			continue;
573 		pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld\n",
574 			rnp->grplo, rnp->grphi, (long)rnp->gp_seq,
575 			(long)rnp->gp_seq_needed);
576 		if (!rcu_is_leaf_node(rnp))
577 			continue;
578 		for_each_leaf_node_possible_cpu(rnp, cpu) {
579 			rdp = per_cpu_ptr(&rcu_data, cpu);
580 			if (rdp->gpwrap ||
581 			    ULONG_CMP_GE(rcu_state.gp_seq,
582 					 rdp->gp_seq_needed))
583 				continue;
584 			pr_info("\tcpu %d ->gp_seq_needed %ld\n",
585 				cpu, (long)rdp->gp_seq_needed);
586 		}
587 	}
588 	/* sched_show_task(rcu_state.gp_kthread); */
589 }
590 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
591 
592 /*
593  * This function checks for grace-period requests that fail to motivate
594  * RCU to come out of its idle mode.
595  */
596 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
597 				     const unsigned long gpssdelay)
598 {
599 	unsigned long flags;
600 	unsigned long j;
601 	struct rcu_node *rnp_root = rcu_get_root();
602 	static atomic_t warned = ATOMIC_INIT(0);
603 
604 	if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
605 	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed))
606 		return;
607 	j = jiffies; /* Expensive access, and in common case don't get here. */
608 	if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
609 	    time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
610 	    atomic_read(&warned))
611 		return;
612 
613 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
614 	j = jiffies;
615 	if (rcu_gp_in_progress() ||
616 	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
617 	    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 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
621 		return;
622 	}
623 	/* Hold onto the leaf lock to make others see warned==1. */
624 
625 	if (rnp_root != rnp)
626 		raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
627 	j = jiffies;
628 	if (rcu_gp_in_progress() ||
629 	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
630 	    time_before(j, rcu_state.gp_req_activity + gpssdelay) ||
631 	    time_before(j, rcu_state.gp_activity + gpssdelay) ||
632 	    atomic_xchg(&warned, 1)) {
633 		if (rnp_root != rnp)
634 			/* irqs remain disabled. */
635 			raw_spin_unlock_rcu_node(rnp_root);
636 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
637 		return;
638 	}
639 	WARN_ON(1);
640 	if (rnp_root != rnp)
641 		raw_spin_unlock_rcu_node(rnp_root);
642 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
643 	show_rcu_gp_kthreads();
644 }
645 
646 /*
647  * Do a forward-progress check for rcutorture.  This is normally invoked
648  * due to an OOM event.  The argument "j" gives the time period during
649  * which rcutorture would like progress to have been made.
650  */
651 void rcu_fwd_progress_check(unsigned long j)
652 {
653 	unsigned long cbs;
654 	int cpu;
655 	unsigned long max_cbs = 0;
656 	int max_cpu = -1;
657 	struct rcu_data *rdp;
658 
659 	if (rcu_gp_in_progress()) {
660 		pr_info("%s: GP age %lu jiffies\n",
661 			__func__, jiffies - rcu_state.gp_start);
662 		show_rcu_gp_kthreads();
663 	} else {
664 		pr_info("%s: Last GP end %lu jiffies ago\n",
665 			__func__, jiffies - rcu_state.gp_end);
666 		preempt_disable();
667 		rdp = this_cpu_ptr(&rcu_data);
668 		rcu_check_gp_start_stall(rdp->mynode, rdp, j);
669 		preempt_enable();
670 	}
671 	for_each_possible_cpu(cpu) {
672 		cbs = rcu_get_n_cbs_cpu(cpu);
673 		if (!cbs)
674 			continue;
675 		if (max_cpu < 0)
676 			pr_info("%s: callbacks", __func__);
677 		pr_cont(" %d: %lu", cpu, cbs);
678 		if (cbs <= max_cbs)
679 			continue;
680 		max_cbs = cbs;
681 		max_cpu = cpu;
682 	}
683 	if (max_cpu >= 0)
684 		pr_cont("\n");
685 }
686 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
687 
688 /* Commandeer a sysrq key to dump RCU's tree. */
689 static bool sysrq_rcu;
690 module_param(sysrq_rcu, bool, 0444);
691 
692 /* Dump grace-period-request information due to commandeered sysrq. */
693 static void sysrq_show_rcu(int key)
694 {
695 	show_rcu_gp_kthreads();
696 }
697 
698 static struct sysrq_key_op sysrq_rcudump_op = {
699 	.handler = sysrq_show_rcu,
700 	.help_msg = "show-rcu(y)",
701 	.action_msg = "Show RCU tree",
702 	.enable_mask = SYSRQ_ENABLE_DUMP,
703 };
704 
705 static int __init rcu_sysrq_init(void)
706 {
707 	if (sysrq_rcu)
708 		return register_sysrq_key('y', &sysrq_rcudump_op);
709 	return 0;
710 }
711 early_initcall(rcu_sysrq_init);
712