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. */
17 int sysctl_panic_on_rcu_stall __read_mostly;
18 int 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
rcu_exp_jiffies_till_stall_check(void)28 int 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 }
54 EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
55
56 /* Limit-check stall timeouts specified at boottime and runtime. */
rcu_jiffies_till_stall_check(void)57 int 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 }
74 EXPORT_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 */
rcu_gp_might_be_stalled(void)89 bool 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. */
rcu_sysrq_start(void)107 void rcu_sysrq_start(void)
108 {
109 if (!rcu_cpu_stall_suppress)
110 rcu_cpu_stall_suppress = 2;
111 }
112
rcu_sysrq_end(void)113 void 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. */
rcu_panic(struct notifier_block * this,unsigned long ev,void * ptr)120 static 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
126 static struct notifier_block rcu_panic_block = {
127 .notifier_call = rcu_panic,
128 };
129
check_cpu_stall_init(void)130 static int __init check_cpu_stall_init(void)
131 {
132 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
133 return 0;
134 }
135 early_initcall(check_cpu_stall_init);
136
137 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
panic_on_rcu_stall(void)138 static 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 */
rcu_cpu_stall_reset(void)154 void 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). */
record_gp_stall_check_time(void)165 static 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. */
zero_cpu_stall_ticks(struct rcu_data * rdp)179 static 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 */
rcu_stall_kick_kthreads(void)190 static 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 */
rcu_iw_handler(struct irq_work * iwp)212 static 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 */
rcu_print_detail_task_stall_rnp(struct rcu_node * rnp)237 static 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.
261 struct 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 */
check_slow_task(struct task_struct * t,void * arg)271 static 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 */
rcu_print_task_stall(struct rcu_node * rnp,unsigned long flags)287 static 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 */
rcu_print_detail_task_stall_rnp(struct rcu_node * rnp)337 static 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 */
rcu_print_task_stall(struct rcu_node * rnp,unsigned long flags)345 static 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 */
rcu_dump_cpu_stacks(void)359 static 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 if (!trigger_single_cpu_backtrace(cpu))
372 dump_cpu_task(cpu);
373 }
374 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
375 }
376 }
377
378 static 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 */
gp_state_getname(short gs)393 static 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? */
rcu_is_gp_kthread_starving(unsigned long * jp)401 static 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
rcu_is_rcuc_kthread_starving(struct rcu_data * rdp,unsigned long * jp)410 static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
411 {
412 unsigned long j = jiffies - READ_ONCE(rdp->rcuc_activity);
413
414 if (jp)
415 *jp = j;
416 return j > 2 * HZ;
417 }
418
419 /*
420 * Print out diagnostic information for the specified stalled CPU.
421 *
422 * If the specified CPU is aware of the current RCU grace period, then
423 * print the number of scheduling clock interrupts the CPU has taken
424 * during the time that it has been aware. Otherwise, print the number
425 * of RCU grace periods that this CPU is ignorant of, for example, "1"
426 * if the CPU was aware of the previous grace period.
427 *
428 * Also print out idle info.
429 */
print_cpu_stall_info(int cpu)430 static void print_cpu_stall_info(int cpu)
431 {
432 unsigned long delta;
433 bool falsepositive;
434 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
435 char *ticks_title;
436 unsigned long ticks_value;
437
438 /*
439 * We could be printing a lot while holding a spinlock. Avoid
440 * triggering hard lockup.
441 */
442 touch_nmi_watchdog();
443
444 ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
445 if (ticks_value) {
446 ticks_title = "GPs behind";
447 } else {
448 ticks_title = "ticks this GP";
449 ticks_value = rdp->ticks_this_gp;
450 }
451 delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
452 falsepositive = rcu_is_gp_kthread_starving(NULL) &&
453 rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp));
454 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s\n",
455 cpu,
456 "O."[!!cpu_online(cpu)],
457 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
458 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
459 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
460 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
461 "!."[!delta],
462 ticks_value, ticks_title,
463 rcu_dynticks_snap(rdp) & 0xfff,
464 rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
465 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
466 data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
467 falsepositive ? " (false positive?)" : "");
468 }
469
rcuc_kthread_dump(struct rcu_data * rdp)470 static void rcuc_kthread_dump(struct rcu_data *rdp)
471 {
472 int cpu;
473 unsigned long j;
474 struct task_struct *rcuc;
475
476 rcuc = rdp->rcu_cpu_kthread_task;
477 if (!rcuc)
478 return;
479
480 cpu = task_cpu(rcuc);
481 if (cpu_is_offline(cpu) || idle_cpu(cpu))
482 return;
483
484 if (!rcu_is_rcuc_kthread_starving(rdp, &j))
485 return;
486
487 pr_err("%s kthread starved for %ld jiffies\n", rcuc->comm, j);
488 sched_show_task(rcuc);
489 if (!trigger_single_cpu_backtrace(cpu))
490 dump_cpu_task(cpu);
491 }
492
493 /* Complain about starvation of grace-period kthread. */
rcu_check_gp_kthread_starvation(void)494 static void rcu_check_gp_kthread_starvation(void)
495 {
496 int cpu;
497 struct task_struct *gpk = rcu_state.gp_kthread;
498 unsigned long j;
499
500 if (rcu_is_gp_kthread_starving(&j)) {
501 cpu = gpk ? task_cpu(gpk) : -1;
502 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
503 rcu_state.name, j,
504 (long)rcu_seq_current(&rcu_state.gp_seq),
505 data_race(READ_ONCE(rcu_state.gp_flags)),
506 gp_state_getname(rcu_state.gp_state),
507 data_race(READ_ONCE(rcu_state.gp_state)),
508 gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
509 if (gpk) {
510 pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
511 pr_err("RCU grace-period kthread stack dump:\n");
512 sched_show_task(gpk);
513 if (cpu >= 0) {
514 if (cpu_is_offline(cpu)) {
515 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
516 } else {
517 pr_err("Stack dump where RCU GP kthread last ran:\n");
518 if (!trigger_single_cpu_backtrace(cpu))
519 dump_cpu_task(cpu);
520 }
521 }
522 wake_up_process(gpk);
523 }
524 }
525 }
526
527 /* Complain about missing wakeups from expired fqs wait timer */
rcu_check_gp_kthread_expired_fqs_timer(void)528 static void rcu_check_gp_kthread_expired_fqs_timer(void)
529 {
530 struct task_struct *gpk = rcu_state.gp_kthread;
531 short gp_state;
532 unsigned long jiffies_fqs;
533 int cpu;
534
535 /*
536 * Order reads of .gp_state and .jiffies_force_qs.
537 * Matching smp_wmb() is present in rcu_gp_fqs_loop().
538 */
539 gp_state = smp_load_acquire(&rcu_state.gp_state);
540 jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
541
542 if (gp_state == RCU_GP_WAIT_FQS &&
543 time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
544 gpk && !READ_ONCE(gpk->on_rq)) {
545 cpu = task_cpu(gpk);
546 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
547 rcu_state.name, (jiffies - jiffies_fqs),
548 (long)rcu_seq_current(&rcu_state.gp_seq),
549 data_race(rcu_state.gp_flags),
550 gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
551 data_race(READ_ONCE(gpk->__state)));
552 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
553 cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
554 }
555 }
556
print_other_cpu_stall(unsigned long gp_seq,unsigned long gps)557 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
558 {
559 int cpu;
560 unsigned long flags;
561 unsigned long gpa;
562 unsigned long j;
563 int ndetected = 0;
564 struct rcu_node *rnp;
565 long totqlen = 0;
566
567 lockdep_assert_irqs_disabled();
568
569 /* Kick and suppress, if so configured. */
570 rcu_stall_kick_kthreads();
571 if (rcu_stall_is_suppressed())
572 return;
573
574 /*
575 * OK, time to rat on our buddy...
576 * See Documentation/RCU/stallwarn.rst for info on how to debug
577 * RCU CPU stall warnings.
578 */
579 trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
580 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
581 rcu_for_each_leaf_node(rnp) {
582 raw_spin_lock_irqsave_rcu_node(rnp, flags);
583 if (rnp->qsmask != 0) {
584 for_each_leaf_node_possible_cpu(rnp, cpu)
585 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
586 print_cpu_stall_info(cpu);
587 ndetected++;
588 }
589 }
590 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
591 lockdep_assert_irqs_disabled();
592 }
593
594 for_each_possible_cpu(cpu)
595 totqlen += rcu_get_n_cbs_cpu(cpu);
596 pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
597 smp_processor_id(), (long)(jiffies - gps),
598 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
599 if (ndetected) {
600 rcu_dump_cpu_stacks();
601
602 /* Complain about tasks blocking the grace period. */
603 rcu_for_each_leaf_node(rnp)
604 rcu_print_detail_task_stall_rnp(rnp);
605 } else {
606 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
607 pr_err("INFO: Stall ended before state dump start\n");
608 } else {
609 j = jiffies;
610 gpa = data_race(READ_ONCE(rcu_state.gp_activity));
611 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
612 rcu_state.name, j - gpa, j, gpa,
613 data_race(READ_ONCE(jiffies_till_next_fqs)),
614 data_race(READ_ONCE(rcu_get_root()->qsmask)));
615 }
616 }
617 /* Rewrite if needed in case of slow consoles. */
618 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
619 WRITE_ONCE(rcu_state.jiffies_stall,
620 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
621
622 rcu_check_gp_kthread_expired_fqs_timer();
623 rcu_check_gp_kthread_starvation();
624
625 panic_on_rcu_stall();
626
627 rcu_force_quiescent_state(); /* Kick them all. */
628 }
629
print_cpu_stall(unsigned long gps)630 static void print_cpu_stall(unsigned long gps)
631 {
632 int cpu;
633 unsigned long flags;
634 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
635 struct rcu_node *rnp = rcu_get_root();
636 long totqlen = 0;
637
638 lockdep_assert_irqs_disabled();
639
640 /* Kick and suppress, if so configured. */
641 rcu_stall_kick_kthreads();
642 if (rcu_stall_is_suppressed())
643 return;
644
645 /*
646 * OK, time to rat on ourselves...
647 * See Documentation/RCU/stallwarn.rst for info on how to debug
648 * RCU CPU stall warnings.
649 */
650 trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
651 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
652 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
653 print_cpu_stall_info(smp_processor_id());
654 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
655 for_each_possible_cpu(cpu)
656 totqlen += rcu_get_n_cbs_cpu(cpu);
657 pr_cont("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
658 jiffies - gps,
659 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen, rcu_state.n_online_cpus);
660
661 rcu_check_gp_kthread_expired_fqs_timer();
662 rcu_check_gp_kthread_starvation();
663
664 if (!use_softirq)
665 rcuc_kthread_dump(rdp);
666
667 rcu_dump_cpu_stacks();
668
669 raw_spin_lock_irqsave_rcu_node(rnp, flags);
670 /* Rewrite if needed in case of slow consoles. */
671 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
672 WRITE_ONCE(rcu_state.jiffies_stall,
673 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
674 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
675
676 panic_on_rcu_stall();
677
678 /*
679 * Attempt to revive the RCU machinery by forcing a context switch.
680 *
681 * A context switch would normally allow the RCU state machine to make
682 * progress and it could be we're stuck in kernel space without context
683 * switches for an entirely unreasonable amount of time.
684 */
685 set_tsk_need_resched(current);
686 set_preempt_need_resched();
687 }
688
check_cpu_stall(struct rcu_data * rdp)689 static void check_cpu_stall(struct rcu_data *rdp)
690 {
691 bool didstall = false;
692 unsigned long gs1;
693 unsigned long gs2;
694 unsigned long gps;
695 unsigned long j;
696 unsigned long jn;
697 unsigned long js;
698 struct rcu_node *rnp;
699
700 lockdep_assert_irqs_disabled();
701 if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
702 !rcu_gp_in_progress())
703 return;
704 rcu_stall_kick_kthreads();
705 j = jiffies;
706
707 /*
708 * Lots of memory barriers to reject false positives.
709 *
710 * The idea is to pick up rcu_state.gp_seq, then
711 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
712 * another copy of rcu_state.gp_seq. These values are updated in
713 * the opposite order with memory barriers (or equivalent) during
714 * grace-period initialization and cleanup. Now, a false positive
715 * can occur if we get an new value of rcu_state.gp_start and a old
716 * value of rcu_state.jiffies_stall. But given the memory barriers,
717 * the only way that this can happen is if one grace period ends
718 * and another starts between these two fetches. This is detected
719 * by comparing the second fetch of rcu_state.gp_seq with the
720 * previous fetch from rcu_state.gp_seq.
721 *
722 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
723 * and rcu_state.gp_start suffice to forestall false positives.
724 */
725 gs1 = READ_ONCE(rcu_state.gp_seq);
726 smp_rmb(); /* Pick up ->gp_seq first... */
727 js = READ_ONCE(rcu_state.jiffies_stall);
728 smp_rmb(); /* ...then ->jiffies_stall before the rest... */
729 gps = READ_ONCE(rcu_state.gp_start);
730 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
731 gs2 = READ_ONCE(rcu_state.gp_seq);
732 if (gs1 != gs2 ||
733 ULONG_CMP_LT(j, js) ||
734 ULONG_CMP_GE(gps, js))
735 return; /* No stall or GP completed since entering function. */
736 rnp = rdp->mynode;
737 jn = jiffies + ULONG_MAX / 2;
738 if (rcu_gp_in_progress() &&
739 (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
740 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
741
742 /*
743 * If a virtual machine is stopped by the host it can look to
744 * the watchdog like an RCU stall. Check to see if the host
745 * stopped the vm.
746 */
747 if (kvm_check_and_clear_guest_paused())
748 return;
749
750 /* We haven't checked in, so go dump stack. */
751 print_cpu_stall(gps);
752 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
753 rcu_ftrace_dump(DUMP_ALL);
754 didstall = true;
755
756 } else if (rcu_gp_in_progress() &&
757 ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
758 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
759
760 /*
761 * If a virtual machine is stopped by the host it can look to
762 * the watchdog like an RCU stall. Check to see if the host
763 * stopped the vm.
764 */
765 if (kvm_check_and_clear_guest_paused())
766 return;
767
768 /* They had a few time units to dump stack, so complain. */
769 print_other_cpu_stall(gs2, gps);
770 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
771 rcu_ftrace_dump(DUMP_ALL);
772 didstall = true;
773 }
774 if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
775 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
776 WRITE_ONCE(rcu_state.jiffies_stall, jn);
777 }
778 }
779
780 //////////////////////////////////////////////////////////////////////////////
781 //
782 // RCU forward-progress mechanisms, including of callback invocation.
783
784
785 /*
786 * Check to see if a failure to end RCU priority inversion was due to
787 * a CPU not passing through a quiescent state. When this happens, there
788 * is nothing that RCU priority boosting can do to help, so we shouldn't
789 * count this as an RCU priority boosting failure. A return of true says
790 * RCU priority boosting is to blame, and false says otherwise. If false
791 * is returned, the first of the CPUs to blame is stored through cpup.
792 * If there was no CPU blocking the current grace period, but also nothing
793 * in need of being boosted, *cpup is set to -1. This can happen in case
794 * of vCPU preemption while the last CPU is reporting its quiscent state,
795 * for example.
796 *
797 * If cpup is NULL, then a lockless quick check is carried out, suitable
798 * for high-rate usage. On the other hand, if cpup is non-NULL, each
799 * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
800 */
rcu_check_boost_fail(unsigned long gp_state,int * cpup)801 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
802 {
803 bool atb = false;
804 int cpu;
805 unsigned long flags;
806 struct rcu_node *rnp;
807
808 rcu_for_each_leaf_node(rnp) {
809 if (!cpup) {
810 if (data_race(READ_ONCE(rnp->qsmask))) {
811 return false;
812 } else {
813 if (READ_ONCE(rnp->gp_tasks))
814 atb = true;
815 continue;
816 }
817 }
818 *cpup = -1;
819 raw_spin_lock_irqsave_rcu_node(rnp, flags);
820 if (rnp->gp_tasks)
821 atb = true;
822 if (!rnp->qsmask) {
823 // No CPUs without quiescent states for this rnp.
824 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
825 continue;
826 }
827 // Find the first holdout CPU.
828 for_each_leaf_node_possible_cpu(rnp, cpu) {
829 if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
830 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
831 *cpup = cpu;
832 return false;
833 }
834 }
835 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
836 }
837 // Can't blame CPUs, so must blame RCU priority boosting.
838 return atb;
839 }
840 EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
841
842 /*
843 * Show the state of the grace-period kthreads.
844 */
show_rcu_gp_kthreads(void)845 void show_rcu_gp_kthreads(void)
846 {
847 unsigned long cbs = 0;
848 int cpu;
849 unsigned long j;
850 unsigned long ja;
851 unsigned long jr;
852 unsigned long js;
853 unsigned long jw;
854 struct rcu_data *rdp;
855 struct rcu_node *rnp;
856 struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
857
858 j = jiffies;
859 ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
860 jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
861 js = j - data_race(READ_ONCE(rcu_state.gp_start));
862 jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
863 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",
864 rcu_state.name, gp_state_getname(rcu_state.gp_state),
865 data_race(READ_ONCE(rcu_state.gp_state)),
866 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
867 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
868 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
869 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
870 data_race(READ_ONCE(rcu_state.gp_max)),
871 data_race(READ_ONCE(rcu_state.gp_flags)));
872 rcu_for_each_node_breadth_first(rnp) {
873 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
874 !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
875 !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
876 continue;
877 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
878 rnp->grplo, rnp->grphi,
879 (long)data_race(READ_ONCE(rnp->gp_seq)),
880 (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
881 data_race(READ_ONCE(rnp->qsmask)),
882 ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
883 ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
884 ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
885 ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
886 data_race(READ_ONCE(rnp->n_boosts)));
887 if (!rcu_is_leaf_node(rnp))
888 continue;
889 for_each_leaf_node_possible_cpu(rnp, cpu) {
890 rdp = per_cpu_ptr(&rcu_data, cpu);
891 if (READ_ONCE(rdp->gpwrap) ||
892 ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
893 READ_ONCE(rdp->gp_seq_needed)))
894 continue;
895 pr_info("\tcpu %d ->gp_seq_needed %ld\n",
896 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
897 }
898 }
899 for_each_possible_cpu(cpu) {
900 rdp = per_cpu_ptr(&rcu_data, cpu);
901 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
902 if (rcu_segcblist_is_offloaded(&rdp->cblist))
903 show_rcu_nocb_state(rdp);
904 }
905 pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
906 show_rcu_tasks_gp_kthreads();
907 }
908 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
909
910 /*
911 * This function checks for grace-period requests that fail to motivate
912 * RCU to come out of its idle mode.
913 */
rcu_check_gp_start_stall(struct rcu_node * rnp,struct rcu_data * rdp,const unsigned long gpssdelay)914 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
915 const unsigned long gpssdelay)
916 {
917 unsigned long flags;
918 unsigned long j;
919 struct rcu_node *rnp_root = rcu_get_root();
920 static atomic_t warned = ATOMIC_INIT(0);
921
922 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
923 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
924 READ_ONCE(rnp_root->gp_seq_needed)) ||
925 !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
926 return;
927 j = jiffies; /* Expensive access, and in common case don't get here. */
928 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
929 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
930 atomic_read(&warned))
931 return;
932
933 raw_spin_lock_irqsave_rcu_node(rnp, flags);
934 j = jiffies;
935 if (rcu_gp_in_progress() ||
936 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
937 READ_ONCE(rnp_root->gp_seq_needed)) ||
938 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
939 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
940 atomic_read(&warned)) {
941 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
942 return;
943 }
944 /* Hold onto the leaf lock to make others see warned==1. */
945
946 if (rnp_root != rnp)
947 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
948 j = jiffies;
949 if (rcu_gp_in_progress() ||
950 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
951 READ_ONCE(rnp_root->gp_seq_needed)) ||
952 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
953 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
954 atomic_xchg(&warned, 1)) {
955 if (rnp_root != rnp)
956 /* irqs remain disabled. */
957 raw_spin_unlock_rcu_node(rnp_root);
958 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
959 return;
960 }
961 WARN_ON(1);
962 if (rnp_root != rnp)
963 raw_spin_unlock_rcu_node(rnp_root);
964 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
965 show_rcu_gp_kthreads();
966 }
967
968 /*
969 * Do a forward-progress check for rcutorture. This is normally invoked
970 * due to an OOM event. The argument "j" gives the time period during
971 * which rcutorture would like progress to have been made.
972 */
rcu_fwd_progress_check(unsigned long j)973 void rcu_fwd_progress_check(unsigned long j)
974 {
975 unsigned long cbs;
976 int cpu;
977 unsigned long max_cbs = 0;
978 int max_cpu = -1;
979 struct rcu_data *rdp;
980
981 if (rcu_gp_in_progress()) {
982 pr_info("%s: GP age %lu jiffies\n",
983 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
984 show_rcu_gp_kthreads();
985 } else {
986 pr_info("%s: Last GP end %lu jiffies ago\n",
987 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
988 preempt_disable();
989 rdp = this_cpu_ptr(&rcu_data);
990 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
991 preempt_enable();
992 }
993 for_each_possible_cpu(cpu) {
994 cbs = rcu_get_n_cbs_cpu(cpu);
995 if (!cbs)
996 continue;
997 if (max_cpu < 0)
998 pr_info("%s: callbacks", __func__);
999 pr_cont(" %d: %lu", cpu, cbs);
1000 if (cbs <= max_cbs)
1001 continue;
1002 max_cbs = cbs;
1003 max_cpu = cpu;
1004 }
1005 if (max_cpu >= 0)
1006 pr_cont("\n");
1007 }
1008 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
1009
1010 /* Commandeer a sysrq key to dump RCU's tree. */
1011 static bool sysrq_rcu;
1012 module_param(sysrq_rcu, bool, 0444);
1013
1014 /* Dump grace-period-request information due to commandeered sysrq. */
sysrq_show_rcu(int key)1015 static void sysrq_show_rcu(int key)
1016 {
1017 show_rcu_gp_kthreads();
1018 }
1019
1020 static const struct sysrq_key_op sysrq_rcudump_op = {
1021 .handler = sysrq_show_rcu,
1022 .help_msg = "show-rcu(y)",
1023 .action_msg = "Show RCU tree",
1024 .enable_mask = SYSRQ_ENABLE_DUMP,
1025 };
1026
rcu_sysrq_init(void)1027 static int __init rcu_sysrq_init(void)
1028 {
1029 if (sysrq_rcu)
1030 return register_sysrq_key('y', &sysrq_rcudump_op);
1031 return 0;
1032 }
1033 early_initcall(rcu_sysrq_init);
1034