1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==============================
4 Using RCU's CPU Stall Detector
5 ==============================
6
7 This document first discusses what sorts of issues RCU's CPU stall
8 detector can locate, and then discusses kernel parameters and Kconfig
9 options that can be used to fine-tune the detector's operation. Finally,
10 this document explains the stall detector's "splat" format.
11
12
13 What Causes RCU CPU Stall Warnings?
14 ===================================
15
16 So your kernel printed an RCU CPU stall warning. The next question is
17 "What caused it?" The following problems can result in RCU CPU stall
18 warnings:
19
20 - A CPU looping in an RCU read-side critical section.
21
22 - A CPU looping with interrupts disabled.
23
24 - A CPU looping with preemption disabled.
25
26 - A CPU looping with bottom halves disabled.
27
28 - For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the
29 kernel without potentially invoking schedule(). If the looping
30 in the kernel is really expected and desirable behavior, you
31 might need to add some calls to cond_resched().
32
33 - Booting Linux using a console connection that is too slow to
34 keep up with the boot-time console-message rate. For example,
35 a 115Kbaud serial console can be *way* too slow to keep up
36 with boot-time message rates, and will frequently result in
37 RCU CPU stall warning messages. Especially if you have added
38 debug printk()s.
39
40 - Anything that prevents RCU's grace-period kthreads from running.
41 This can result in the "All QSes seen" console-log message.
42 This message will include information on when the kthread last
43 ran and how often it should be expected to run. It can also
44 result in the ``rcu_.*kthread starved for`` console-log message,
45 which will include additional debugging information.
46
47 - A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might
48 happen to preempt a low-priority task in the middle of an RCU
49 read-side critical section. This is especially damaging if
50 that low-priority task is not permitted to run on any other CPU,
51 in which case the next RCU grace period can never complete, which
52 will eventually cause the system to run out of memory and hang.
53 While the system is in the process of running itself out of
54 memory, you might see stall-warning messages.
55
56 - A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
57 is running at a higher priority than the RCU softirq threads.
58 This will prevent RCU callbacks from ever being invoked,
59 and in a CONFIG_PREEMPT_RCU kernel will further prevent
60 RCU grace periods from ever completing. Either way, the
61 system will eventually run out of memory and hang. In the
62 CONFIG_PREEMPT_RCU case, you might see stall-warning
63 messages.
64
65 You can use the rcutree.kthread_prio kernel boot parameter to
66 increase the scheduling priority of RCU's kthreads, which can
67 help avoid this problem. However, please note that doing this
68 can increase your system's context-switch rate and thus degrade
69 performance.
70
71 - A periodic interrupt whose handler takes longer than the time
72 interval between successive pairs of interrupts. This can
73 prevent RCU's kthreads and softirq handlers from running.
74 Note that certain high-overhead debugging options, for example
75 the function_graph tracer, can result in interrupt handler taking
76 considerably longer than normal, which can in turn result in
77 RCU CPU stall warnings.
78
79 - Testing a workload on a fast system, tuning the stall-warning
80 timeout down to just barely avoid RCU CPU stall warnings, and then
81 running the same workload with the same stall-warning timeout on a
82 slow system. Note that thermal throttling and on-demand governors
83 can cause a single system to be sometimes fast and sometimes slow!
84
85 - A hardware or software issue shuts off the scheduler-clock
86 interrupt on a CPU that is not in dyntick-idle mode. This
87 problem really has happened, and seems to be most likely to
88 result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
89
90 - A hardware or software issue that prevents time-based wakeups
91 from occurring. These issues can range from misconfigured or
92 buggy timer hardware through bugs in the interrupt or exception
93 path (whether hardware, firmware, or software) through bugs
94 in Linux's timer subsystem through bugs in the scheduler, and,
95 yes, even including bugs in RCU itself. It can also result in
96 the ``rcu_.*timer wakeup didn't happen for`` console-log message,
97 which will include additional debugging information.
98
99 - A low-level kernel issue that either fails to invoke one of the
100 variants of rcu_eqs_enter(true), rcu_eqs_exit(true), ct_idle_enter(),
101 ct_idle_exit(), ct_irq_enter(), or ct_irq_exit() on the one
102 hand, or that invokes one of them too many times on the other.
103 Historically, the most frequent issue has been an omission
104 of either irq_enter() or irq_exit(), which in turn invoke
105 ct_irq_enter() or ct_irq_exit(), respectively. Building your
106 kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types
107 of issues, which sometimes arise in architecture-specific code.
108
109 - A bug in the RCU implementation.
110
111 - A hardware failure. This is quite unlikely, but is not at all
112 uncommon in large datacenter. In one memorable case some decades
113 back, a CPU failed in a running system, becoming unresponsive,
114 but not causing an immediate crash. This resulted in a series
115 of RCU CPU stall warnings, eventually leading the realization
116 that the CPU had failed.
117
118 The RCU, RCU-sched, RCU-tasks, and RCU-tasks-trace implementations have
119 CPU stall warning. Note that SRCU does *not* have CPU stall warnings.
120 Please note that RCU only detects CPU stalls when there is a grace period
121 in progress. No grace period, no CPU stall warnings.
122
123 To diagnose the cause of the stall, inspect the stack traces.
124 The offending function will usually be near the top of the stack.
125 If you have a series of stall warnings from a single extended stall,
126 comparing the stack traces can often help determine where the stall
127 is occurring, which will usually be in the function nearest the top of
128 that portion of the stack which remains the same from trace to trace.
129 If you can reliably trigger the stall, ftrace can be quite helpful.
130
131 RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
132 and with RCU's event tracing. For information on RCU's event tracing,
133 see include/trace/events/rcu.h.
134
135
136 Fine-Tuning the RCU CPU Stall Detector
137 ======================================
138
139 The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
140 CPU stall detector, which detects conditions that unduly delay RCU grace
141 periods. This module parameter enables CPU stall detection by default,
142 but may be overridden via boot-time parameter or at runtime via sysfs.
143 The stall detector's idea of what constitutes "unduly delayed" is
144 controlled by a set of kernel configuration variables and cpp macros:
145
146 CONFIG_RCU_CPU_STALL_TIMEOUT
147 ----------------------------
148
149 This kernel configuration parameter defines the period of time
150 that RCU will wait from the beginning of a grace period until it
151 issues an RCU CPU stall warning. This time period is normally
152 21 seconds.
153
154 This configuration parameter may be changed at runtime via the
155 /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
156 this parameter is checked only at the beginning of a cycle.
157 So if you are 10 seconds into a 40-second stall, setting this
158 sysfs parameter to (say) five will shorten the timeout for the
159 *next* stall, or the following warning for the current stall
160 (assuming the stall lasts long enough). It will not affect the
161 timing of the next warning for the current stall.
162
163 Stall-warning messages may be enabled and disabled completely via
164 /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
165
166 CONFIG_RCU_EXP_CPU_STALL_TIMEOUT
167 --------------------------------
168
169 Same as the CONFIG_RCU_CPU_STALL_TIMEOUT parameter but only for
170 the expedited grace period. This parameter defines the period
171 of time that RCU will wait from the beginning of an expedited
172 grace period until it issues an RCU CPU stall warning. This time
173 period is normally 20 milliseconds on Android devices. A zero
174 value causes the CONFIG_RCU_CPU_STALL_TIMEOUT value to be used,
175 after conversion to milliseconds.
176
177 This configuration parameter may be changed at runtime via the
178 /sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout, however
179 this parameter is checked only at the beginning of a cycle. If you
180 are in a current stall cycle, setting it to a new value will change
181 the timeout for the -next- stall.
182
183 Stall-warning messages may be enabled and disabled completely via
184 /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
185
186 RCU_STALL_DELAY_DELTA
187 ---------------------
188
189 Although the lockdep facility is extremely useful, it does add
190 some overhead. Therefore, under CONFIG_PROVE_RCU, the
191 RCU_STALL_DELAY_DELTA macro allows five extra seconds before
192 giving an RCU CPU stall warning message. (This is a cpp
193 macro, not a kernel configuration parameter.)
194
195 RCU_STALL_RAT_DELAY
196 -------------------
197
198 The CPU stall detector tries to make the offending CPU print its
199 own warnings, as this often gives better-quality stack traces.
200 However, if the offending CPU does not detect its own stall in
201 the number of jiffies specified by RCU_STALL_RAT_DELAY, then
202 some other CPU will complain. This delay is normally set to
203 two jiffies. (This is a cpp macro, not a kernel configuration
204 parameter.)
205
206 rcupdate.rcu_task_stall_timeout
207 -------------------------------
208
209 This boot/sysfs parameter controls the RCU-tasks and
210 RCU-tasks-trace stall warning intervals. A value of zero or less
211 suppresses RCU-tasks stall warnings. A positive value sets the
212 stall-warning interval in seconds. An RCU-tasks stall warning
213 starts with the line:
214
215 INFO: rcu_tasks detected stalls on tasks:
216
217 And continues with the output of sched_show_task() for each
218 task stalling the current RCU-tasks grace period.
219
220 An RCU-tasks-trace stall warning starts (and continues) similarly:
221
222 INFO: rcu_tasks_trace detected stalls on tasks
223
224
225 Interpreting RCU's CPU Stall-Detector "Splats"
226 ==============================================
227
228 For non-RCU-tasks flavors of RCU, when a CPU detects that some other
229 CPU is stalling, it will print a message similar to the following::
230
231 INFO: rcu_sched detected stalls on CPUs/tasks:
232 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
233 16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
234 (detected by 32, t=2603 jiffies, g=7075, q=625)
235
236 This message indicates that CPU 32 detected that CPUs 2 and 16 were both
237 causing stalls, and that the stall was affecting RCU-sched. This message
238 will normally be followed by stack dumps for each CPU. Please note that
239 PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that
240 the tasks will be indicated by PID, for example, "P3421". It is even
241 possible for an rcu_state stall to be caused by both CPUs *and* tasks,
242 in which case the offending CPUs and tasks will all be called out in the list.
243 In some cases, CPUs will detect themselves stalling, which will result
244 in a self-detected stall.
245
246 CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
247 the RCU core for the past three grace periods. In contrast, CPU 16's "(0
248 ticks this GP)" indicates that this CPU has not taken any scheduling-clock
249 interrupts during the current stalled grace period.
250
251 The "idle=" portion of the message prints the dyntick-idle state.
252 The hex number before the first "/" is the low-order 12 bits of the
253 dynticks counter, which will have an even-numbered value if the CPU
254 is in dyntick-idle mode and an odd-numbered value otherwise. The hex
255 number between the two "/"s is the value of the nesting, which will be
256 a small non-negative number if in the idle loop (as shown above) and a
257 very large positive number otherwise. The number following the final
258 "/" is the NMI nesting, which will be a small non-negative number.
259
260 The "softirq=" portion of the message tracks the number of RCU softirq
261 handlers that the stalled CPU has executed. The number before the "/"
262 is the number that had executed since boot at the time that this CPU
263 last noted the beginning of a grace period, which might be the current
264 (stalled) grace period, or it might be some earlier grace period (for
265 example, if the CPU might have been in dyntick-idle mode for an extended
266 time period). The number after the "/" is the number that have executed
267 since boot until the current time. If this latter number stays constant
268 across repeated stall-warning messages, it is possible that RCU's softirq
269 handlers are no longer able to execute on this CPU. This can happen if
270 the stalled CPU is spinning with interrupts are disabled, or, in -rt
271 kernels, if a high-priority process is starving RCU's softirq handler.
272
273 The "fqs=" shows the number of force-quiescent-state idle/offline
274 detection passes that the grace-period kthread has made across this
275 CPU since the last time that this CPU noted the beginning of a grace
276 period.
277
278 The "detected by" line indicates which CPU detected the stall (in this
279 case, CPU 32), how many jiffies have elapsed since the start of the grace
280 period (in this case 2603), the grace-period sequence number (7075), and
281 an estimate of the total number of RCU callbacks queued across all CPUs
282 (625 in this case).
283
284 If the grace period ends just as the stall warning starts printing,
285 there will be a spurious stall-warning message, which will include
286 the following::
287
288 INFO: Stall ended before state dump start
289
290 This is rare, but does happen from time to time in real life. It is also
291 possible for a zero-jiffy stall to be flagged in this case, depending
292 on how the stall warning and the grace-period initialization happen to
293 interact. Please note that it is not possible to entirely eliminate this
294 sort of false positive without resorting to things like stop_machine(),
295 which is overkill for this sort of problem.
296
297 If all CPUs and tasks have passed through quiescent states, but the
298 grace period has nevertheless failed to end, the stall-warning splat
299 will include something like the following::
300
301 All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0
302
303 The "23807" indicates that it has been more than 23 thousand jiffies
304 since the grace-period kthread ran. The "jiffies_till_next_fqs"
305 indicates how frequently that kthread should run, giving the number
306 of jiffies between force-quiescent-state scans, in this case three,
307 which is way less than 23807. Finally, the root rcu_node structure's
308 ->qsmask field is printed, which will normally be zero.
309
310 If the relevant grace-period kthread has been unable to run prior to
311 the stall warning, as was the case in the "All QSes seen" line above,
312 the following additional line is printed::
313
314 rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
315 Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
316
317 Starving the grace-period kthreads of CPU time can of course result
318 in RCU CPU stall warnings even when all CPUs and tasks have passed
319 through the required quiescent states. The "g" number shows the current
320 grace-period sequence number, the "f" precedes the ->gp_flags command
321 to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the
322 kthread is waiting for a short timeout, the "state" precedes value of the
323 task_struct ->state field, and the "cpu" indicates that the grace-period
324 kthread last ran on CPU 5.
325
326 If the relevant grace-period kthread does not wake from FQS wait in a
327 reasonable time, then the following additional line is printed::
328
329 kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
330
331 The "23804" indicates that kthread's timer expired more than 23 thousand
332 jiffies ago. The rest of the line has meaning similar to the kthread
333 starvation case.
334
335 Additionally, the following line is printed::
336
337 Possible timer handling issue on cpu=4 timer-softirq=11142
338
339 Here "cpu" indicates that the grace-period kthread last ran on CPU 4,
340 where it queued the fqs timer. The number following the "timer-softirq"
341 is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not
342 change on successive RCU CPU stall warnings, there is further reason to
343 suspect a timer problem.
344
345 These messages are usually followed by stack dumps of the CPUs and tasks
346 involved in the stall. These stack traces can help you locate the cause
347 of the stall, keeping in mind that the CPU detecting the stall will have
348 an interrupt frame that is mainly devoted to detecting the stall.
349
350
351 Multiple Warnings From One Stall
352 ================================
353
354 If a stall lasts long enough, multiple stall-warning messages will
355 be printed for it. The second and subsequent messages are printed at
356 longer intervals, so that the time between (say) the first and second
357 message will be about three times the interval between the beginning
358 of the stall and the first message. It can be helpful to compare the
359 stack dumps for the different messages for the same stalled grace period.
360
361
362 Stall Warnings for Expedited Grace Periods
363 ==========================================
364
365 If an expedited grace period detects a stall, it will place a message
366 like the following in dmesg::
367
368 INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
369
370 This indicates that CPU 7 has failed to respond to a reschedule IPI.
371 The three periods (".") following the CPU number indicate that the CPU
372 is online (otherwise the first period would instead have been "O"),
373 that the CPU was online at the beginning of the expedited grace period
374 (otherwise the second period would have instead been "o"), and that
375 the CPU has been online at least once since boot (otherwise, the third
376 period would instead have been "N"). The number before the "jiffies"
377 indicates that the expedited grace period has been going on for 21,119
378 jiffies. The number following the "s:" indicates that the expedited
379 grace-period sequence counter is 73. The fact that this last value is
380 odd indicates that an expedited grace period is in flight. The number
381 following "root:" is a bitmask that indicates which children of the root
382 rcu_node structure correspond to CPUs and/or tasks that are blocking the
383 current expedited grace period. If the tree had more than one level,
384 additional hex numbers would be printed for the states of the other
385 rcu_node structures in the tree.
386
387 As with normal grace periods, PREEMPT_RCU builds can be stalled by
388 tasks as well as by CPUs, and that the tasks will be indicated by PID,
389 for example, "P3421".
390
391 It is entirely possible to see stall warnings from normal and from
392 expedited grace periods at about the same time during the same run.
393
394 RCU_CPU_STALL_CPUTIME
395 =====================
396
397 In kernels built with CONFIG_RCU_CPU_STALL_CPUTIME=y or booted with
398 rcupdate.rcu_cpu_stall_cputime=1, the following additional information
399 is supplied with each RCU CPU stall warning::
400
401 rcu: hardirqs softirqs csw/system
402 rcu: number: 624 45 0
403 rcu: cputime: 69 1 2425 ==> 2500(ms)
404
405 These statistics are collected during the sampling period. The values
406 in row "number:" are the number of hard interrupts, number of soft
407 interrupts, and number of context switches on the stalled CPU. The
408 first three values in row "cputime:" indicate the CPU time in
409 milliseconds consumed by hard interrupts, soft interrupts, and tasks
410 on the stalled CPU. The last number is the measurement interval, again
411 in milliseconds. Because user-mode tasks normally do not cause RCU CPU
412 stalls, these tasks are typically kernel tasks, which is why only the
413 system CPU time are considered.
414
415 The sampling period is shown as follows::
416
417 |<------------first timeout---------->|<-----second timeout----->|
418 |<--half timeout-->|<--half timeout-->| |
419 | |<--first period-->| |
420 | |<-----------second sampling period---------->|
421 | | | |
422 snapshot time point 1st-stall 2nd-stall
423
424 The following describes four typical scenarios:
425
426 1. A CPU looping with interrupts disabled.
427
428 ::
429
430 rcu: hardirqs softirqs csw/system
431 rcu: number: 0 0 0
432 rcu: cputime: 0 0 0 ==> 2500(ms)
433
434 Because interrupts have been disabled throughout the measurement
435 interval, there are no interrupts and no context switches.
436 Furthermore, because CPU time consumption was measured using interrupt
437 handlers, the system CPU consumption is misleadingly measured as zero.
438 This scenario will normally also have "(0 ticks this GP)" printed on
439 this CPU's summary line.
440
441 2. A CPU looping with bottom halves disabled.
442
443 This is similar to the previous example, but with non-zero number of
444 and CPU time consumed by hard interrupts, along with non-zero CPU
445 time consumed by in-kernel execution::
446
447 rcu: hardirqs softirqs csw/system
448 rcu: number: 624 0 0
449 rcu: cputime: 49 0 2446 ==> 2500(ms)
450
451 The fact that there are zero softirqs gives a hint that these were
452 disabled, perhaps via local_bh_disable(). It is of course possible
453 that there were no softirqs, perhaps because all events that would
454 result in softirq execution are confined to other CPUs. In this case,
455 the diagnosis should continue as shown in the next example.
456
457 3. A CPU looping with preemption disabled.
458
459 Here, only the number of context switches is zero::
460
461 rcu: hardirqs softirqs csw/system
462 rcu: number: 624 45 0
463 rcu: cputime: 69 1 2425 ==> 2500(ms)
464
465 This situation hints that the stalled CPU was looping with preemption
466 disabled.
467
468 4. No looping, but massive hard and soft interrupts.
469
470 ::
471
472 rcu: hardirqs softirqs csw/system
473 rcu: number: xx xx 0
474 rcu: cputime: xx xx 0 ==> 2500(ms)
475
476 Here, the number and CPU time of hard interrupts are all non-zero,
477 but the number of context switches and the in-kernel CPU time consumed
478 are zero. The number and cputime of soft interrupts will usually be
479 non-zero, but could be zero, for example, if the CPU was spinning
480 within a single hard interrupt handler.
481
482 If this type of RCU CPU stall warning can be reproduced, you can
483 narrow it down by looking at /proc/interrupts or by writing code to
484 trace each interrupt, for example, by referring to show_interrupts().
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.