1 Using RCU's CPU Stall Detector
3 This document first discusses what sorts of issues RCU's CPU stall
4 detector can locate, and then discusses kernel parameters and Kconfig
5 options that can be used to fine-tune the detector's operation. Finally,
6 this document explains the stall detector's "splat" format.
9 What Causes RCU CPU Stall Warnings?
11 So your kernel printed an RCU CPU stall warning. The next question is
12 "What caused it?" The following problems can result in RCU CPU stall
15 o A CPU looping in an RCU read-side critical section.
17 o A CPU looping with interrupts disabled.
19 o A CPU looping with preemption disabled. This condition can
20 result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
23 o A CPU looping with bottom halves disabled. This condition can
24 result in RCU-sched and RCU-bh stalls.
26 o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
27 kernel without invoking schedule(). Note that cond_resched()
28 does not necessarily prevent RCU CPU stall warnings. Therefore,
29 if the looping in the kernel is really expected and desirable
30 behavior, you might need to replace some of the cond_resched()
31 calls with calls to cond_resched_rcu_qs().
33 o 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
40 o 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.
47 o A CPU-bound real-time task in a CONFIG_PREEMPT 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.
56 o 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
65 o A periodic interrupt whose handler takes longer than the time
66 interval between successive pairs of interrupts. This can
67 prevent RCU's kthreads and softirq handlers from running.
68 Note that certain high-overhead debugging options, for example
69 the function_graph tracer, can result in interrupt handler taking
70 considerably longer than normal, which can in turn result in
71 RCU CPU stall warnings.
73 o Testing a workload on a fast system, tuning the stall-warning
74 timeout down to just barely avoid RCU CPU stall warnings, and then
75 running the same workload with the same stall-warning timeout on a
76 slow system. Note that thermal throttling and on-demand governors
77 can cause a single system to be sometimes fast and sometimes slow!
79 o A hardware or software issue shuts off the scheduler-clock
80 interrupt on a CPU that is not in dyntick-idle mode. This
81 problem really has happened, and seems to be most likely to
82 result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
84 o A bug in the RCU implementation.
86 o A hardware failure. This is quite unlikely, but has occurred
87 at least once in real life. A CPU failed in a running system,
88 becoming unresponsive, but not causing an immediate crash.
89 This resulted in a series of RCU CPU stall warnings, eventually
90 leading the realization that the CPU had failed.
92 The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
93 warning. Note that SRCU does -not- have CPU stall warnings. Please note
94 that RCU only detects CPU stalls when there is a grace period in progress.
95 No grace period, no CPU stall warnings.
97 To diagnose the cause of the stall, inspect the stack traces.
98 The offending function will usually be near the top of the stack.
99 If you have a series of stall warnings from a single extended stall,
100 comparing the stack traces can often help determine where the stall
101 is occurring, which will usually be in the function nearest the top of
102 that portion of the stack which remains the same from trace to trace.
103 If you can reliably trigger the stall, ftrace can be quite helpful.
105 RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
106 and with RCU's event tracing. For information on RCU's event tracing,
107 see include/trace/events/rcu.h.
110 Fine-Tuning the RCU CPU Stall Detector
112 The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
113 CPU stall detector, which detects conditions that unduly delay RCU grace
114 periods. This module parameter enables CPU stall detection by default,
115 but may be overridden via boot-time parameter or at runtime via sysfs.
116 The stall detector's idea of what constitutes "unduly delayed" is
117 controlled by a set of kernel configuration variables and cpp macros:
119 CONFIG_RCU_CPU_STALL_TIMEOUT
121 This kernel configuration parameter defines the period of time
122 that RCU will wait from the beginning of a grace period until it
123 issues an RCU CPU stall warning. This time period is normally
126 This configuration parameter may be changed at runtime via the
127 /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
128 this parameter is checked only at the beginning of a cycle.
129 So if you are 10 seconds into a 40-second stall, setting this
130 sysfs parameter to (say) five will shorten the timeout for the
131 -next- stall, or the following warning for the current stall
132 (assuming the stall lasts long enough). It will not affect the
133 timing of the next warning for the current stall.
135 Stall-warning messages may be enabled and disabled completely via
136 /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
138 RCU_STALL_DELAY_DELTA
140 Although the lockdep facility is extremely useful, it does add
141 some overhead. Therefore, under CONFIG_PROVE_RCU, the
142 RCU_STALL_DELAY_DELTA macro allows five extra seconds before
143 giving an RCU CPU stall warning message. (This is a cpp
144 macro, not a kernel configuration parameter.)
148 The CPU stall detector tries to make the offending CPU print its
149 own warnings, as this often gives better-quality stack traces.
150 However, if the offending CPU does not detect its own stall in
151 the number of jiffies specified by RCU_STALL_RAT_DELAY, then
152 some other CPU will complain. This delay is normally set to
153 two jiffies. (This is a cpp macro, not a kernel configuration
156 rcupdate.rcu_task_stall_timeout
158 This boot/sysfs parameter controls the RCU-tasks stall warning
159 interval. A value of zero or less suppresses RCU-tasks stall
160 warnings. A positive value sets the stall-warning interval
161 in jiffies. An RCU-tasks stall warning starts with the line:
163 INFO: rcu_tasks detected stalls on tasks:
165 And continues with the output of sched_show_task() for each
166 task stalling the current RCU-tasks grace period.
169 Interpreting RCU's CPU Stall-Detector "Splats"
171 For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
172 it will print a message similar to the following:
174 INFO: rcu_sched detected stalls on CPUs/tasks:
175 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
176 16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
177 (detected by 32, t=2603 jiffies, g=7073, c=7072, q=625)
179 This message indicates that CPU 32 detected that CPUs 2 and 16 were both
180 causing stalls, and that the stall was affecting RCU-sched. This message
181 will normally be followed by stack dumps for each CPU. Please note that
182 PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that
183 the tasks will be indicated by PID, for example, "P3421". It is even
184 possible for a rcu_preempt_state stall to be caused by both CPUs -and-
185 tasks, in which case the offending CPUs and tasks will all be called
188 CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
189 the RCU core for the past three grace periods. In contrast, CPU 16's "(0
190 ticks this GP)" indicates that this CPU has not taken any scheduling-clock
191 interrupts during the current stalled grace period.
193 The "idle=" portion of the message prints the dyntick-idle state.
194 The hex number before the first "/" is the low-order 12 bits of the
195 dynticks counter, which will have an even-numbered value if the CPU
196 is in dyntick-idle mode and an odd-numbered value otherwise. The hex
197 number between the two "/"s is the value of the nesting, which will be
198 a small non-negative number if in the idle loop (as shown above) and a
199 very large positive number otherwise.
201 The "softirq=" portion of the message tracks the number of RCU softirq
202 handlers that the stalled CPU has executed. The number before the "/"
203 is the number that had executed since boot at the time that this CPU
204 last noted the beginning of a grace period, which might be the current
205 (stalled) grace period, or it might be some earlier grace period (for
206 example, if the CPU might have been in dyntick-idle mode for an extended
207 time period. The number after the "/" is the number that have executed
208 since boot until the current time. If this latter number stays constant
209 across repeated stall-warning messages, it is possible that RCU's softirq
210 handlers are no longer able to execute on this CPU. This can happen if
211 the stalled CPU is spinning with interrupts are disabled, or, in -rt
212 kernels, if a high-priority process is starving RCU's softirq handler.
214 The "fps=" shows the number of force-quiescent-state idle/offline
215 detection passes that the grace-period kthread has made across this
216 CPU since the last time that this CPU noted the beginning of a grace
219 The "detected by" line indicates which CPU detected the stall (in this
220 case, CPU 32), how many jiffies have elapsed since the start of the
221 grace period (in this case 2603), the number of the last grace period
222 to start and to complete (7073 and 7072, respectively), and an estimate
223 of the total number of RCU callbacks queued across all CPUs (625 in
226 In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
229 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
231 The "last_accelerate:" prints the low-order 16 bits (in hex) of the
232 jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
233 from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
234 rcu_prepare_for_idle(). The "nonlazy_posted:" prints the number
235 of non-lazy callbacks posted since the last call to rcu_needs_cpu().
236 Finally, an "L" indicates that there are currently no non-lazy callbacks
237 ("." is printed otherwise, as shown above) and "D" indicates that
238 dyntick-idle processing is enabled ("." is printed otherwise, for example,
239 if disabled via the "nohz=" kernel boot parameter).
241 If the grace period ends just as the stall warning starts printing,
242 there will be a spurious stall-warning message, which will include
245 INFO: Stall ended before state dump start
247 This is rare, but does happen from time to time in real life. It is also
248 possible for a zero-jiffy stall to be flagged in this case, depending
249 on how the stall warning and the grace-period initialization happen to
250 interact. Please note that it is not possible to entirely eliminate this
251 sort of false positive without resorting to things like stop_machine(),
252 which is overkill for this sort of problem.
254 If all CPUs and tasks have passed through quiescent states, but the
255 grace period has nevertheless failed to end, the stall-warning splat
256 will include something like the following:
258 All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0
260 The "23807" indicates that it has been more than 23 thousand jiffies
261 since the grace-period kthread ran. The "jiffies_till_next_fqs"
262 indicates how frequently that kthread should run, giving the number
263 of jiffies between force-quiescent-state scans, in this case three,
264 which is way less than 23807. Finally, the root rcu_node structure's
265 ->qsmask field is printed, which will normally be zero.
267 If the relevant grace-period kthread has been unable to run prior to
268 the stall warning, as was the case in the "All QSes seen" line above,
269 the following additional line is printed:
271 kthread starved for 23807 jiffies! g7073 c7072 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
273 Starving the grace-period kthreads of CPU time can of course result
274 in RCU CPU stall warnings even when all CPUs and tasks have passed
275 through the required quiescent states. The "g" and "c" numbers flag the
276 number of the last grace period started and completed, respectively,
277 the "f" precedes the ->gp_flags command to the grace-period kthread,
278 the "RCU_GP_WAIT_FQS" indicates that the kthread is waiting for a short
279 timeout, and the "state" precedes value of the task_struct ->state field.
282 Multiple Warnings From One Stall
284 If a stall lasts long enough, multiple stall-warning messages will be
285 printed for it. The second and subsequent messages are printed at
286 longer intervals, so that the time between (say) the first and second
287 message will be about three times the interval between the beginning
288 of the stall and the first message.
291 Stall Warnings for Expedited Grace Periods
293 If an expedited grace period detects a stall, it will place a message
294 like the following in dmesg:
296 INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
298 This indicates that CPU 7 has failed to respond to a reschedule IPI.
299 The three periods (".") following the CPU number indicate that the CPU
300 is online (otherwise the first period would instead have been "O"),
301 that the CPU was online at the beginning of the expedited grace period
302 (otherwise the second period would have instead been "o"), and that
303 the CPU has been online at least once since boot (otherwise, the third
304 period would instead have been "N"). The number before the "jiffies"
305 indicates that the expedited grace period has been going on for 21,119
306 jiffies. The number following the "s:" indicates that the expedited
307 grace-period sequence counter is 73. The fact that this last value is
308 odd indicates that an expedited grace period is in flight. The number
309 following "root:" is a bitmask that indicates which children of the root
310 rcu_node structure correspond to CPUs and/or tasks that are blocking the
311 current expedited grace period. If the tree had more than one level,
312 additional hex numbers would be printed for the states of the other
313 rcu_node structures in the tree.
315 As with normal grace periods, PREEMPT_RCU builds can be stalled by
316 tasks as well as by CPUs, and that the tasks will be indicated by PID,
317 for example, "P3421".
319 It is entirely possible to see stall warnings from normal and from
320 expedited grace periods at about the same time during the same run.