1.. SPDX-License-Identifier: GPL-2.0
2
3==============================
4Using RCU's CPU Stall Detector
5==============================
6
7This document first discusses what sorts of issues RCU's CPU stall
8detector can locate, and then discusses kernel parameters and Kconfig
9options that can be used to fine-tune the detector's operation.  Finally,
10this document explains the stall detector's "splat" format.
11
12
13What Causes RCU CPU Stall Warnings?
14===================================
15
16So 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
18warnings:
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 kernel
29	without invoking schedule().  If the looping in the kernel is
30	really expected and desirable behavior, you might need to add
31	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_user_enter(), rcu_user_exit(), rcu_idle_enter(),
101	rcu_idle_exit(), rcu_irq_enter(), or rcu_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	rcu_irq_enter() or rcu_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 has occurred
112	at least once in real life.  A CPU failed in a running system,
113	becoming unresponsive, but not causing an immediate crash.
114	This resulted in a series of RCU CPU stall warnings, eventually
115	leading the realization that the CPU had failed.
116
117The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning.
118Note that SRCU does *not* have CPU stall warnings.  Please note that
119RCU only detects CPU stalls when there is a grace period in progress.
120No grace period, no CPU stall warnings.
121
122To diagnose the cause of the stall, inspect the stack traces.
123The offending function will usually be near the top of the stack.
124If you have a series of stall warnings from a single extended stall,
125comparing the stack traces can often help determine where the stall
126is occurring, which will usually be in the function nearest the top of
127that portion of the stack which remains the same from trace to trace.
128If you can reliably trigger the stall, ftrace can be quite helpful.
129
130RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
131and with RCU's event tracing.  For information on RCU's event tracing,
132see include/trace/events/rcu.h.
133
134
135Fine-Tuning the RCU CPU Stall Detector
136======================================
137
138The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
139CPU stall detector, which detects conditions that unduly delay RCU grace
140periods.  This module parameter enables CPU stall detection by default,
141but may be overridden via boot-time parameter or at runtime via sysfs.
142The stall detector's idea of what constitutes "unduly delayed" is
143controlled by a set of kernel configuration variables and cpp macros:
144
145CONFIG_RCU_CPU_STALL_TIMEOUT
146----------------------------
147
148	This kernel configuration parameter defines the period of time
149	that RCU will wait from the beginning of a grace period until it
150	issues an RCU CPU stall warning.  This time period is normally
151	21 seconds.
152
153	This configuration parameter may be changed at runtime via the
154	/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
155	this parameter is checked only at the beginning of a cycle.
156	So if you are 10 seconds into a 40-second stall, setting this
157	sysfs parameter to (say) five will shorten the timeout for the
158	*next* stall, or the following warning for the current stall
159	(assuming the stall lasts long enough).  It will not affect the
160	timing of the next warning for the current stall.
161
162	Stall-warning messages may be enabled and disabled completely via
163	/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
164
165CONFIG_RCU_EXP_CPU_STALL_TIMEOUT
166--------------------------------
167
168	Same as the CONFIG_RCU_CPU_STALL_TIMEOUT parameter but only for
169	the expedited grace period. This parameter defines the period
170	of time that RCU will wait from the beginning of an expedited
171	grace period until it issues an RCU CPU stall warning. This time
172	period is normally 20 milliseconds on Android devices.	A zero
173	value causes the CONFIG_RCU_CPU_STALL_TIMEOUT value to be used,
174	after conversion to milliseconds.
175
176	This configuration parameter may be changed at runtime via the
177	/sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout, however
178	this parameter is checked only at the beginning of a cycle. If you
179	are in a current stall cycle, setting it to a new value will change
180	the timeout for the -next- stall.
181
182	Stall-warning messages may be enabled and disabled completely via
183	/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
184
185RCU_STALL_DELAY_DELTA
186---------------------
187
188	Although the lockdep facility is extremely useful, it does add
189	some overhead.  Therefore, under CONFIG_PROVE_RCU, the
190	RCU_STALL_DELAY_DELTA macro allows five extra seconds before
191	giving an RCU CPU stall warning message.  (This is a cpp
192	macro, not a kernel configuration parameter.)
193
194RCU_STALL_RAT_DELAY
195-------------------
196
197	The CPU stall detector tries to make the offending CPU print its
198	own warnings, as this often gives better-quality stack traces.
199	However, if the offending CPU does not detect its own stall in
200	the number of jiffies specified by RCU_STALL_RAT_DELAY, then
201	some other CPU will complain.  This delay is normally set to
202	two jiffies.  (This is a cpp macro, not a kernel configuration
203	parameter.)
204
205rcupdate.rcu_task_stall_timeout
206-------------------------------
207
208	This boot/sysfs parameter controls the RCU-tasks stall warning
209	interval.  A value of zero or less suppresses RCU-tasks stall
210	warnings.  A positive value sets the stall-warning interval
211	in seconds.  An RCU-tasks stall warning starts with the line:
212
213		INFO: rcu_tasks detected stalls on tasks:
214
215	And continues with the output of sched_show_task() for each
216	task stalling the current RCU-tasks grace period.
217
218
219Interpreting RCU's CPU Stall-Detector "Splats"
220==============================================
221
222For non-RCU-tasks flavors of RCU, when a CPU detects that some other
223CPU is stalling, it will print a message similar to the following::
224
225	INFO: rcu_sched detected stalls on CPUs/tasks:
226	2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
227	16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
228	(detected by 32, t=2603 jiffies, g=7075, q=625)
229
230This message indicates that CPU 32 detected that CPUs 2 and 16 were both
231causing stalls, and that the stall was affecting RCU-sched.  This message
232will normally be followed by stack dumps for each CPU.  Please note that
233PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that
234the tasks will be indicated by PID, for example, "P3421".  It is even
235possible for an rcu_state stall to be caused by both CPUs *and* tasks,
236in which case the offending CPUs and tasks will all be called out in the list.
237In some cases, CPUs will detect themselves stalling, which will result
238in a self-detected stall.
239
240CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
241the RCU core for the past three grace periods.  In contrast, CPU 16's "(0
242ticks this GP)" indicates that this CPU has not taken any scheduling-clock
243interrupts during the current stalled grace period.
244
245The "idle=" portion of the message prints the dyntick-idle state.
246The hex number before the first "/" is the low-order 12 bits of the
247dynticks counter, which will have an even-numbered value if the CPU
248is in dyntick-idle mode and an odd-numbered value otherwise.  The hex
249number between the two "/"s is the value of the nesting, which will be
250a small non-negative number if in the idle loop (as shown above) and a
251very large positive number otherwise.
252
253The "softirq=" portion of the message tracks the number of RCU softirq
254handlers that the stalled CPU has executed.  The number before the "/"
255is the number that had executed since boot at the time that this CPU
256last noted the beginning of a grace period, which might be the current
257(stalled) grace period, or it might be some earlier grace period (for
258example, if the CPU might have been in dyntick-idle mode for an extended
259time period).  The number after the "/" is the number that have executed
260since boot until the current time.  If this latter number stays constant
261across repeated stall-warning messages, it is possible that RCU's softirq
262handlers are no longer able to execute on this CPU.  This can happen if
263the stalled CPU is spinning with interrupts are disabled, or, in -rt
264kernels, if a high-priority process is starving RCU's softirq handler.
265
266The "fqs=" shows the number of force-quiescent-state idle/offline
267detection passes that the grace-period kthread has made across this
268CPU since the last time that this CPU noted the beginning of a grace
269period.
270
271The "detected by" line indicates which CPU detected the stall (in this
272case, CPU 32), how many jiffies have elapsed since the start of the grace
273period (in this case 2603), the grace-period sequence number (7075), and
274an estimate of the total number of RCU callbacks queued across all CPUs
275(625 in this case).
276
277If the grace period ends just as the stall warning starts printing,
278there will be a spurious stall-warning message, which will include
279the following::
280
281	INFO: Stall ended before state dump start
282
283This is rare, but does happen from time to time in real life.  It is also
284possible for a zero-jiffy stall to be flagged in this case, depending
285on how the stall warning and the grace-period initialization happen to
286interact.  Please note that it is not possible to entirely eliminate this
287sort of false positive without resorting to things like stop_machine(),
288which is overkill for this sort of problem.
289
290If all CPUs and tasks have passed through quiescent states, but the
291grace period has nevertheless failed to end, the stall-warning splat
292will include something like the following::
293
294	All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0
295
296The "23807" indicates that it has been more than 23 thousand jiffies
297since the grace-period kthread ran.  The "jiffies_till_next_fqs"
298indicates how frequently that kthread should run, giving the number
299of jiffies between force-quiescent-state scans, in this case three,
300which is way less than 23807.  Finally, the root rcu_node structure's
301->qsmask field is printed, which will normally be zero.
302
303If the relevant grace-period kthread has been unable to run prior to
304the stall warning, as was the case in the "All QSes seen" line above,
305the following additional line is printed::
306
307	rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
308	Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
309
310Starving the grace-period kthreads of CPU time can of course result
311in RCU CPU stall warnings even when all CPUs and tasks have passed
312through the required quiescent states.  The "g" number shows the current
313grace-period sequence number, the "f" precedes the ->gp_flags command
314to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the
315kthread is waiting for a short timeout, the "state" precedes value of the
316task_struct ->state field, and the "cpu" indicates that the grace-period
317kthread last ran on CPU 5.
318
319If the relevant grace-period kthread does not wake from FQS wait in a
320reasonable time, then the following additional line is printed::
321
322	kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
323
324The "23804" indicates that kthread's timer expired more than 23 thousand
325jiffies ago.  The rest of the line has meaning similar to the kthread
326starvation case.
327
328Additionally, the following line is printed::
329
330	Possible timer handling issue on cpu=4 timer-softirq=11142
331
332Here "cpu" indicates that the grace-period kthread last ran on CPU 4,
333where it queued the fqs timer.  The number following the "timer-softirq"
334is the current ``TIMER_SOFTIRQ`` count on cpu 4.  If this value does not
335change on successive RCU CPU stall warnings, there is further reason to
336suspect a timer problem.
337
338These messages are usually followed by stack dumps of the CPUs and tasks
339involved in the stall.  These stack traces can help you locate the cause
340of the stall, keeping in mind that the CPU detecting the stall will have
341an interrupt frame that is mainly devoted to detecting the stall.
342
343
344Multiple Warnings From One Stall
345================================
346
347If a stall lasts long enough, multiple stall-warning messages will
348be printed for it.  The second and subsequent messages are printed at
349longer intervals, so that the time between (say) the first and second
350message will be about three times the interval between the beginning
351of the stall and the first message.  It can be helpful to compare the
352stack dumps for the different messages for the same stalled grace period.
353
354
355Stall Warnings for Expedited Grace Periods
356==========================================
357
358If an expedited grace period detects a stall, it will place a message
359like the following in dmesg::
360
361	INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
362
363This indicates that CPU 7 has failed to respond to a reschedule IPI.
364The three periods (".") following the CPU number indicate that the CPU
365is online (otherwise the first period would instead have been "O"),
366that the CPU was online at the beginning of the expedited grace period
367(otherwise the second period would have instead been "o"), and that
368the CPU has been online at least once since boot (otherwise, the third
369period would instead have been "N").  The number before the "jiffies"
370indicates that the expedited grace period has been going on for 21,119
371jiffies.  The number following the "s:" indicates that the expedited
372grace-period sequence counter is 73.  The fact that this last value is
373odd indicates that an expedited grace period is in flight.  The number
374following "root:" is a bitmask that indicates which children of the root
375rcu_node structure correspond to CPUs and/or tasks that are blocking the
376current expedited grace period.  If the tree had more than one level,
377additional hex numbers would be printed for the states of the other
378rcu_node structures in the tree.
379
380As with normal grace periods, PREEMPT_RCU builds can be stalled by
381tasks as well as by CPUs, and that the tasks will be indicated by PID,
382for example, "P3421".
383
384It is entirely possible to see stall warnings from normal and from
385expedited grace periods at about the same time during the same run.
386