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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	Using RCU's CPU Stall Detector
2	
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.
7	
8	
9	What Causes RCU CPU Stall Warnings?
10	
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
13	warnings:
14	
15	o	A CPU looping in an RCU read-side critical section.
16	
17	o	A CPU looping with interrupts disabled.
18	
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
21		stalls.
22	
23	o	A CPU looping with bottom halves disabled.  This condition can
24		result in RCU-sched and RCU-bh stalls.
25	
26	o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
27		without invoking schedule().  If the looping in the kernel is
28		really expected and desirable behavior, you might need to add
29		some calls to cond_resched().
30	
31	o	Booting Linux using a console connection that is too slow to
32		keep up with the boot-time console-message rate.  For example,
33		a 115Kbaud serial console can be -way- too slow to keep up
34		with boot-time message rates, and will frequently result in
35		RCU CPU stall warning messages.  Especially if you have added
36		debug printk()s.
37	
38	o	Anything that prevents RCU's grace-period kthreads from running.
39		This can result in the "All QSes seen" console-log message.
40		This message will include information on when the kthread last
41		ran and how often it should be expected to run.  It can also
42		result in the "rcu_.*kthread starved for" console-log message,
43		which will include additional debugging information.
44	
45	o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
46		happen to preempt a low-priority task in the middle of an RCU
47		read-side critical section.   This is especially damaging if
48		that low-priority task is not permitted to run on any other CPU,
49		in which case the next RCU grace period can never complete, which
50		will eventually cause the system to run out of memory and hang.
51		While the system is in the process of running itself out of
52		memory, you might see stall-warning messages.
53	
54	o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
55		is running at a higher priority than the RCU softirq threads.
56		This will prevent RCU callbacks from ever being invoked,
57		and in a CONFIG_PREEMPT_RCU kernel will further prevent
58		RCU grace periods from ever completing.  Either way, the
59		system will eventually run out of memory and hang.  In the
60		CONFIG_PREEMPT_RCU case, you might see stall-warning
61		messages.
62	
63	o	A periodic interrupt whose handler takes longer than the time
64		interval between successive pairs of interrupts.  This can
65		prevent RCU's kthreads and softirq handlers from running.
66		Note that certain high-overhead debugging options, for example
67		the function_graph tracer, can result in interrupt handler taking
68		considerably longer than normal, which can in turn result in
69		RCU CPU stall warnings.
70	
71	o	Testing a workload on a fast system, tuning the stall-warning
72		timeout down to just barely avoid RCU CPU stall warnings, and then
73		running the same workload with the same stall-warning timeout on a
74		slow system.  Note that thermal throttling and on-demand governors
75		can cause a single system to be sometimes fast and sometimes slow!
76	
77	o	A hardware or software issue shuts off the scheduler-clock
78		interrupt on a CPU that is not in dyntick-idle mode.  This
79		problem really has happened, and seems to be most likely to
80		result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
81	
82	o	A bug in the RCU implementation.
83	
84	o	A hardware failure.  This is quite unlikely, but has occurred
85		at least once in real life.  A CPU failed in a running system,
86		becoming unresponsive, but not causing an immediate crash.
87		This resulted in a series of RCU CPU stall warnings, eventually
88		leading the realization that the CPU had failed.
89	
90	The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
91	warning.  Note that SRCU does -not- have CPU stall warnings.  Please note
92	that RCU only detects CPU stalls when there is a grace period in progress.
93	No grace period, no CPU stall warnings.
94	
95	To diagnose the cause of the stall, inspect the stack traces.
96	The offending function will usually be near the top of the stack.
97	If you have a series of stall warnings from a single extended stall,
98	comparing the stack traces can often help determine where the stall
99	is occurring, which will usually be in the function nearest the top of
100	that portion of the stack which remains the same from trace to trace.
101	If you can reliably trigger the stall, ftrace can be quite helpful.
102	
103	RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
104	and with RCU's event tracing.  For information on RCU's event tracing,
105	see include/trace/events/rcu.h.
106	
107	
108	Fine-Tuning the RCU CPU Stall Detector
109	
110	The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
111	CPU stall detector, which detects conditions that unduly delay RCU grace
112	periods.  This module parameter enables CPU stall detection by default,
113	but may be overridden via boot-time parameter or at runtime via sysfs.
114	The stall detector's idea of what constitutes "unduly delayed" is
115	controlled by a set of kernel configuration variables and cpp macros:
116	
117	CONFIG_RCU_CPU_STALL_TIMEOUT
118	
119		This kernel configuration parameter defines the period of time
120		that RCU will wait from the beginning of a grace period until it
121		issues an RCU CPU stall warning.  This time period is normally
122		21 seconds.
123	
124		This configuration parameter may be changed at runtime via the
125		/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
126		this parameter is checked only at the beginning of a cycle.
127		So if you are 10 seconds into a 40-second stall, setting this
128		sysfs parameter to (say) five will shorten the timeout for the
129		-next- stall, or the following warning for the current stall
130		(assuming the stall lasts long enough).  It will not affect the
131		timing of the next warning for the current stall.
132	
133		Stall-warning messages may be enabled and disabled completely via
134		/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
135	
136	RCU_STALL_DELAY_DELTA
137	
138		Although the lockdep facility is extremely useful, it does add
139		some overhead.  Therefore, under CONFIG_PROVE_RCU, the
140		RCU_STALL_DELAY_DELTA macro allows five extra seconds before
141		giving an RCU CPU stall warning message.  (This is a cpp
142		macro, not a kernel configuration parameter.)
143	
144	RCU_STALL_RAT_DELAY
145	
146		The CPU stall detector tries to make the offending CPU print its
147		own warnings, as this often gives better-quality stack traces.
148		However, if the offending CPU does not detect its own stall in
149		the number of jiffies specified by RCU_STALL_RAT_DELAY, then
150		some other CPU will complain.  This delay is normally set to
151		two jiffies.  (This is a cpp macro, not a kernel configuration
152		parameter.)
153	
154	rcupdate.rcu_task_stall_timeout
155	
156		This boot/sysfs parameter controls the RCU-tasks stall warning
157		interval.  A value of zero or less suppresses RCU-tasks stall
158		warnings.  A positive value sets the stall-warning interval
159		in jiffies.  An RCU-tasks stall warning starts with the line:
160	
161			INFO: rcu_tasks detected stalls on tasks:
162	
163		And continues with the output of sched_show_task() for each
164		task stalling the current RCU-tasks grace period.
165	
166	
167	Interpreting RCU's CPU Stall-Detector "Splats"
168	
169	For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
170	it will print a message similar to the following:
171	
172		INFO: rcu_sched detected stalls on CPUs/tasks:
173		2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
174		16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
175		(detected by 32, t=2603 jiffies, g=7073, c=7072, q=625)
176	
177	This message indicates that CPU 32 detected that CPUs 2 and 16 were both
178	causing stalls, and that the stall was affecting RCU-sched.  This message
179	will normally be followed by stack dumps for each CPU.  Please note that
180	PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that
181	the tasks will be indicated by PID, for example, "P3421".  It is even
182	possible for a rcu_preempt_state stall to be caused by both CPUs -and-
183	tasks, in which case the offending CPUs and tasks will all be called
184	out in the list.
185	
186	CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
187	the RCU core for the past three grace periods.  In contrast, CPU 16's "(0
188	ticks this GP)" indicates that this CPU has not taken any scheduling-clock
189	interrupts during the current stalled grace period.
190	
191	The "idle=" portion of the message prints the dyntick-idle state.
192	The hex number before the first "/" is the low-order 12 bits of the
193	dynticks counter, which will have an even-numbered value if the CPU
194	is in dyntick-idle mode and an odd-numbered value otherwise.  The hex
195	number between the two "/"s is the value of the nesting, which will be
196	a small non-negative number if in the idle loop (as shown above) and a
197	very large positive number otherwise.
198	
199	The "softirq=" portion of the message tracks the number of RCU softirq
200	handlers that the stalled CPU has executed.  The number before the "/"
201	is the number that had executed since boot at the time that this CPU
202	last noted the beginning of a grace period, which might be the current
203	(stalled) grace period, or it might be some earlier grace period (for
204	example, if the CPU might have been in dyntick-idle mode for an extended
205	time period.  The number after the "/" is the number that have executed
206	since boot until the current time.  If this latter number stays constant
207	across repeated stall-warning messages, it is possible that RCU's softirq
208	handlers are no longer able to execute on this CPU.  This can happen if
209	the stalled CPU is spinning with interrupts are disabled, or, in -rt
210	kernels, if a high-priority process is starving RCU's softirq handler.
211	
212	The "fps=" shows the number of force-quiescent-state idle/offline
213	detection passes that the grace-period kthread has made across this
214	CPU since the last time that this CPU noted the beginning of a grace
215	period.
216	
217	The "detected by" line indicates which CPU detected the stall (in this
218	case, CPU 32), how many jiffies have elapsed since the start of the
219	grace period (in this case 2603), the number of the last grace period
220	to start and to complete (7073 and 7072, respectively), and an estimate
221	of the total number of RCU callbacks queued across all CPUs (625 in
222	this case).
223	
224	In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
225	for each CPU:
226	
227		0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
228	
229	The "last_accelerate:" prints the low-order 16 bits (in hex) of the
230	jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
231	from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
232	rcu_prepare_for_idle().  The "nonlazy_posted:" prints the number
233	of non-lazy callbacks posted since the last call to rcu_needs_cpu().
234	Finally, an "L" indicates that there are currently no non-lazy callbacks
235	("." is printed otherwise, as shown above) and "D" indicates that
236	dyntick-idle processing is enabled ("." is printed otherwise, for example,
237	if disabled via the "nohz=" kernel boot parameter).
238	
239	If the grace period ends just as the stall warning starts printing,
240	there will be a spurious stall-warning message, which will include
241	the following:
242	
243		INFO: Stall ended before state dump start
244	
245	This is rare, but does happen from time to time in real life.  It is also
246	possible for a zero-jiffy stall to be flagged in this case, depending
247	on how the stall warning and the grace-period initialization happen to
248	interact.  Please note that it is not possible to entirely eliminate this
249	sort of false positive without resorting to things like stop_machine(),
250	which is overkill for this sort of problem.
251	
252	If all CPUs and tasks have passed through quiescent states, but the
253	grace period has nevertheless failed to end, the stall-warning splat
254	will include something like the following:
255	
256		All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0
257	
258	The "23807" indicates that it has been more than 23 thousand jiffies
259	since the grace-period kthread ran.  The "jiffies_till_next_fqs"
260	indicates how frequently that kthread should run, giving the number
261	of jiffies between force-quiescent-state scans, in this case three,
262	which is way less than 23807.  Finally, the root rcu_node structure's
263	->qsmask field is printed, which will normally be zero.
264	
265	If the relevant grace-period kthread has been unable to run prior to
266	the stall warning, as was the case in the "All QSes seen" line above,
267	the following additional line is printed:
268	
269		kthread starved for 23807 jiffies! g7073 c7072 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
270	
271	Starving the grace-period kthreads of CPU time can of course result
272	in RCU CPU stall warnings even when all CPUs and tasks have passed
273	through the required quiescent states.  The "g" and "c" numbers flag the
274	number of the last grace period started and completed, respectively,
275	the "f" precedes the ->gp_flags command to the grace-period kthread,
276	the "RCU_GP_WAIT_FQS" indicates that the kthread is waiting for a short
277	timeout, and the "state" precedes value of the task_struct ->state field.
278	
279	
280	Multiple Warnings From One Stall
281	
282	If a stall lasts long enough, multiple stall-warning messages will be
283	printed for it.  The second and subsequent messages are printed at
284	longer intervals, so that the time between (say) the first and second
285	message will be about three times the interval between the beginning
286	of the stall and the first message.
287	
288	
289	Stall Warnings for Expedited Grace Periods
290	
291	If an expedited grace period detects a stall, it will place a message
292	like the following in dmesg:
293	
294		INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
295	
296	This indicates that CPU 7 has failed to respond to a reschedule IPI.
297	The three periods (".") following the CPU number indicate that the CPU
298	is online (otherwise the first period would instead have been "O"),
299	that the CPU was online at the beginning of the expedited grace period
300	(otherwise the second period would have instead been "o"), and that
301	the CPU has been online at least once since boot (otherwise, the third
302	period would instead have been "N").  The number before the "jiffies"
303	indicates that the expedited grace period has been going on for 21,119
304	jiffies.  The number following the "s:" indicates that the expedited
305	grace-period sequence counter is 73.  The fact that this last value is
306	odd indicates that an expedited grace period is in flight.  The number
307	following "root:" is a bitmask that indicates which children of the root
308	rcu_node structure correspond to CPUs and/or tasks that are blocking the
309	current expedited grace period.  If the tree had more than one level,
310	additional hex numbers would be printed for the states of the other
311	rcu_node structures in the tree.
312	
313	As with normal grace periods, PREEMPT_RCU builds can be stalled by
314	tasks as well as by CPUs, and that the tasks will be indicated by PID,
315	for example, "P3421".
316	
317	It is entirely possible to see stall warnings from normal and from
318	expedited grace periods at about the same time during the same run.
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