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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

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
13	warnings:
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
21		stalls.
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
38		debug printk()s.
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.
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.
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.
63	o	A hardware or software issue shuts off the scheduler-clock
64		interrupt on a CPU that is not in dyntick-idle mode.  This
65		problem really has happened, and seems to be most likely to
66		result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
68	o	A bug in the RCU implementation.
70	o	A hardware failure.  This is quite unlikely, but has occurred
71		at least once in real life.  A CPU failed in a running system,
72		becoming unresponsive, but not causing an immediate crash.
73		This resulted in a series of RCU CPU stall warnings, eventually
74		leading the realization that the CPU had failed.
76	The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
77	warning.  Note that SRCU does -not- have CPU stall warnings.  Please note
78	that RCU only detects CPU stalls when there is a grace period in progress.
79	No grace period, no CPU stall warnings.
81	To diagnose the cause of the stall, inspect the stack traces.
82	The offending function will usually be near the top of the stack.
83	If you have a series of stall warnings from a single extended stall,
84	comparing the stack traces can often help determine where the stall
85	is occurring, which will usually be in the function nearest the top of
86	that portion of the stack which remains the same from trace to trace.
87	If you can reliably trigger the stall, ftrace can be quite helpful.
89	RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
90	and with RCU's event tracing.  For information on RCU's event tracing,
91	see include/trace/events/rcu.h.
94	Fine-Tuning the RCU CPU Stall Detector
96	The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
97	CPU stall detector, which detects conditions that unduly delay RCU grace
98	periods.  This module parameter enables CPU stall detection by default,
99	but may be overridden via boot-time parameter or at runtime via sysfs.
100	The stall detector's idea of what constitutes "unduly delayed" is
101	controlled by a set of kernel configuration variables and cpp macros:
105		This kernel configuration parameter defines the period of time
106		that RCU will wait from the beginning of a grace period until it
107		issues an RCU CPU stall warning.  This time period is normally
108		21 seconds.
110		This configuration parameter may be changed at runtime via the
111		/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
112		this parameter is checked only at the beginning of a cycle.
113		So if you are 10 seconds into a 40-second stall, setting this
114		sysfs parameter to (say) five will shorten the timeout for the
115		-next- stall, or the following warning for the current stall
116		(assuming the stall lasts long enough).  It will not affect the
117		timing of the next warning for the current stall.
119		Stall-warning messages may be enabled and disabled completely via
120		/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
124		Although the lockdep facility is extremely useful, it does add
125		some overhead.  Therefore, under CONFIG_PROVE_RCU, the
126		RCU_STALL_DELAY_DELTA macro allows five extra seconds before
127		giving an RCU CPU stall warning message.  (This is a cpp
128		macro, not a kernel configuration parameter.)
132		The CPU stall detector tries to make the offending CPU print its
133		own warnings, as this often gives better-quality stack traces.
134		However, if the offending CPU does not detect its own stall in
135		the number of jiffies specified by RCU_STALL_RAT_DELAY, then
136		some other CPU will complain.  This delay is normally set to
137		two jiffies.  (This is a cpp macro, not a kernel configuration
138		parameter.)
140	rcupdate.rcu_task_stall_timeout
142		This boot/sysfs parameter controls the RCU-tasks stall warning
143		interval.  A value of zero or less suppresses RCU-tasks stall
144		warnings.  A positive value sets the stall-warning interval
145		in jiffies.  An RCU-tasks stall warning starts with the line:
147			INFO: rcu_tasks detected stalls on tasks:
149		And continues with the output of sched_show_task() for each
150		task stalling the current RCU-tasks grace period.
153	Interpreting RCU's CPU Stall-Detector "Splats"
155	For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
156	it will print a message similar to the following:
158	INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
160	This message indicates that CPU 5 detected that it was causing a stall,
161	and that the stall was affecting RCU-sched.  This message will normally be
162	followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,
163	RCU and RCU-sched are implemented by the same underlying mechanism,
164	while on PREEMPT_RCU kernel builds, RCU is instead implemented
165	by rcu_preempt_state.
167	On the other hand, if the offending CPU fails to print out a stall-warning
168	message quickly enough, some other CPU will print a message similar to
169	the following:
171	INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
173	This message indicates that CPU 2 detected that CPUs 3 and 5 were both
174	causing stalls, and that the stall was affecting RCU-bh.  This message
175	will normally be followed by stack dumps for each CPU.  Please note that
176	PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
177	and that the tasks will be indicated by PID, for example, "P3421".
178	It is even possible for a rcu_preempt_state stall to be caused by both
179	CPUs -and- tasks, in which case the offending CPUs and tasks will all
180	be called out in the list.
182	Finally, if the grace period ends just as the stall warning starts
183	printing, there will be a spurious stall-warning message:
185	INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
187	This is rare, but does happen from time to time in real life.  It is also
188	possible for a zero-jiffy stall to be flagged in this case, depending
189	on how the stall warning and the grace-period initialization happen to
190	interact.  Please note that it is not possible to entirely eliminate this
191	sort of false positive without resorting to things like stop_machine(),
192	which is overkill for this sort of problem.
194	Recent kernels will print a long form of the stall-warning message:
196		INFO: rcu_preempt detected stall on CPU
197		0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 softirq=82/543
198		   (t=65000 jiffies)
200	In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed:
202		INFO: rcu_preempt detected stall on CPU
203		0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
204		   (t=65000 jiffies)
206	The "(64628 ticks this GP)" indicates that this CPU has taken more
207	than 64,000 scheduling-clock interrupts during the current stalled
208	grace period.  If the CPU was not yet aware of the current grace
209	period (for example, if it was offline), then this part of the message
210	indicates how many grace periods behind the CPU is.
212	The "idle=" portion of the message prints the dyntick-idle state.
213	The hex number before the first "/" is the low-order 12 bits of the
214	dynticks counter, which will have an even-numbered value if the CPU is
215	in dyntick-idle mode and an odd-numbered value otherwise.  The hex
216	number between the two "/"s is the value of the nesting, which will
217	be a small positive number if in the idle loop and a very large positive
218	number (as shown above) otherwise.
220	The "softirq=" portion of the message tracks the number of RCU softirq
221	handlers that the stalled CPU has executed.  The number before the "/"
222	is the number that had executed since boot at the time that this CPU
223	last noted the beginning of a grace period, which might be the current
224	(stalled) grace period, or it might be some earlier grace period (for
225	example, if the CPU might have been in dyntick-idle mode for an extended
226	time period.  The number after the "/" is the number that have executed
227	since boot until the current time.  If this latter number stays constant
228	across repeated stall-warning messages, it is possible that RCU's softirq
229	handlers are no longer able to execute on this CPU.  This can happen if
230	the stalled CPU is spinning with interrupts are disabled, or, in -rt
231	kernels, if a high-priority process is starving RCU's softirq handler.
233	For CONFIG_RCU_FAST_NO_HZ kernels, the "last_accelerate:" prints the
234	low-order 16 bits (in hex) of the jiffies counter when this CPU last
235	invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked
236	rcu_accelerate_cbs() from rcu_prepare_for_idle().  The "nonlazy_posted:"
237	prints the number of non-lazy callbacks posted since the last call to
238	rcu_needs_cpu().  Finally, an "L" indicates that there are currently
239	no non-lazy callbacks ("." is printed otherwise, as shown above) and
240	"D" indicates that dyntick-idle processing is enabled ("." is printed
241	otherwise, for example, if disabled via the "nohz=" kernel boot parameter).
243	If the relevant grace-period kthread has been unable to run prior to
244	the stall warning, the following additional line is printed:
246		rcu_preempt kthread starved for 2023 jiffies!
248	Starving the grace-period kthreads of CPU time can of course result in
249	RCU CPU stall warnings even when all CPUs and tasks have passed through
250	the required quiescent states.
253	Multiple Warnings From One Stall
255	If a stall lasts long enough, multiple stall-warning messages will be
256	printed for it.  The second and subsequent messages are printed at
257	longer intervals, so that the time between (say) the first and second
258	message will be about three times the interval between the beginning
259	of the stall and the first message.
262	Stall Warnings for Expedited Grace Periods
264	If an expedited grace period detects a stall, it will place a message
265	like the following in dmesg:
267		INFO: rcu_sched detected expedited stalls on CPUs: { 1 2 6 } 26009 jiffies s: 1043
269	This indicates that CPUs 1, 2, and 6 have failed to respond to a
270	reschedule IPI, that the expedited grace period has been going on for
271	26,009 jiffies, and that the expedited grace-period sequence counter is
272	1043.  The fact that this last value is odd indicates that an expedited
273	grace period is in flight.
275	It is entirely possible to see stall warnings from normal and from
276	expedited grace periods at about the same time from the same run.
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