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Based on kernel version 4.3. Page generated on 2015-11-02 12:50 EST.

1	Using RCU's CPU Stall Detector
3	The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall
4	detector, which detects conditions that unduly delay RCU grace periods.
5	This module parameter enables CPU stall detection by default, but
6	may be overridden via boot-time parameter or at runtime via sysfs.
7	The stall detector's idea of what constitutes "unduly delayed" is
8	controlled by a set of kernel configuration variables and cpp macros:
12		This kernel configuration parameter defines the period of time
13		that RCU will wait from the beginning of a grace period until it
14		issues an RCU CPU stall warning.  This time period is normally
15		21 seconds.
17		This configuration parameter may be changed at runtime via the
18		/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
19		this parameter is checked only at the beginning of a cycle.
20		So if you are 10 seconds into a 40-second stall, setting this
21		sysfs parameter to (say) five will shorten the timeout for the
22		-next- stall, or the following warning for the current stall
23		(assuming the stall lasts long enough).  It will not affect the
24		timing of the next warning for the current stall.
26		Stall-warning messages may be enabled and disabled completely via
27		/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
31		Although the lockdep facility is extremely useful, it does add
32		some overhead.  Therefore, under CONFIG_PROVE_RCU, the
33		RCU_STALL_DELAY_DELTA macro allows five extra seconds before
34		giving an RCU CPU stall warning message.  (This is a cpp
35		macro, not a kernel configuration parameter.)
39		The CPU stall detector tries to make the offending CPU print its
40		own warnings, as this often gives better-quality stack traces.
41		However, if the offending CPU does not detect its own stall in
42		the number of jiffies specified by RCU_STALL_RAT_DELAY, then
43		some other CPU will complain.  This delay is normally set to
44		two jiffies.  (This is a cpp macro, not a kernel configuration
45		parameter.)
47	rcupdate.rcu_task_stall_timeout
49		This boot/sysfs parameter controls the RCU-tasks stall warning
50		interval.  A value of zero or less suppresses RCU-tasks stall
51		warnings.  A positive value sets the stall-warning interval
52		in jiffies.  An RCU-tasks stall warning starts wtih the line:
54			INFO: rcu_tasks detected stalls on tasks:
56		And continues with the output of sched_show_task() for each
57		task stalling the current RCU-tasks grace period.
59	For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
60	it will print a message similar to the following:
62	INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
64	This message indicates that CPU 5 detected that it was causing a stall,
65	and that the stall was affecting RCU-sched.  This message will normally be
66	followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,
67	RCU and RCU-sched are implemented by the same underlying mechanism,
68	while on PREEMPT_RCU kernel builds, RCU is instead implemented
69	by rcu_preempt_state.
71	On the other hand, if the offending CPU fails to print out a stall-warning
72	message quickly enough, some other CPU will print a message similar to
73	the following:
75	INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
77	This message indicates that CPU 2 detected that CPUs 3 and 5 were both
78	causing stalls, and that the stall was affecting RCU-bh.  This message
79	will normally be followed by stack dumps for each CPU.  Please note that
80	PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
81	and that the tasks will be indicated by PID, for example, "P3421".
82	It is even possible for a rcu_preempt_state stall to be caused by both
83	CPUs -and- tasks, in which case the offending CPUs and tasks will all
84	be called out in the list.
86	Finally, if the grace period ends just as the stall warning starts
87	printing, there will be a spurious stall-warning message:
89	INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
91	This is rare, but does happen from time to time in real life.  It is also
92	possible for a zero-jiffy stall to be flagged in this case, depending
93	on how the stall warning and the grace-period initialization happen to
94	interact.  Please note that it is not possible to entirely eliminate this
95	sort of false positive without resorting to things like stop_machine(),
96	which is overkill for this sort of problem.
98	Recent kernels will print a long form of the stall-warning message:
100		INFO: rcu_preempt detected stall on CPU
101		0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 softirq=82/543
102		   (t=65000 jiffies)
104	In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed:
106		INFO: rcu_preempt detected stall on CPU
107		0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
108		   (t=65000 jiffies)
110	The "(64628 ticks this GP)" indicates that this CPU has taken more
111	than 64,000 scheduling-clock interrupts during the current stalled
112	grace period.  If the CPU was not yet aware of the current grace
113	period (for example, if it was offline), then this part of the message
114	indicates how many grace periods behind the CPU is.
116	The "idle=" portion of the message prints the dyntick-idle state.
117	The hex number before the first "/" is the low-order 12 bits of the
118	dynticks counter, which will have an even-numbered value if the CPU is
119	in dyntick-idle mode and an odd-numbered value otherwise.  The hex
120	number between the two "/"s is the value of the nesting, which will
121	be a small positive number if in the idle loop and a very large positive
122	number (as shown above) otherwise.
124	The "softirq=" portion of the message tracks the number of RCU softirq
125	handlers that the stalled CPU has executed.  The number before the "/"
126	is the number that had executed since boot at the time that this CPU
127	last noted the beginning of a grace period, which might be the current
128	(stalled) grace period, or it might be some earlier grace period (for
129	example, if the CPU might have been in dyntick-idle mode for an extended
130	time period.  The number after the "/" is the number that have executed
131	since boot until the current time.  If this latter number stays constant
132	across repeated stall-warning messages, it is possible that RCU's softirq
133	handlers are no longer able to execute on this CPU.  This can happen if
134	the stalled CPU is spinning with interrupts are disabled, or, in -rt
135	kernels, if a high-priority process is starving RCU's softirq handler.
137	For CONFIG_RCU_FAST_NO_HZ kernels, the "last_accelerate:" prints the
138	low-order 16 bits (in hex) of the jiffies counter when this CPU last
139	invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked
140	rcu_accelerate_cbs() from rcu_prepare_for_idle().  The "nonlazy_posted:"
141	prints the number of non-lazy callbacks posted since the last call to
142	rcu_needs_cpu().  Finally, an "L" indicates that there are currently
143	no non-lazy callbacks ("." is printed otherwise, as shown above) and
144	"D" indicates that dyntick-idle processing is enabled ("." is printed
145	otherwise, for example, if disabled via the "nohz=" kernel boot parameter).
147	If the relevant grace-period kthread has been unable to run prior to
148	the stall warning, the following additional line is printed:
150		rcu_preempt kthread starved for 2023 jiffies!
152	Starving the grace-period kthreads of CPU time can of course result in
153	RCU CPU stall warnings even when all CPUs and tasks have passed through
154	the required quiescent states.
157	Multiple Warnings From One Stall
159	If a stall lasts long enough, multiple stall-warning messages will be
160	printed for it.  The second and subsequent messages are printed at
161	longer intervals, so that the time between (say) the first and second
162	message will be about three times the interval between the beginning
163	of the stall and the first message.
166	Stall Warnings for Expedited Grace Periods
168	If an expedited grace period detects a stall, it will place a message
169	like the following in dmesg:
171		INFO: rcu_sched detected expedited stalls on CPUs: { 1 2 6 } 26009 jiffies s: 1043
173	This indicates that CPUs 1, 2, and 6 have failed to respond to a
174	reschedule IPI, that the expedited grace period has been going on for
175	26,009 jiffies, and that the expedited grace-period sequence counter is
176	1043.  The fact that this last value is odd indicates that an expedited
177	grace period is in flight.
179	It is entirely possible to see stall warnings from normal and from
180	expedited grace periods at about the same time from the same run.
183	What Causes RCU CPU Stall Warnings?
185	So your kernel printed an RCU CPU stall warning.  The next question is
186	"What caused it?"  The following problems can result in RCU CPU stall
187	warnings:
189	o	A CPU looping in an RCU read-side critical section.
191	o	A CPU looping with interrupts disabled.  This condition can
192		result in RCU-sched and RCU-bh stalls.
194	o	A CPU looping with preemption disabled.  This condition can
195		result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
196		stalls.
198	o	A CPU looping with bottom halves disabled.  This condition can
199		result in RCU-sched and RCU-bh stalls.
201	o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
202		kernel without invoking schedule().  Note that cond_resched()
203		does not necessarily prevent RCU CPU stall warnings.  Therefore,
204		if the looping in the kernel is really expected and desirable
205		behavior, you might need to replace some of the cond_resched()
206		calls with calls to cond_resched_rcu_qs().
208	o	Anything that prevents RCU's grace-period kthreads from running.
209		This can result in the "All QSes seen" console-log message.
210		This message will include information on when the kthread last
211		ran and how often it should be expected to run.
213	o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
214		happen to preempt a low-priority task in the middle of an RCU
215		read-side critical section.   This is especially damaging if
216		that low-priority task is not permitted to run on any other CPU,
217		in which case the next RCU grace period can never complete, which
218		will eventually cause the system to run out of memory and hang.
219		While the system is in the process of running itself out of
220		memory, you might see stall-warning messages.
222	o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
223		is running at a higher priority than the RCU softirq threads.
224		This will prevent RCU callbacks from ever being invoked,
225		and in a CONFIG_PREEMPT_RCU kernel will further prevent
226		RCU grace periods from ever completing.  Either way, the
227		system will eventually run out of memory and hang.  In the
228		CONFIG_PREEMPT_RCU case, you might see stall-warning
229		messages.
231	o	A hardware or software issue shuts off the scheduler-clock
232		interrupt on a CPU that is not in dyntick-idle mode.  This
233		problem really has happened, and seems to be most likely to
234		result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
236	o	A bug in the RCU implementation.
238	o	A hardware failure.  This is quite unlikely, but has occurred
239		at least once in real life.  A CPU failed in a running system,
240		becoming unresponsive, but not causing an immediate crash.
241		This resulted in a series of RCU CPU stall warnings, eventually
242		leading the realization that the CPU had failed.
244	The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
245	warning.  Note that SRCU does -not- have CPU stall warnings.  Please note
246	that RCU only detects CPU stalls when there is a grace period in progress.
247	No grace period, no CPU stall warnings.
249	To diagnose the cause of the stall, inspect the stack traces.
250	The offending function will usually be near the top of the stack.
251	If you have a series of stall warnings from a single extended stall,
252	comparing the stack traces can often help determine where the stall
253	is occurring, which will usually be in the function nearest the top of
254	that portion of the stack which remains the same from trace to trace.
255	If you can reliably trigger the stall, ftrace can be quite helpful.
257	RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
258	and with RCU's event tracing.  For information on RCU's event tracing,
259	see include/trace/events/rcu.h.
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