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Based on kernel version 4.2. Page generated on 2015-09-09 12:14 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		This kernel configuration parameter causes the stall warning to
32		print out additional per-CPU diagnostic information, including
33		information on scheduling-clock ticks and RCU's idle-CPU tracking.
37		Although the lockdep facility is extremely useful, it does add
38		some overhead.  Therefore, under CONFIG_PROVE_RCU, the
39		RCU_STALL_DELAY_DELTA macro allows five extra seconds before
40		giving an RCU CPU stall warning message.  (This is a cpp
41		macro, not a kernel configuration parameter.)
45		The CPU stall detector tries to make the offending CPU print its
46		own warnings, as this often gives better-quality stack traces.
47		However, if the offending CPU does not detect its own stall in
48		the number of jiffies specified by RCU_STALL_RAT_DELAY, then
49		some other CPU will complain.  This delay is normally set to
50		two jiffies.  (This is a cpp macro, not a kernel configuration
51		parameter.)
53	rcupdate.rcu_task_stall_timeout
55		This boot/sysfs parameter controls the RCU-tasks stall warning
56		interval.  A value of zero or less suppresses RCU-tasks stall
57		warnings.  A positive value sets the stall-warning interval
58		in jiffies.  An RCU-tasks stall warning starts wtih the line:
60			INFO: rcu_tasks detected stalls on tasks:
62		And continues with the output of sched_show_task() for each
63		task stalling the current RCU-tasks grace period.
65	For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
66	it will print a message similar to the following:
68	INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
70	This message indicates that CPU 5 detected that it was causing a stall,
71	and that the stall was affecting RCU-sched.  This message will normally be
72	followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,
73	RCU and RCU-sched are implemented by the same underlying mechanism,
74	while on PREEMPT_RCU kernel builds, RCU is instead implemented
75	by rcu_preempt_state.
77	On the other hand, if the offending CPU fails to print out a stall-warning
78	message quickly enough, some other CPU will print a message similar to
79	the following:
81	INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
83	This message indicates that CPU 2 detected that CPUs 3 and 5 were both
84	causing stalls, and that the stall was affecting RCU-bh.  This message
85	will normally be followed by stack dumps for each CPU.  Please note that
86	PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
87	and that the tasks will be indicated by PID, for example, "P3421".
88	It is even possible for a rcu_preempt_state stall to be caused by both
89	CPUs -and- tasks, in which case the offending CPUs and tasks will all
90	be called out in the list.
92	Finally, if the grace period ends just as the stall warning starts
93	printing, there will be a spurious stall-warning message:
95	INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
97	This is rare, but does happen from time to time in real life.  It is also
98	possible for a zero-jiffy stall to be flagged in this case, depending
99	on how the stall warning and the grace-period initialization happen to
100	interact.  Please note that it is not possible to entirely eliminate this
101	sort of false positive without resorting to things like stop_machine(),
102	which is overkill for this sort of problem.
104	If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set,
105	more information is printed with the stall-warning message, for example:
107		INFO: rcu_preempt detected stall on CPU
108		0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 softirq=82/543
109		   (t=65000 jiffies)
111	In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is
112	printed:
114		INFO: rcu_preempt detected stall on CPU
115		0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
116		   (t=65000 jiffies)
118	The "(64628 ticks this GP)" indicates that this CPU has taken more
119	than 64,000 scheduling-clock interrupts during the current stalled
120	grace period.  If the CPU was not yet aware of the current grace
121	period (for example, if it was offline), then this part of the message
122	indicates how many grace periods behind the CPU is.
124	The "idle=" portion of the message prints the dyntick-idle state.
125	The hex number before the first "/" is the low-order 12 bits of the
126	dynticks counter, which will have an even-numbered value if the CPU is
127	in dyntick-idle mode and an odd-numbered value otherwise.  The hex
128	number between the two "/"s is the value of the nesting, which will
129	be a small positive number if in the idle loop and a very large positive
130	number (as shown above) otherwise.
132	The "softirq=" portion of the message tracks the number of RCU softirq
133	handlers that the stalled CPU has executed.  The number before the "/"
134	is the number that had executed since boot at the time that this CPU
135	last noted the beginning of a grace period, which might be the current
136	(stalled) grace period, or it might be some earlier grace period (for
137	example, if the CPU might have been in dyntick-idle mode for an extended
138	time period.  The number after the "/" is the number that have executed
139	since boot until the current time.  If this latter number stays constant
140	across repeated stall-warning messages, it is possible that RCU's softirq
141	handlers are no longer able to execute on this CPU.  This can happen if
142	the stalled CPU is spinning with interrupts are disabled, or, in -rt
143	kernels, if a high-priority process is starving RCU's softirq handler.
145	For CONFIG_RCU_FAST_NO_HZ kernels, the "last_accelerate:" prints the
146	low-order 16 bits (in hex) of the jiffies counter when this CPU last
147	invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked
148	rcu_accelerate_cbs() from rcu_prepare_for_idle().  The "nonlazy_posted:"
149	prints the number of non-lazy callbacks posted since the last call to
150	rcu_needs_cpu().  Finally, an "L" indicates that there are currently
151	no non-lazy callbacks ("." is printed otherwise, as shown above) and
152	"D" indicates that dyntick-idle processing is enabled ("." is printed
153	otherwise, for example, if disabled via the "nohz=" kernel boot parameter).
155	If the relevant grace-period kthread has been unable to run prior to
156	the stall warning, the following additional line is printed:
158		rcu_preempt kthread starved for 2023 jiffies!
160	Starving the grace-period kthreads of CPU time can of course result in
161	RCU CPU stall warnings even when all CPUs and tasks have passed through
162	the required quiescent states.
165	Multiple Warnings From One Stall
167	If a stall lasts long enough, multiple stall-warning messages will be
168	printed for it.  The second and subsequent messages are printed at
169	longer intervals, so that the time between (say) the first and second
170	message will be about three times the interval between the beginning
171	of the stall and the first message.
174	What Causes RCU CPU Stall Warnings?
176	So your kernel printed an RCU CPU stall warning.  The next question is
177	"What caused it?"  The following problems can result in RCU CPU stall
178	warnings:
180	o	A CPU looping in an RCU read-side critical section.
182	o	A CPU looping with interrupts disabled.  This condition can
183		result in RCU-sched and RCU-bh stalls.
185	o	A CPU looping with preemption disabled.  This condition can
186		result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
187		stalls.
189	o	A CPU looping with bottom halves disabled.  This condition can
190		result in RCU-sched and RCU-bh stalls.
192	o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
193		kernel without invoking schedule().  Note that cond_resched()
194		does not necessarily prevent RCU CPU stall warnings.  Therefore,
195		if the looping in the kernel is really expected and desirable
196		behavior, you might need to replace some of the cond_resched()
197		calls with calls to cond_resched_rcu_qs().
199	o	Anything that prevents RCU's grace-period kthreads from running.
200		This can result in the "All QSes seen" console-log message.
201		This message will include information on when the kthread last
202		ran and how often it should be expected to run.
204	o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
205		happen to preempt a low-priority task in the middle of an RCU
206		read-side critical section.   This is especially damaging if
207		that low-priority task is not permitted to run on any other CPU,
208		in which case the next RCU grace period can never complete, which
209		will eventually cause the system to run out of memory and hang.
210		While the system is in the process of running itself out of
211		memory, you might see stall-warning messages.
213	o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
214		is running at a higher priority than the RCU softirq threads.
215		This will prevent RCU callbacks from ever being invoked,
216		and in a CONFIG_PREEMPT_RCU kernel will further prevent
217		RCU grace periods from ever completing.  Either way, the
218		system will eventually run out of memory and hang.  In the
219		CONFIG_PREEMPT_RCU case, you might see stall-warning
220		messages.
222	o	A hardware or software issue shuts off the scheduler-clock
223		interrupt on a CPU that is not in dyntick-idle mode.  This
224		problem really has happened, and seems to be most likely to
225		result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
227	o	A bug in the RCU implementation.
229	o	A hardware failure.  This is quite unlikely, but has occurred
230		at least once in real life.  A CPU failed in a running system,
231		becoming unresponsive, but not causing an immediate crash.
232		This resulted in a series of RCU CPU stall warnings, eventually
233		leading the realization that the CPU had failed.
235	The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall
236	warning.  Note that SRCU does -not- have CPU stall warnings.  Please note
237	that RCU only detects CPU stalls when there is a grace period in progress.
238	No grace period, no CPU stall warnings.
240	To diagnose the cause of the stall, inspect the stack traces.
241	The offending function will usually be near the top of the stack.
242	If you have a series of stall warnings from a single extended stall,
243	comparing the stack traces can often help determine where the stall
244	is occurring, which will usually be in the function nearest the top of
245	that portion of the stack which remains the same from trace to trace.
246	If you can reliably trigger the stall, ftrace can be quite helpful.
248	RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
249	and with RCU's event tracing.  For information on RCU's event tracing,
250	see include/trace/events/rcu.h.
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