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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	Using RCU's CPU Stall Detector
2	
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:
9	
10	CONFIG_RCU_CPU_STALL_TIMEOUT
11	
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.
16	
17		This configuration parameter may be changed at runtime via the
18		/sys/module/rcutree/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.
25	
26		Stall-warning messages may be enabled and disabled completely via
27		/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
28	
29	CONFIG_RCU_CPU_STALL_VERBOSE
30	
31		This kernel configuration parameter causes the stall warning to
32		also dump the stacks of any tasks that are blocking the current
33		RCU-preempt grace period.
34	
35	CONFIG_RCU_CPU_STALL_INFO
36	
37		This kernel configuration parameter causes the stall warning to
38		print out additional per-CPU diagnostic information, including
39		information on scheduling-clock ticks and RCU's idle-CPU tracking.
40	
41	RCU_STALL_DELAY_DELTA
42	
43		Although the lockdep facility is extremely useful, it does add
44		some overhead.  Therefore, under CONFIG_PROVE_RCU, the
45		RCU_STALL_DELAY_DELTA macro allows five extra seconds before
46		giving an RCU CPU stall warning message.  (This is a cpp
47		macro, not a kernel configuration parameter.)
48	
49	RCU_STALL_RAT_DELAY
50	
51		The CPU stall detector tries to make the offending CPU print its
52		own warnings, as this often gives better-quality stack traces.
53		However, if the offending CPU does not detect its own stall in
54		the number of jiffies specified by RCU_STALL_RAT_DELAY, then
55		some other CPU will complain.  This delay is normally set to
56		two jiffies.  (This is a cpp macro, not a kernel configuration
57		parameter.)
58	
59	When a CPU detects that it is stalling, it will print a message similar
60	to the following:
61	
62	INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
63	
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 TREE_PREEMPT_RCU kernel builds, RCU is instead implemented
69	by rcu_preempt_state.
70	
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:
74	
75	INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
76	
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	TREE_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.
85	
86	Finally, if the grace period ends just as the stall warning starts
87	printing, there will be a spurious stall-warning message:
88	
89	INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
90	
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.
97	
98	If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set,
99	more information is printed with the stall-warning message, for example:
100	
101		INFO: rcu_preempt detected stall on CPU
102		0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 softirq=82/543
103		   (t=65000 jiffies)
104	
105	In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is
106	printed:
107	
108		INFO: rcu_preempt detected stall on CPU
109		0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
110		   (t=65000 jiffies)
111	
112	The "(64628 ticks this GP)" indicates that this CPU has taken more
113	than 64,000 scheduling-clock interrupts during the current stalled
114	grace period.  If the CPU was not yet aware of the current grace
115	period (for example, if it was offline), then this part of the message
116	indicates how many grace periods behind the CPU is.
117	
118	The "idle=" portion of the message prints the dyntick-idle state.
119	The hex number before the first "/" is the low-order 12 bits of the
120	dynticks counter, which will have an even-numbered value if the CPU is
121	in dyntick-idle mode and an odd-numbered value otherwise.  The hex
122	number between the two "/"s is the value of the nesting, which will
123	be a small positive number if in the idle loop and a very large positive
124	number (as shown above) otherwise.
125	
126	The "softirq=" portion of the message tracks the number of RCU softirq
127	handlers that the stalled CPU has executed.  The number before the "/"
128	is the number that had executed since boot at the time that this CPU
129	last noted the beginning of a grace period, which might be the current
130	(stalled) grace period, or it might be some earlier grace period (for
131	example, if the CPU might have been in dyntick-idle mode for an extended
132	time period.  The number after the "/" is the number that have executed
133	since boot until the current time.  If this latter number stays constant
134	across repeated stall-warning messages, it is possible that RCU's softirq
135	handlers are no longer able to execute on this CPU.  This can happen if
136	the stalled CPU is spinning with interrupts are disabled, or, in -rt
137	kernels, if a high-priority process is starving RCU's softirq handler.
138	
139	For CONFIG_RCU_FAST_NO_HZ kernels, the "last_accelerate:" prints the
140	low-order 16 bits (in hex) of the jiffies counter when this CPU last
141	invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked
142	rcu_accelerate_cbs() from rcu_prepare_for_idle().  The "nonlazy_posted:"
143	prints the number of non-lazy callbacks posted since the last call to
144	rcu_needs_cpu().  Finally, an "L" indicates that there are currently
145	no non-lazy callbacks ("." is printed otherwise, as shown above) and
146	"D" indicates that dyntick-idle processing is enabled ("." is printed
147	otherwise, for example, if disabled via the "nohz=" kernel boot parameter).
148	
149	
150	Multiple Warnings From One Stall
151	
152	If a stall lasts long enough, multiple stall-warning messages will be
153	printed for it.  The second and subsequent messages are printed at
154	longer intervals, so that the time between (say) the first and second
155	message will be about three times the interval between the beginning
156	of the stall and the first message.
157	
158	
159	What Causes RCU CPU Stall Warnings?
160	
161	So your kernel printed an RCU CPU stall warning.  The next question is
162	"What caused it?"  The following problems can result in RCU CPU stall
163	warnings:
164	
165	o	A CPU looping in an RCU read-side critical section.
166		
167	o	A CPU looping with interrupts disabled.  This condition can
168		result in RCU-sched and RCU-bh stalls.
169	
170	o	A CPU looping with preemption disabled.  This condition can
171		result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
172		stalls.
173	
174	o	A CPU looping with bottom halves disabled.  This condition can
175		result in RCU-sched and RCU-bh stalls.
176	
177	o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
178		without invoking schedule().
179	
180	o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
181		happen to preempt a low-priority task in the middle of an RCU
182		read-side critical section.   This is especially damaging if
183		that low-priority task is not permitted to run on any other CPU,
184		in which case the next RCU grace period can never complete, which
185		will eventually cause the system to run out of memory and hang.
186		While the system is in the process of running itself out of
187		memory, you might see stall-warning messages.
188	
189	o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
190		is running at a higher priority than the RCU softirq threads.
191		This will prevent RCU callbacks from ever being invoked,
192		and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
193		RCU grace periods from ever completing.  Either way, the
194		system will eventually run out of memory and hang.  In the
195		CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
196		messages.
197	
198	o	A hardware or software issue shuts off the scheduler-clock
199		interrupt on a CPU that is not in dyntick-idle mode.  This
200		problem really has happened, and seems to be most likely to
201		result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
202	
203	o	A bug in the RCU implementation.
204	
205	o	A hardware failure.  This is quite unlikely, but has occurred
206		at least once in real life.  A CPU failed in a running system,
207		becoming unresponsive, but not causing an immediate crash.
208		This resulted in a series of RCU CPU stall warnings, eventually
209		leading the realization that the CPU had failed.
210	
211	The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.
212	SRCU does not have its own CPU stall warnings, but its calls to
213	synchronize_sched() will result in RCU-sched detecting RCU-sched-related
214	CPU stalls.  Please note that RCU only detects CPU stalls when there is
215	a grace period in progress.  No grace period, no CPU stall warnings.
216	
217	To diagnose the cause of the stall, inspect the stack traces.
218	The offending function will usually be near the top of the stack.
219	If you have a series of stall warnings from a single extended stall,
220	comparing the stack traces can often help determine where the stall
221	is occurring, which will usually be in the function nearest the top of
222	that portion of the stack which remains the same from trace to trace.
223	If you can reliably trigger the stall, ftrace can be quite helpful.
224	
225	RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
226	and with RCU's event tracing.  For information on RCU's event tracing,
227	see include/trace/events/rcu.h.
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