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Based on kernel version 3.2. Page generated on 2012-01-05 23:29 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		ten seconds.
16	
17	RCU_SECONDS_TILL_STALL_RECHECK
18	
19		This macro defines the period of time that RCU will wait after
20		issuing a stall warning until it issues another stall warning
21		for the same stall.  This time period is normally set to three
22		times the check interval plus thirty seconds.
23	
24	RCU_STALL_RAT_DELAY
25	
26		The CPU stall detector tries to make the offending CPU print its
27		own warnings, as this often gives better-quality stack traces.
28		However, if the offending CPU does not detect its own stall in
29		the number of jiffies specified by RCU_STALL_RAT_DELAY, then
30		some other CPU will complain.  This delay is normally set to
31		two jiffies.
32	
33	When a CPU detects that it is stalling, it will print a message similar
34	to the following:
35	
36	INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
37	
38	This message indicates that CPU 5 detected that it was causing a stall,
39	and that the stall was affecting RCU-sched.  This message will normally be
40	followed by a stack dump of the offending CPU.  On TREE_RCU kernel builds,
41	RCU and RCU-sched are implemented by the same underlying mechanism,
42	while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented
43	by rcu_preempt_state.
44	
45	On the other hand, if the offending CPU fails to print out a stall-warning
46	message quickly enough, some other CPU will print a message similar to
47	the following:
48	
49	INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
50	
51	This message indicates that CPU 2 detected that CPUs 3 and 5 were both
52	causing stalls, and that the stall was affecting RCU-bh.  This message
53	will normally be followed by stack dumps for each CPU.  Please note that
54	TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
55	and that the tasks will be indicated by PID, for example, "P3421".
56	It is even possible for a rcu_preempt_state stall to be caused by both
57	CPUs -and- tasks, in which case the offending CPUs and tasks will all
58	be called out in the list.
59	
60	Finally, if the grace period ends just as the stall warning starts
61	printing, there will be a spurious stall-warning message:
62	
63	INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
64	
65	This is rare, but does happen from time to time in real life.
66	
67	So your kernel printed an RCU CPU stall warning.  The next question is
68	"What caused it?"  The following problems can result in RCU CPU stall
69	warnings:
70	
71	o	A CPU looping in an RCU read-side critical section.
72		
73	o	A CPU looping with interrupts disabled.  This condition can
74		result in RCU-sched and RCU-bh stalls.
75	
76	o	A CPU looping with preemption disabled.  This condition can
77		result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
78		stalls.
79	
80	o	A CPU looping with bottom halves disabled.  This condition can
81		result in RCU-sched and RCU-bh stalls.
82	
83	o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
84		without invoking schedule().
85	
86	o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
87		happen to preempt a low-priority task in the middle of an RCU
88		read-side critical section.   This is especially damaging if
89		that low-priority task is not permitted to run on any other CPU,
90		in which case the next RCU grace period can never complete, which
91		will eventually cause the system to run out of memory and hang.
92		While the system is in the process of running itself out of
93		memory, you might see stall-warning messages.
94	
95	o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
96		is running at a higher priority than the RCU softirq threads.
97		This will prevent RCU callbacks from ever being invoked,
98		and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
99		RCU grace periods from ever completing.  Either way, the
100		system will eventually run out of memory and hang.  In the
101		CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
102		messages.
103	
104	o	A bug in the RCU implementation.
105	
106	o	A hardware failure.  This is quite unlikely, but has occurred
107		at least once in real life.  A CPU failed in a running system,
108		becoming unresponsive, but not causing an immediate crash.
109		This resulted in a series of RCU CPU stall warnings, eventually
110		leading the realization that the CPU had failed.
111	
112	The RCU, RCU-sched, and RCU-bh implementations have CPU stall
113	warning.  SRCU does not have its own CPU stall warnings, but its
114	calls to synchronize_sched() will result in RCU-sched detecting
115	RCU-sched-related CPU stalls.  Please note that RCU only detects
116	CPU stalls when there is a grace period in progress.  No grace period,
117	no CPU stall warnings.
118	
119	To diagnose the cause of the stall, inspect the stack traces.
120	The offending function will usually be near the top of the stack.
121	If you have a series of stall warnings from a single extended stall,
122	comparing the stack traces can often help determine where the stall
123	is occurring, which will usually be in the function nearest the top of
124	that portion of the stack which remains the same from trace to trace.
125	If you can reliably trigger the stall, ftrace can be quite helpful.
126	
127	RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE.
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