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Paul E. McKenney4c540052010-01-14 16:10:57 -08001Using RCU's CPU Stall Detector
2
Paul E. McKenneya00e0d712011-02-08 17:14:39 -08003The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall
4detector, which detects conditions that unduly delay RCU grace periods.
5This module parameter enables CPU stall detection by default, but
6may be overridden via boot-time parameter or at runtime via sysfs.
7The stall detector's idea of what constitutes "unduly delayed" is
8controlled by a set of kernel configuration variables and cpp macros:
Paul E. McKenney4c540052010-01-14 16:10:57 -08009
Paul E. McKenneya00e0d712011-02-08 17:14:39 -080010CONFIG_RCU_CPU_STALL_TIMEOUT
Paul E. McKenney4c540052010-01-14 16:10:57 -080011
Paul E. McKenneya00e0d712011-02-08 17:14:39 -080012 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
Paul E. McKenney24cd7fd2012-01-20 17:35:55 -080015 sixty seconds.
Paul E. McKenney4c540052010-01-14 16:10:57 -080016
Paul E. McKenney24cd7fd2012-01-20 17:35:55 -080017 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 30 seconds into a 70-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.
Paul E. McKenney4c540052010-01-14 16:10:57 -080025
Paul E. McKenney24cd7fd2012-01-20 17:35:55 -080026 Stall-warning messages may be enabled and disabled completely via
27 /sys/module/rcutree/parameters/rcu_cpu_stall_suppress.
28
29CONFIG_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
35RCU_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
41RCU_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.
Paul E. McKenney4c540052010-01-14 16:10:57 -080047
48RCU_STALL_RAT_DELAY
49
Paul E. McKenneyf1d507b2010-04-15 15:49:46 -070050 The CPU stall detector tries to make the offending CPU print its
51 own warnings, as this often gives better-quality stack traces.
52 However, if the offending CPU does not detect its own stall in
53 the number of jiffies specified by RCU_STALL_RAT_DELAY, then
54 some other CPU will complain. This delay is normally set to
55 two jiffies.
Paul E. McKenney4c540052010-01-14 16:10:57 -080056
Paul E. McKenneyf1d507b2010-04-15 15:49:46 -070057When a CPU detects that it is stalling, it will print a message similar
58to the following:
59
60INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
61
62This message indicates that CPU 5 detected that it was causing a stall,
63and that the stall was affecting RCU-sched. This message will normally be
64followed by a stack dump of the offending CPU. On TREE_RCU kernel builds,
65RCU and RCU-sched are implemented by the same underlying mechanism,
66while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented
67by rcu_preempt_state.
68
69On the other hand, if the offending CPU fails to print out a stall-warning
70message quickly enough, some other CPU will print a message similar to
71the following:
72
73INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
74
75This message indicates that CPU 2 detected that CPUs 3 and 5 were both
76causing stalls, and that the stall was affecting RCU-bh. This message
77will normally be followed by stack dumps for each CPU. Please note that
78TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
79and that the tasks will be indicated by PID, for example, "P3421".
80It is even possible for a rcu_preempt_state stall to be caused by both
81CPUs -and- tasks, in which case the offending CPUs and tasks will all
82be called out in the list.
83
84Finally, if the grace period ends just as the stall warning starts
85printing, there will be a spurious stall-warning message:
86
87INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
88
89This is rare, but does happen from time to time in real life.
90
Paul E. McKenney24cd7fd2012-01-20 17:35:55 -080091If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set,
92more information is printed with the stall-warning message, for example:
93
94 INFO: rcu_preempt detected stall on CPU
95 0: (63959 ticks this GP) idle=241/3fffffffffffffff/0
96 (t=65000 jiffies)
97
98In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is
99printed:
100
101 INFO: rcu_preempt detected stall on CPU
102 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 drain=0 . timer=-1
103 (t=65000 jiffies)
104
105The "(64628 ticks this GP)" indicates that this CPU has taken more
106than 64,000 scheduling-clock interrupts during the current stalled
107grace period. If the CPU was not yet aware of the current grace
108period (for example, if it was offline), then this part of the message
109indicates how many grace periods behind the CPU is.
110
111The "idle=" portion of the message prints the dyntick-idle state.
112The hex number before the first "/" is the low-order 12 bits of the
113dynticks counter, which will have an even-numbered value if the CPU is
114in dyntick-idle mode and an odd-numbered value otherwise. The hex
115number between the two "/"s is the value of the nesting, which will
116be a small positive number if in the idle loop and a very large positive
117number (as shown above) otherwise.
118
119For CONFIG_RCU_FAST_NO_HZ kernels, the "drain=0" indicates that the
120CPU is not in the process of trying to force itself into dyntick-idle
121state, the "." indicates that the CPU has not given up forcing RCU
122into dyntick-idle mode (it would be "H" otherwise), and the "timer=-1"
123indicates that the CPU has not recented forced RCU into dyntick-idle
124mode (it would otherwise indicate the number of microseconds remaining
125in this forced state).
126
127
128Multiple Warnings From One Stall
129
130If a stall lasts long enough, multiple stall-warning messages will be
131printed for it. The second and subsequent messages are printed at
132longer intervals, so that the time between (say) the first and second
133message will be about three times the interval between the beginning
134of the stall and the first message.
135
136
137What Causes RCU CPU Stall Warnings?
138
Paul E. McKenneyf1d507b2010-04-15 15:49:46 -0700139So your kernel printed an RCU CPU stall warning. The next question is
140"What caused it?" The following problems can result in RCU CPU stall
141warnings:
Paul E. McKenney4c540052010-01-14 16:10:57 -0800142
143o A CPU looping in an RCU read-side critical section.
144
Paul E. McKenneyf1d507b2010-04-15 15:49:46 -0700145o A CPU looping with interrupts disabled. This condition can
146 result in RCU-sched and RCU-bh stalls.
Paul E. McKenney4c540052010-01-14 16:10:57 -0800147
Paul E. McKenneyf1d507b2010-04-15 15:49:46 -0700148o A CPU looping with preemption disabled. This condition can
149 result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
150 stalls.
151
152o A CPU looping with bottom halves disabled. This condition can
153 result in RCU-sched and RCU-bh stalls.
Paul E. McKenney4c540052010-01-14 16:10:57 -0800154
155o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
156 without invoking schedule().
157
Paul E. McKenney2c96c772010-08-23 16:34:02 -0700158o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
159 happen to preempt a low-priority task in the middle of an RCU
160 read-side critical section. This is especially damaging if
161 that low-priority task is not permitted to run on any other CPU,
162 in which case the next RCU grace period can never complete, which
163 will eventually cause the system to run out of memory and hang.
164 While the system is in the process of running itself out of
165 memory, you might see stall-warning messages.
166
167o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
168 is running at a higher priority than the RCU softirq threads.
169 This will prevent RCU callbacks from ever being invoked,
170 and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
171 RCU grace periods from ever completing. Either way, the
172 system will eventually run out of memory and hang. In the
173 CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
174 messages.
175
Paul E. McKenney2c015312011-10-02 17:21:18 -0700176o A hardware or software issue shuts off the scheduler-clock
177 interrupt on a CPU that is not in dyntick-idle mode. This
178 problem really has happened, and seems to be most likely to
179 result in RCU CPU stall warnings for CONFIG_NO_HZ=n kernels.
180
Paul E. McKenney4c540052010-01-14 16:10:57 -0800181o A bug in the RCU implementation.
182
183o A hardware failure. This is quite unlikely, but has occurred
Paul E. McKenneyf1d507b2010-04-15 15:49:46 -0700184 at least once in real life. A CPU failed in a running system,
Paul E. McKenney4c540052010-01-14 16:10:57 -0800185 becoming unresponsive, but not causing an immediate crash.
186 This resulted in a series of RCU CPU stall warnings, eventually
187 leading the realization that the CPU had failed.
188
Paul E. McKenney9ceae0e2011-11-03 13:43:24 -0700189The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.
190SRCU does not have its own CPU stall warnings, but its calls to
191synchronize_sched() will result in RCU-sched detecting RCU-sched-related
192CPU stalls. Please note that RCU only detects CPU stalls when there is
193a grace period in progress. No grace period, no CPU stall warnings.
Paul E. McKenney4c540052010-01-14 16:10:57 -0800194
Paul E. McKenneyf1d507b2010-04-15 15:49:46 -0700195To diagnose the cause of the stall, inspect the stack traces.
196The offending function will usually be near the top of the stack.
197If you have a series of stall warnings from a single extended stall,
198comparing the stack traces can often help determine where the stall
199is occurring, which will usually be in the function nearest the top of
200that portion of the stack which remains the same from trace to trace.
201If you can reliably trigger the stall, ftrace can be quite helpful.
Paul E. McKenney4c540052010-01-14 16:10:57 -0800202
Paul E. McKenney24cd7fd2012-01-20 17:35:55 -0800203RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
204and with RCU's event tracing.