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1 | Using RCU's CPU Stall Detector |
2 | ||
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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: | |
4c54005c | 9 | |
a00e0d71 | 10 | CONFIG_RCU_CPU_STALL_TIMEOUT |
4c54005c | 11 | |
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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. | |
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16 | |
17 | RCU_SECONDS_TILL_STALL_RECHECK | |
18 | ||
19 | This macro defines the period of time that RCU will wait after | |
f1d507be | 20 | issuing a stall warning until it issues another stall warning |
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21 | for the same stall. This time period is normally set to three |
22 | times the check interval plus thirty seconds. | |
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23 | |
24 | RCU_STALL_RAT_DELAY | |
25 | ||
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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. | |
4c54005c | 32 | |
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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: | |
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70 | |
71 | o A CPU looping in an RCU read-side critical section. | |
72 | ||
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73 | o A CPU looping with interrupts disabled. This condition can |
74 | result in RCU-sched and RCU-bh stalls. | |
4c54005c | 75 | |
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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. | |
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82 | |
83 | o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel | |
84 | without invoking schedule(). | |
85 | ||
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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 | ||
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104 | o A hardware or software issue shuts off the scheduler-clock |
105 | interrupt on a CPU that is not in dyntick-idle mode. This | |
106 | problem really has happened, and seems to be most likely to | |
107 | result in RCU CPU stall warnings for CONFIG_NO_HZ=n kernels. | |
108 | ||
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109 | o A bug in the RCU implementation. |
110 | ||
111 | o A hardware failure. This is quite unlikely, but has occurred | |
f1d507be | 112 | at least once in real life. A CPU failed in a running system, |
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113 | becoming unresponsive, but not causing an immediate crash. |
114 | This resulted in a series of RCU CPU stall warnings, eventually | |
115 | leading the realization that the CPU had failed. | |
116 | ||
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117 | The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning. |
118 | SRCU does not have its own CPU stall warnings, but its calls to | |
119 | synchronize_sched() will result in RCU-sched detecting RCU-sched-related | |
120 | CPU stalls. Please note that RCU only detects CPU stalls when there is | |
121 | a grace period in progress. No grace period, no CPU stall warnings. | |
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122 | |
123 | To diagnose the cause of the stall, inspect the stack traces. | |
124 | The offending function will usually be near the top of the stack. | |
125 | If you have a series of stall warnings from a single extended stall, | |
126 | comparing the stack traces can often help determine where the stall | |
127 | is occurring, which will usually be in the function nearest the top of | |
128 | that portion of the stack which remains the same from trace to trace. | |
129 | If you can reliably trigger the stall, ftrace can be quite helpful. | |
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130 | |
131 | RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE. |