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1Using RCU's CPU Stall Detector
2
3The CONFIG_RCU_CPU_STALL_DETECTOR kernel config parameter enables
4RCU's CPU stall detector, which detects conditions that unduly delay
5RCU grace periods. The stall detector's idea of what constitutes
6"unduly delayed" is controlled by a pair of C preprocessor macros:
7
8RCU_SECONDS_TILL_STALL_CHECK
9
10 This macro defines the period of time that RCU will wait from
11 the beginning of a grace period until it issues an RCU CPU
12 stall warning. It is normally ten seconds.
13
14RCU_SECONDS_TILL_STALL_RECHECK
15
16 This macro defines the period of time that RCU will wait after
17 issuing a stall warning until it issues another stall warning.
18 It is normally set to thirty seconds.
19
20RCU_STALL_RAT_DELAY
21
22 The CPU stall detector tries to make the offending CPU rat on itself,
23 as this often gives better-quality stack traces. However, if
24 the offending CPU does not detect its own stall in the number
25 of jiffies specified by RCU_STALL_RAT_DELAY, then other CPUs will
26 complain. This is normally set to two jiffies.
27
28The following problems can result in an RCU CPU stall warning:
29
30o A CPU looping in an RCU read-side critical section.
31
32o A CPU looping with interrupts disabled.
33
34o A CPU looping with preemption disabled.
35
36o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
37 without invoking schedule().
38
39o A bug in the RCU implementation.
40
41o A hardware failure. This is quite unlikely, but has occurred
42 at least once in a former life. A CPU failed in a running system,
43 becoming unresponsive, but not causing an immediate crash.
44 This resulted in a series of RCU CPU stall warnings, eventually
45 leading the realization that the CPU had failed.
46
47The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.
48SRCU does not do so directly, but its calls to synchronize_sched() will
49result in RCU-sched detecting any CPU stalls that might be occurring.
50
51To diagnose the cause of the stall, inspect the stack traces. The offending
52function will usually be near the top of the stack. If you have a series
53of stall warnings from a single extended stall, comparing the stack traces
54can often help determine where the stall is occurring, which will usually
55be in the function nearest the top of the stack that stays the same from
56trace to trace.
57
58RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE.