125 lines
5.4 KiB
Plaintext
125 lines
5.4 KiB
Plaintext
Using RCU's CPU Stall Detector
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The CONFIG_RCU_CPU_STALL_DETECTOR kernel config parameter enables
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RCU's CPU stall detector, which detects conditions that unduly delay
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RCU grace periods. The stall detector's idea of what constitutes
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"unduly delayed" is controlled by a set of C preprocessor macros:
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RCU_SECONDS_TILL_STALL_CHECK
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This macro defines the period of time that RCU will wait from
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the beginning of a grace period until it issues an RCU CPU
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stall warning. This time period is normally ten seconds.
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RCU_SECONDS_TILL_STALL_RECHECK
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This macro defines the period of time that RCU will wait after
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issuing a stall warning until it issues another stall warning
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for the same stall. This time period is normally set to thirty
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seconds.
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RCU_STALL_RAT_DELAY
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The CPU stall detector tries to make the offending CPU print its
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own warnings, as this often gives better-quality stack traces.
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However, if the offending CPU does not detect its own stall in
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the number of jiffies specified by RCU_STALL_RAT_DELAY, then
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some other CPU will complain. This delay is normally set to
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two jiffies.
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When a CPU detects that it is stalling, it will print a message similar
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to the following:
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INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies)
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This message indicates that CPU 5 detected that it was causing a stall,
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and that the stall was affecting RCU-sched. This message will normally be
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followed by a stack dump of the offending CPU. On TREE_RCU kernel builds,
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RCU and RCU-sched are implemented by the same underlying mechanism,
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while on TREE_PREEMPT_RCU kernel builds, RCU is instead implemented
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by rcu_preempt_state.
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On the other hand, if the offending CPU fails to print out a stall-warning
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message quickly enough, some other CPU will print a message similar to
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the following:
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INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies)
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This message indicates that CPU 2 detected that CPUs 3 and 5 were both
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causing stalls, and that the stall was affecting RCU-bh. This message
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will normally be followed by stack dumps for each CPU. Please note that
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TREE_PREEMPT_RCU builds can be stalled by tasks as well as by CPUs,
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and that the tasks will be indicated by PID, for example, "P3421".
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It is even possible for a rcu_preempt_state stall to be caused by both
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CPUs -and- tasks, in which case the offending CPUs and tasks will all
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be called out in the list.
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Finally, if the grace period ends just as the stall warning starts
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printing, there will be a spurious stall-warning message:
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INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies)
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This is rare, but does happen from time to time in real life.
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So your kernel printed an RCU CPU stall warning. The next question is
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"What caused it?" The following problems can result in RCU CPU stall
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warnings:
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o A CPU looping in an RCU read-side critical section.
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o A CPU looping with interrupts disabled. This condition can
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result in RCU-sched and RCU-bh stalls.
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o A CPU looping with preemption disabled. This condition can
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result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh
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stalls.
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o A CPU looping with bottom halves disabled. This condition can
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result in RCU-sched and RCU-bh stalls.
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o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
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without invoking schedule().
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o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
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happen to preempt a low-priority task in the middle of an RCU
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read-side critical section. This is especially damaging if
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that low-priority task is not permitted to run on any other CPU,
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in which case the next RCU grace period can never complete, which
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will eventually cause the system to run out of memory and hang.
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While the system is in the process of running itself out of
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memory, you might see stall-warning messages.
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o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
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is running at a higher priority than the RCU softirq threads.
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This will prevent RCU callbacks from ever being invoked,
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and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
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RCU grace periods from ever completing. Either way, the
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system will eventually run out of memory and hang. In the
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CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
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messages.
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o A bug in the RCU implementation.
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o A hardware failure. This is quite unlikely, but has occurred
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at least once in real life. A CPU failed in a running system,
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becoming unresponsive, but not causing an immediate crash.
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This resulted in a series of RCU CPU stall warnings, eventually
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leading the realization that the CPU had failed.
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The RCU, RCU-sched, and RCU-bh implementations have CPU stall
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warning. SRCU does not have its own CPU stall warnings, but its
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calls to synchronize_sched() will result in RCU-sched detecting
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RCU-sched-related CPU stalls. Please note that RCU only detects
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CPU stalls when there is a grace period in progress. No grace period,
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no CPU stall warnings.
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To diagnose the cause of the stall, inspect the stack traces.
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The offending function will usually be near the top of the stack.
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If you have a series of stall warnings from a single extended stall,
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comparing the stack traces can often help determine where the stall
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is occurring, which will usually be in the function nearest the top of
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that portion of the stack which remains the same from trace to trace.
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If you can reliably trigger the stall, ftrace can be quite helpful.
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RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE.
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