59 lines
2.3 KiB
Plaintext
59 lines
2.3 KiB
Plaintext
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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 pair 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. It 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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It is normally set to thirty seconds.
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RCU_STALL_RAT_DELAY
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The CPU stall detector tries to make the offending CPU rat on itself,
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as this often gives better-quality stack traces. However, if
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the offending CPU does not detect its own stall in the number
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of jiffies specified by RCU_STALL_RAT_DELAY, then other CPUs will
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complain. This is normally set to two jiffies.
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The following problems can result in an RCU CPU stall warning:
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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.
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o A CPU looping with preemption disabled.
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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 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 a former 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 warning.
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SRCU does not do so directly, but its calls to synchronize_sched() will
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result in RCU-sched detecting any CPU stalls that might be occurring.
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To diagnose the cause of the stall, inspect the stack traces. The offending
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function will usually be near the top of the stack. If you have a series
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of stall warnings from a single extended stall, comparing the stack traces
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can often help determine where the stall is occurring, which will usually
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be in the function nearest the top of the stack that stays the same from
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trace to trace.
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RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE.
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