319 lines
15 KiB
Plaintext
319 lines
15 KiB
Plaintext
Using RCU's CPU Stall Detector
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This document first discusses what sorts of issues RCU's CPU stall
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detector can locate, and then discusses kernel parameters and Kconfig
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options that can be used to fine-tune the detector's operation. Finally,
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this document explains the stall detector's "splat" format.
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What Causes RCU CPU Stall Warnings?
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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.
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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(). If the looping in the kernel is
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really expected and desirable behavior, you might need to add
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some calls to cond_resched().
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o Booting Linux using a console connection that is too slow to
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keep up with the boot-time console-message rate. For example,
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a 115Kbaud serial console can be -way- too slow to keep up
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with boot-time message rates, and will frequently result in
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RCU CPU stall warning messages. Especially if you have added
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debug printk()s.
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o Anything that prevents RCU's grace-period kthreads from running.
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This can result in the "All QSes seen" console-log message.
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This message will include information on when the kthread last
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ran and how often it should be expected to run. It can also
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result in the "rcu_.*kthread starved for" console-log message,
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which will include additional debugging information.
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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_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_PREEMPT_RCU case, you might see stall-warning
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messages.
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o A periodic interrupt whose handler takes longer than the time
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interval between successive pairs of interrupts. This can
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prevent RCU's kthreads and softirq handlers from running.
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Note that certain high-overhead debugging options, for example
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the function_graph tracer, can result in interrupt handler taking
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considerably longer than normal, which can in turn result in
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RCU CPU stall warnings.
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o Testing a workload on a fast system, tuning the stall-warning
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timeout down to just barely avoid RCU CPU stall warnings, and then
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running the same workload with the same stall-warning timeout on a
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slow system. Note that thermal throttling and on-demand governors
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can cause a single system to be sometimes fast and sometimes slow!
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o A hardware or software issue shuts off the scheduler-clock
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interrupt on a CPU that is not in dyntick-idle mode. This
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problem really has happened, and seems to be most likely to
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result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
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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, RCU-bh, and RCU-tasks implementations have CPU stall
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warning. Note that SRCU does -not- have CPU stall warnings. Please note
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that RCU only detects CPU stalls when there is a grace period in progress.
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No grace period, 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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and with RCU's event tracing. For information on RCU's event tracing,
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see include/trace/events/rcu.h.
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Fine-Tuning the RCU CPU Stall Detector
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The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
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CPU stall detector, which detects conditions that unduly delay RCU grace
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periods. This module parameter enables CPU stall detection by default,
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but may be overridden via boot-time parameter or at runtime via sysfs.
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The stall detector's idea of what constitutes "unduly delayed" is
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controlled by a set of kernel configuration variables and cpp macros:
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CONFIG_RCU_CPU_STALL_TIMEOUT
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This kernel configuration parameter defines the period of time
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that RCU will wait from the beginning of a grace period until it
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issues an RCU CPU stall warning. This time period is normally
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21 seconds.
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This configuration parameter may be changed at runtime via the
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/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
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this parameter is checked only at the beginning of a cycle.
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So if you are 10 seconds into a 40-second stall, setting this
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sysfs parameter to (say) five will shorten the timeout for the
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-next- stall, or the following warning for the current stall
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(assuming the stall lasts long enough). It will not affect the
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timing of the next warning for the current stall.
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Stall-warning messages may be enabled and disabled completely via
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/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
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RCU_STALL_DELAY_DELTA
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Although the lockdep facility is extremely useful, it does add
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some overhead. Therefore, under CONFIG_PROVE_RCU, the
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RCU_STALL_DELAY_DELTA macro allows five extra seconds before
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giving an RCU CPU stall warning message. (This is a cpp
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macro, not a kernel configuration parameter.)
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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. (This is a cpp macro, not a kernel configuration
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parameter.)
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rcupdate.rcu_task_stall_timeout
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This boot/sysfs parameter controls the RCU-tasks stall warning
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interval. A value of zero or less suppresses RCU-tasks stall
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warnings. A positive value sets the stall-warning interval
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in jiffies. An RCU-tasks stall warning starts with the line:
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INFO: rcu_tasks detected stalls on tasks:
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And continues with the output of sched_show_task() for each
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task stalling the current RCU-tasks grace period.
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Interpreting RCU's CPU Stall-Detector "Splats"
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For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling,
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it will print a message similar to the following:
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INFO: rcu_sched detected stalls on CPUs/tasks:
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2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
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16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
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(detected by 32, t=2603 jiffies, g=7075, q=625)
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This message indicates that CPU 32 detected that CPUs 2 and 16 were both
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causing stalls, and that the stall was affecting RCU-sched. This message
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will normally be followed by stack dumps for each CPU. Please note that
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PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that
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the tasks will be indicated by PID, for example, "P3421". It is even
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possible for a rcu_preempt_state stall to be caused by both CPUs -and-
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tasks, in which case the offending CPUs and tasks will all be called
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out in the list.
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CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
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the RCU core for the past three grace periods. In contrast, CPU 16's "(0
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ticks this GP)" indicates that this CPU has not taken any scheduling-clock
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interrupts during the current stalled grace period.
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The "idle=" portion of the message prints the dyntick-idle state.
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The hex number before the first "/" is the low-order 12 bits of the
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dynticks counter, which will have an even-numbered value if the CPU
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is in dyntick-idle mode and an odd-numbered value otherwise. The hex
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number between the two "/"s is the value of the nesting, which will be
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a small non-negative number if in the idle loop (as shown above) and a
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very large positive number otherwise.
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The "softirq=" portion of the message tracks the number of RCU softirq
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handlers that the stalled CPU has executed. The number before the "/"
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is the number that had executed since boot at the time that this CPU
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last noted the beginning of a grace period, which might be the current
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(stalled) grace period, or it might be some earlier grace period (for
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example, if the CPU might have been in dyntick-idle mode for an extended
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time period. The number after the "/" is the number that have executed
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since boot until the current time. If this latter number stays constant
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across repeated stall-warning messages, it is possible that RCU's softirq
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handlers are no longer able to execute on this CPU. This can happen if
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the stalled CPU is spinning with interrupts are disabled, or, in -rt
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kernels, if a high-priority process is starving RCU's softirq handler.
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The "fps=" shows the number of force-quiescent-state idle/offline
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detection passes that the grace-period kthread has made across this
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CPU since the last time that this CPU noted the beginning of a grace
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period.
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The "detected by" line indicates which CPU detected the stall (in this
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case, CPU 32), how many jiffies have elapsed since the start of the grace
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period (in this case 2603), the grace-period sequence number (7075), and
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an estimate of the total number of RCU callbacks queued across all CPUs
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(625 in this case).
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In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
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for each CPU:
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0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
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The "last_accelerate:" prints the low-order 16 bits (in hex) of the
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jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
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from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
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rcu_prepare_for_idle(). The "nonlazy_posted:" prints the number
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of non-lazy callbacks posted since the last call to rcu_needs_cpu().
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Finally, an "L" indicates that there are currently no non-lazy callbacks
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("." is printed otherwise, as shown above) and "D" indicates that
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dyntick-idle processing is enabled ("." is printed otherwise, for example,
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if disabled via the "nohz=" kernel boot parameter).
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If the grace period ends just as the stall warning starts printing,
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there will be a spurious stall-warning message, which will include
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the following:
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INFO: Stall ended before state dump start
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This is rare, but does happen from time to time in real life. It is also
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possible for a zero-jiffy stall to be flagged in this case, depending
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on how the stall warning and the grace-period initialization happen to
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interact. Please note that it is not possible to entirely eliminate this
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sort of false positive without resorting to things like stop_machine(),
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which is overkill for this sort of problem.
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If all CPUs and tasks have passed through quiescent states, but the
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grace period has nevertheless failed to end, the stall-warning splat
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will include something like the following:
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All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0
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The "23807" indicates that it has been more than 23 thousand jiffies
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since the grace-period kthread ran. The "jiffies_till_next_fqs"
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indicates how frequently that kthread should run, giving the number
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of jiffies between force-quiescent-state scans, in this case three,
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which is way less than 23807. Finally, the root rcu_node structure's
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->qsmask field is printed, which will normally be zero.
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If the relevant grace-period kthread has been unable to run prior to
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the stall warning, as was the case in the "All QSes seen" line above,
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the following additional line is printed:
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kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
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Starving the grace-period kthreads of CPU time can of course result
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in RCU CPU stall warnings even when all CPUs and tasks have passed
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through the required quiescent states. The "g" number shows the current
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grace-period sequence number, the "f" precedes the ->gp_flags command
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to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the
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kthread is waiting for a short timeout, the "state" precedes value of the
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task_struct ->state field, and the "cpu" indicates that the grace-period
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kthread last ran on CPU 5.
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Multiple Warnings From One Stall
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If a stall lasts long enough, multiple stall-warning messages will be
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printed for it. The second and subsequent messages are printed at
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longer intervals, so that the time between (say) the first and second
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message will be about three times the interval between the beginning
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of the stall and the first message.
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Stall Warnings for Expedited Grace Periods
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If an expedited grace period detects a stall, it will place a message
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like the following in dmesg:
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INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
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This indicates that CPU 7 has failed to respond to a reschedule IPI.
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The three periods (".") following the CPU number indicate that the CPU
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is online (otherwise the first period would instead have been "O"),
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that the CPU was online at the beginning of the expedited grace period
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(otherwise the second period would have instead been "o"), and that
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the CPU has been online at least once since boot (otherwise, the third
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period would instead have been "N"). The number before the "jiffies"
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indicates that the expedited grace period has been going on for 21,119
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jiffies. The number following the "s:" indicates that the expedited
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grace-period sequence counter is 73. The fact that this last value is
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odd indicates that an expedited grace period is in flight. The number
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following "root:" is a bitmask that indicates which children of the root
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rcu_node structure correspond to CPUs and/or tasks that are blocking the
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current expedited grace period. If the tree had more than one level,
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additional hex numbers would be printed for the states of the other
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rcu_node structures in the tree.
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As with normal grace periods, PREEMPT_RCU builds can be stalled by
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tasks as well as by CPUs, and that the tasks will be indicated by PID,
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for example, "P3421".
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It is entirely possible to see stall warnings from normal and from
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expedited grace periods at about the same time during the same run.
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