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ReStructuredText
386 lines
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ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==============================
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Using RCU's CPU Stall Detector
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==============================
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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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===================================
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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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- A CPU looping in an RCU read-side critical section.
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- A CPU looping with interrupts disabled.
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- A CPU looping with preemption disabled.
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- A CPU looping with bottom halves disabled.
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- For !CONFIG_PREEMPTION 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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- 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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- 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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- A CPU-bound real-time task in a CONFIG_PREEMPTION 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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- 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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You can use the rcutree.kthread_prio kernel boot parameter to
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increase the scheduling priority of RCU's kthreads, which can
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help avoid this problem. However, please note that doing this
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can increase your system's context-switch rate and thus degrade
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performance.
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- 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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- 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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- 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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- A hardware or software issue that prevents time-based wakeups
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from occurring. These issues can range from misconfigured or
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buggy timer hardware through bugs in the interrupt or exception
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path (whether hardware, firmware, or software) through bugs
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in Linux's timer subsystem through bugs in the scheduler, and,
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yes, even including bugs in RCU itself. It can also result in
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the ``rcu_.*timer wakeup didn't happen for`` console-log message,
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which will include additional debugging information.
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- A low-level kernel issue that either fails to invoke one of the
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variants of rcu_eqs_enter(true), rcu_eqs_exit(true), ct_idle_enter(),
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ct_idle_exit(), ct_irq_enter(), or ct_irq_exit() on the one
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hand, or that invokes one of them too many times on the other.
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Historically, the most frequent issue has been an omission
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of either irq_enter() or irq_exit(), which in turn invoke
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ct_irq_enter() or ct_irq_exit(), respectively. Building your
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kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types
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of issues, which sometimes arise in architecture-specific code.
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- A bug in the RCU implementation.
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- 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-tasks implementations have CPU stall warning.
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Note that SRCU does *not* have CPU stall warnings. Please note that
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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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======================================
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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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----------------------------
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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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CONFIG_RCU_EXP_CPU_STALL_TIMEOUT
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--------------------------------
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Same as the CONFIG_RCU_CPU_STALL_TIMEOUT parameter but only for
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the expedited grace period. This parameter defines the period
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of time that RCU will wait from the beginning of an expedited
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grace period until it issues an RCU CPU stall warning. This time
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period is normally 20 milliseconds on Android devices. A zero
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value causes the CONFIG_RCU_CPU_STALL_TIMEOUT value to be used,
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after conversion to milliseconds.
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This configuration parameter may be changed at runtime via the
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/sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout, however
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this parameter is checked only at the beginning of a cycle. If you
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are in a current stall cycle, setting it to a new value will change
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the timeout for the -next- 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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---------------------
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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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-------------------
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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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-------------------------------
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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 seconds. 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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==============================================
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For non-RCU-tasks flavors of RCU, when a CPU detects that some other
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CPU is stalling, 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 an rcu_state stall to be caused by both CPUs *and* tasks,
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in which case the offending CPUs and tasks will all be called out in the list.
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In some cases, CPUs will detect themselves stalling, which will result
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in a self-detected stall.
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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 "fqs=" 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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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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rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
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Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
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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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If the relevant grace-period kthread does not wake from FQS wait in a
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reasonable time, then the following additional line is printed::
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kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
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The "23804" indicates that kthread's timer expired more than 23 thousand
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jiffies ago. The rest of the line has meaning similar to the kthread
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starvation case.
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Additionally, the following line is printed::
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Possible timer handling issue on cpu=4 timer-softirq=11142
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Here "cpu" indicates that the grace-period kthread last ran on CPU 4,
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where it queued the fqs timer. The number following the "timer-softirq"
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is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not
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change on successive RCU CPU stall warnings, there is further reason to
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suspect a timer problem.
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These messages are usually followed by stack dumps of the CPUs and tasks
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involved in the stall. These stack traces can help you locate the cause
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of the stall, keeping in mind that the CPU detecting the stall will have
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an interrupt frame that is mainly devoted to detecting the stall.
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Multiple Warnings From One Stall
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================================
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If a stall lasts long enough, multiple stall-warning messages will
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be 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. It can be helpful to compare the
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stack dumps for the different messages for the same stalled grace period.
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Stall Warnings for Expedited Grace Periods
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==========================================
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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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