84 lines
4.1 KiB
ReStructuredText
84 lines
4.1 KiB
ReStructuredText
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===============================================================
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Softlockup detector and hardlockup detector (aka nmi_watchdog)
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===============================================================
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The Linux kernel can act as a watchdog to detect both soft and hard
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lockups.
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A 'softlockup' is defined as a bug that causes the kernel to loop in
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kernel mode for more than 20 seconds (see "Implementation" below for
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details), without giving other tasks a chance to run. The current
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stack trace is displayed upon detection and, by default, the system
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will stay locked up. Alternatively, the kernel can be configured to
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panic; a sysctl, "kernel.softlockup_panic", a kernel parameter,
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"softlockup_panic" (see "Documentation/admin-guide/kernel-parameters.rst" for
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details), and a compile option, "BOOTPARAM_SOFTLOCKUP_PANIC", are
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provided for this.
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A 'hardlockup' is defined as a bug that causes the CPU to loop in
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kernel mode for more than 10 seconds (see "Implementation" below for
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details), without letting other interrupts have a chance to run.
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Similarly to the softlockup case, the current stack trace is displayed
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upon detection and the system will stay locked up unless the default
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behavior is changed, which can be done through a sysctl,
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'hardlockup_panic', a compile time knob, "BOOTPARAM_HARDLOCKUP_PANIC",
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and a kernel parameter, "nmi_watchdog"
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(see "Documentation/admin-guide/kernel-parameters.rst" for details).
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The panic option can be used in combination with panic_timeout (this
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timeout is set through the confusingly named "kernel.panic" sysctl),
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to cause the system to reboot automatically after a specified amount
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of time.
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Implementation
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==============
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The soft and hard lockup detectors are built on top of the hrtimer and
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perf subsystems, respectively. A direct consequence of this is that,
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in principle, they should work in any architecture where these
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subsystems are present.
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A periodic hrtimer runs to generate interrupts and kick the watchdog
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task. An NMI perf event is generated every "watchdog_thresh"
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(compile-time initialized to 10 and configurable through sysctl of the
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same name) seconds to check for hardlockups. If any CPU in the system
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does not receive any hrtimer interrupt during that time the
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'hardlockup detector' (the handler for the NMI perf event) will
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generate a kernel warning or call panic, depending on the
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configuration.
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The watchdog task is a high priority kernel thread that updates a
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timestamp every time it is scheduled. If that timestamp is not updated
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for 2*watchdog_thresh seconds (the softlockup threshold) the
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'softlockup detector' (coded inside the hrtimer callback function)
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will dump useful debug information to the system log, after which it
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will call panic if it was instructed to do so or resume execution of
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other kernel code.
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The period of the hrtimer is 2*watchdog_thresh/5, which means it has
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two or three chances to generate an interrupt before the hardlockup
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detector kicks in.
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As explained above, a kernel knob is provided that allows
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administrators to configure the period of the hrtimer and the perf
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event. The right value for a particular environment is a trade-off
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between fast response to lockups and detection overhead.
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By default, the watchdog runs on all online cores. However, on a
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kernel configured with NO_HZ_FULL, by default the watchdog runs only
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on the housekeeping cores, not the cores specified in the "nohz_full"
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boot argument. If we allowed the watchdog to run by default on
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the "nohz_full" cores, we would have to run timer ticks to activate
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the scheduler, which would prevent the "nohz_full" functionality
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from protecting the user code on those cores from the kernel.
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Of course, disabling it by default on the nohz_full cores means that
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when those cores do enter the kernel, by default we will not be
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able to detect if they lock up. However, allowing the watchdog
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to continue to run on the housekeeping (non-tickless) cores means
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that we will continue to detect lockups properly on those cores.
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In either case, the set of cores excluded from running the watchdog
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may be adjusted via the kernel.watchdog_cpumask sysctl. For
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nohz_full cores, this may be useful for debugging a case where the
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kernel seems to be hanging on the nohz_full cores.
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