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