Documentation/admin-guide/lockup-watchdogs.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/lockup-watchdogs.rst

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Documentation/admin-guide/lockup-watchdogs.rst
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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===============================================================
Softlockup detector and hardlockup detector (aka nmi_watchdog)
===============================================================

The Linux kernel can act as a watchdog to detect both soft and hard
lockups.

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 several 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
of time.

Configuration
=============

A kernel knob is provided that allows administrators to configure
this period. The "watchdog_thresh" parameter (default 10 seconds)
controls the threshold. The right value for a particular environment
is a trade-off between fast response to lockups and detection overhead.

Implementation
==============

The soft and hard lockup detectors are built around an hrtimer.
In addition, the softlockup detector regularly schedules a job, and
the hard lockup detector might use Perf/NMI events on architectures
that support it.

Frequency and Heartbeats
------------------------

The core of the detectors is an hrtimer. It serves multiple purposes:

- schedules watchdog job for the softlockup detector
- bumps the interrupt counter for hardlockup detectors (heartbeat)
- detects softlockups
- detects hardlockups in Buddy mode

The period of this hrtimer is 2*watchdog_thresh/5, which is 4 seconds
by default. The hrtimer has two or three chances to generate an interrupt
(heartbeat) before the hardlockup detector kicks in.

Softlockup Detector
-------------------

The watchdog job is scheduled by the hrtimer and runs in a stop scheduling
thread. It 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

Annotation

Implementation Notes