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doc: watchdog: document buddy detector

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The current documentation generalizes the hardlockup detector as primarily
NMI-perf-based and lacks details on the SMP "Buddy" detector.

Update the documentation to add a detailed description of the Buddy
detector, and also restructure the "Implementation" section to explicitly
separate "Softlockup Detector", "Hardlockup Detector (NMI/Perf)", and
"Hardlockup Detector (Buddy)".

Clarify that the softlockup hrtimer acts as the heartbeat generator for
both hardlockup mechanisms and centralize the configuration details in a
"Frequency and Heartbeats" section.

Link: https://lkml.kernel.org/r/20260312-hardlockup-watchdog-fixes-v2-5-45bd8a0cc7ed@google.com
Signed-off-by: Mayank Rungta <mrungta@google.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Li Huafei <lihuafei1@huawei.com>
Cc: Max Kellermann <max.kellermann@ionos.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Stephane Erainan <eranian@google.com>
Cc: Wang Jinchao <wangjinchao600@gmail.com>
Cc: Yunhui Cui <cuiyunhui@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Mayank Rungta
2026-03-12 16:22:06 -07:00
committed by Andrew Morton
parent 077ba03600
commit cb8615f3cb
1 changed files with 101 additions and 48 deletions
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149
Documentation/admin-guide/lockup-watchdogs.rst
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@@ -30,22 +30,23 @@ 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 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.
The soft lockup detector is built on top of the hrtimer subsystem.
The hard lockup detector is built on top of the perf subsystem
(on architectures that support it) or uses an SMP "buddy" system.
A periodic hrtimer runs to generate interrupts and kick the watchdog
job. 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
configuration.
Softlockup Detector
-------------------
The watchdog job runs in a stop scheduling thread that updates a
timestamp every time it is scheduled. If that timestamp is not updated
@@ -55,53 +56,105 @@ 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.
Frequency and Heartbeats
------------------------
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.
The hrtimer used by the softlockup detector serves a dual purpose:
it detects softlockups, and it also generates the interrupts
(heartbeats) that the hardlockup detectors use to verify CPU liveness.
Detection Overhead
------------------
The period of this hrtimer is 2*watchdog_thresh/5. This means the
hrtimer has two or three chances to generate an interrupt before the
NMI hardlockup detector kicks in.
The hardlockup detector checks for lockups using a periodic NMI perf
event. This means the time to detect a lockup can vary depending on
when the lockup occurs relative to the NMI check window.
Hardlockup Detector (NMI/Perf)
------------------------------
**Best Case:**
In the best case scenario, the lockup occurs just before the first
heartbeat is due. The detector will notice the missing hrtimer
interrupt almost immediately during the next check.
On architectures that support NMI (Non-Maskable Interrupt) perf events,
a periodic NMI is generated every "watchdog_thresh" seconds.
::
If any CPU in the system does not receive any hrtimer interrupt
(heartbeat) during the "watchdog_thresh" window, the 'hardlockup
detector' (the handler for the NMI perf event) will generate a kernel
warning or call panic.
Time 100.0: cpu 1 heartbeat
Time 100.1: hardlockup_check, cpu1 stores its state
Time 103.9: Hard Lockup on cpu1
Time 104.0: cpu 1 heartbeat never comes
Time 110.1: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
**Detection Overhead (NMI):**
Time to detection: ~6 seconds
The time to detect a lockup can vary depending on when the lockup
occurs relative to the NMI check window. Examples below assume a watchdog_thresh of 10.
**Worst Case:**
In the worst case scenario, the lockup occurs shortly after a valid
interrupt (heartbeat) which itself happened just after the NMI check.
The next NMI check sees that the interrupt count has changed (due to
that one heartbeat), assumes the CPU is healthy, and resets the
baseline. The lockup is only detected at the subsequent check.
* **Best Case:** The lockup occurs just before the first heartbeat is
due. The detector will notice the missing hrtimer interrupt almost
immediately during the next check.
::
::
Time 100.0: hardlockup_check, cpu1 stores its state
Time 100.1: cpu 1 heartbeat
Time 100.2: Hard Lockup on cpu1
Time 110.0: hardlockup_check, cpu1 stores its state (misses lockup as state changed)
Time 120.0: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
Time 100.0: cpu 1 heartbeat
Time 100.1: hardlockup_check, cpu1 stores its state
Time 103.9: Hard Lockup on cpu1
Time 104.0: cpu 1 heartbeat never comes
Time 110.1: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
Time to detection: ~20 seconds
Time to detection: ~6 seconds
* **Worst Case:** The lockup occurs shortly after a valid interrupt
(heartbeat) which itself happened just after the NMI check. The next
NMI check sees that the interrupt count has changed (due to that one
heartbeat), assumes the CPU is healthy, and resets the baseline. The
lockup is only detected at the subsequent check.
::
Time 100.0: hardlockup_check, cpu1 stores its state
Time 100.1: cpu 1 heartbeat
Time 100.2: Hard Lockup on cpu1
Time 110.0: hardlockup_check, cpu1 stores its state (misses lockup as state changed)
Time 120.0: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
Time to detection: ~20 seconds
Hardlockup Detector (Buddy)
---------------------------
On architectures or configurations where NMI perf events are not
available (or disabled), the kernel may use the "buddy" hardlockup
detector. This mechanism requires SMP (Symmetric Multi-Processing).
In this mode, each CPU is assigned a "buddy" CPU to monitor. The
monitoring CPU runs its own hrtimer (the same one used for softlockup
detection) and checks if the buddy CPU's hrtimer interrupt count has
increased.
To ensure timeliness and avoid false positives, the buddy system performs
checks at every hrtimer interval (2*watchdog_thresh/5, which is 4 seconds
by default). It uses a missed-interrupt threshold of 3. If the buddy's
interrupt count has not changed for 3 consecutive checks, it is assumed
that the buddy CPU is hardlocked (interrupts disabled). The monitoring
CPU will then trigger the hardlockup response (warning or panic).
**Detection Overhead (Buddy):**
With a default check interval of 4 seconds (watchdog_thresh = 10):
* **Best case:** Lockup occurs just before a check.
Detected in ~8s (0s till 1st check + 4s till 2nd + 4s till 3rd).
* **Worst case:** Lockup occurs just after a check.
Detected in ~12s (4s till 1st check + 4s till 2nd + 4s till 3rd).
**Limitations of the Buddy Detector:**
1. **All-CPU Lockup:** If all CPUs lock up simultaneously, the buddy
detector cannot detect the condition because the monitoring CPUs
are also frozen.
2. **Stack Traces:** Unlike the NMI detector, the buddy detector
cannot directly interrupt the locked CPU to grab a stack trace.
It relies on architecture-specific mechanisms (like NMI backtrace
support) to try and retrieve the status of the locked CPU. If
such support is missing, the log may only show that a lockup
occurred without providing the locked CPU's stack.
Watchdog Core Exclusion
=======================
By default, the watchdog runs on all online cores. However, on a
kernel configured with NO_HZ_FULL, by default the watchdog runs only