Re: [PATCH v2 5/5] doc: watchdog: Document buddy detector

[Petr Mladek <pmladek@suse.com>]

On Thu 2026-03-12 16:22:06, Mayank Rungta via B4 Relay wrote:
> From: Mayank Rungta <mrungta@google.com>
> 
> 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.

This is a great step forward. See few nits below:

> --- a/Documentation/admin-guide/lockup-watchdogs.rst
> +++ b/Documentation/admin-guide/lockup-watchdogs.rst
> @@ -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.

This looks like a too big simplification. In fact, the hrtimer is
the core of all these detectors. The buddy detector uses only
the hrtimer. Also it would be nice to mention the scheduled
job used by softlockup detector.

See below for a proposal.

> -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
> +------------------------
> +
> +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.
> +
> +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.

As I said, the hrtimer is the core of all detectors. I would explain
this first.

I propose the following changes on top of this one:

From f1cfdc330cfbc68568dfe6bf2513bde9373c89d7 Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Mon, 23 Mar 2026 18:21:38 +0100
Subject: [PATCH] doc: watchdog: Futher improvements

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 .../admin-guide/lockup-watchdogs.rst          | 44 ++++++++++---------
 1 file changed, 24 insertions(+), 20 deletions(-)

diff --git a/Documentation/admin-guide/lockup-watchdogs.rst b/Documentation/admin-guide/lockup-watchdogs.rst
index 7ae7ce3abd2c..d0773edf3396 100644
--- a/Documentation/admin-guide/lockup-watchdogs.rst
+++ b/Documentation/admin-guide/lockup-watchdogs.rst
@@ -41,31 +41,35 @@ is a trade-off between fast response to lockups and detection overhead.
 Implementation
 ==============
 
-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.
-
-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
-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 soft and hard lockup detectors are built around a 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 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.
+The core of the detectors in a hrtimer. It servers multiple purpose:
 
-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.
+- 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
+will call panic if it was instructed to do so or resume execution of
+other kernel code.
 
 Hardlockup Detector (NMI/Perf)
 ------------------------------
-- 
2.53.0


Best Regards,
Petr