Re: [PATCH 3/4] watchdog/hardlockup: improve buddy system detection timeliness

[Petr Mladek <pmladek@suse.com>]

On Thu 2026-02-12 14:12:12, Mayank Rungta via B4 Relay wrote:
> From: Mayank Rungta <mrungta@google.com>
> 
> Currently, the buddy system only performs checks every 3rd sample. With
> a 4-second interval. If a check window is missed, the next check occurs
> 12 seconds later, potentially delaying hard lockup detection for up to
> 24 seconds.
> 
> Modify the buddy system to perform checks at every interval (4s).
> Introduce a missed-interrupt threshold to maintain the existing grace
> period while reducing the detection window to 8-12 seconds.
> 
> Best and worst case detection scenarios:
> 
> Before (12s check window):
> - Best case: Lockup occurs after first check but just before heartbeat
>   interval. Detected in ~8s (8s till next check).
> - Worst case: Lockup occurs just after a check.
>   Detected in ~24s (missed check + 12s till next check + 12s logic).
> 
> After (4s check window with threshold of 3):
> - 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).

One might argue that the interval <8s,24s> is not much worse than
<6s,20s> achieved by the perf detector.

But I personally like that the disperse of <8s,12s> is lower so that
the result is more predictable. And it is relatively cheap.

People might have different option. But I am fine with this change.

> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -163,8 +171,13 @@ static bool is_hardlockup(unsigned int cpu)
>  {
>  	int hrint = atomic_read(&per_cpu(hrtimer_interrupts, cpu));
>  
> -	if (per_cpu(hrtimer_interrupts_saved, cpu) == hrint)
> -		return true;
> +	if (per_cpu(hrtimer_interrupts_saved, cpu) == hrint) {
> +		per_cpu(hrtimer_interrupts_missed, cpu)++;
> +		if (per_cpu(hrtimer_interrupts_missed, cpu) >= watchdog_hardlockup_miss_thresh)

This would return true for every check when missed >= 3.
As a result, the hardlockup would be reported every 4s.

I would keep the 12s cadence and change this to:

		if (per_cpu(hrtimer_interrupts_missed, cpu) % watchdog_hardlockup_miss_thresh == 0)

> +			return true;
> +
> +		return false;
> +	}
>  
>  	/*
>  	 * NOTE: we don't need any fancy atomic_t or READ_ONCE/WRITE_ONCE
> --- a/kernel/watchdog_buddy.c
> +++ b/kernel/watchdog_buddy.c
> @@ -86,14 +87,6 @@ void watchdog_buddy_check_hardlockup(int hrtimer_interrupts)
>  {
>  	unsigned int next_cpu;
>  
> -	/*
> -	 * Test for hardlockups every 3 samples. The sample period is
> -	 *  watchdog_thresh * 2 / 5, so 3 samples gets us back to slightly over
> -	 *  watchdog_thresh (over by 20%).
> -	 */
> -	if (hrtimer_interrupts % 3 != 0)
> -		return;

It would be symetric with the "% 3" above.

> -
>  	/* check for a hardlockup on the next CPU */
>  	next_cpu = watchdog_next_cpu(smp_processor_id());
>  	if (next_cpu >= nr_cpu_ids)

Best Regards,
Petr