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

[Doug Anderson]

Hi,

On Thu, Mar 5, 2026 at 5:47 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> > --- 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)

I could be confused, but I don't think this is needed because we clear
"hrtimer_interrupts_missed" to 0 any time we save the timer count.
While I believe the "%" will functionally work, it seems harder to
understand, at least to me.


> > +                     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.

Here we weren't resetting the count, so the "%" _was_ important. In
the new code where we're resetting the count back to 0...

-Doug