Re: [PATCH 1/4] watchdog/hardlockup: Always update saved interrupts during check

[Petr Mladek <pmladek@suse.com>]

On Tue 2026-03-10 19:51:21, Mayank Rungta wrote:
> > On Mon, Mar 9, 2026 at 6:33 AM Petr Mladek <pmladek@suse.com> wrote:
> > On Thu 2026-03-05 08:13:39, Doug Anderson wrote:
> > > On Thu, Mar 5, 2026 at 3:27 AM Petr Mladek <pmladek@suse.com> wrote:
> > > > A better solution might be to separate the check and update/reset
> > > > of the values. Something like (on top of this patchset, just
> > > > compilation tested):
> > > >
> > > > diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> > > > index 30199eaeb5d7..4d0851f0f412 100644
> > > > --- a/kernel/watchdog.c
> > > > +++ b/kernel/watchdog.c
> > > > @@ -167,18 +167,10 @@ void watchdog_hardlockup_touch_cpu(unsigned int cpu)
> > > >         per_cpu(watchdog_hardlockup_touched, cpu) = true;
> > > >  }
> > > >
> > > > -static bool is_hardlockup(unsigned int cpu)
> > > > +static void watchdog_hardlockup_update_reset(unsigned int cpu)
> > > >  {
> > > >         int hrint = atomic_read(&per_cpu(hrtimer_interrupts, cpu));
> > > >
> > > > -       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)
> > > > -                       return true;
> > > > -
> > > > -               return false;
> > > > -       }
> > > > -
> > > >         /*
> > > >          * NOTE: we don't need any fancy atomic_t or READ_ONCE/WRITE_ONCE
> > > >          * for hrtimer_interrupts_saved. hrtimer_interrupts_saved is
> > > > @@ -186,8 +178,20 @@ static bool is_hardlockup(unsigned int cpu)
> > > >          */
> > > >         per_cpu(hrtimer_interrupts_saved, cpu) = hrint;
> > > >         per_cpu(hrtimer_interrupts_missed, cpu) = 0;
> > > > +}
> > > >
> > > > -       return false;
> > > > +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 false;
> > > > +
> > > > +       per_cpu(hrtimer_interrupts_missed, cpu)++;
> > > > +       if (per_cpu(hrtimer_interrupts_missed, cpu) < watchdog_hardlockup_miss_thresh)
> > > > +               return false;
> > > > +
> > > > +       return true;
> > > >  }
> > > >
> > > >  static void watchdog_hardlockup_kick(void)
> > > > @@ -200,23 +204,10 @@ static void watchdog_hardlockup_kick(void)
> > > >
> > > >  void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
> > > >  {
> > > > -       bool is_hl;
> > > >         int hardlockup_all_cpu_backtrace;
> > > > -       /*
> > > > -        * Check for a hardlockup by making sure the CPU's timer
> > > > -        * interrupt is incrementing. The timer interrupt should have
> > > > -        * fired multiple times before we overflow'd. If it hasn't
> > > > -        * then this is a good indication the cpu is stuck
> > > > -        *
> > > > -        * Purposely check this _before_ checking watchdog_hardlockup_touched
> > > > -        * so we make sure we still update the saved value of the interrupts.
> > > > -        * Without that we'll take an extra round through this function before
> > > > -        * we can detect a lockup.
> > > > -        */
> > > > -
> > > > -       is_hl = is_hardlockup(cpu);
> > > >
> > > >         if (per_cpu(watchdog_hardlockup_touched, cpu)) {
> > > > +               watchdog_hardlockup_update_reset(cpu);
> > > >                 per_cpu(watchdog_hardlockup_touched, cpu) = false;
> > > >                 return;
> > > >         }
> > > > @@ -224,7 +215,13 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
> > > >         hardlockup_all_cpu_backtrace = (hardlockup_si_mask & SYS_INFO_ALL_BT) ?
> > > >                                         1 : sysctl_hardlockup_all_cpu_backtrace;
> > > >
> > > > -       if (is_hl) {
> > > > +       /*
> > > > +        * Check for a hardlockup by making sure the CPU's timer
> > > > +        * interrupt is incrementing. The timer interrupt should have
> > > > +        * fired multiple times before we overflow'd. If it hasn't
> > > > +        * then this is a good indication the cpu is stuck
> > > > +        */
> > > > +       if (is_hardlockup(cpu)) {
> > > >                 unsigned int this_cpu = smp_processor_id();
> > > >                 unsigned long flags;
> > > >
> > > > @@ -290,6 +287,7 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
> > > >
> > > >                 per_cpu(watchdog_hardlockup_warned, cpu) = true;
> > > >         } else {
> > > > +               watchdog_hardlockup_update_reset(cpu);
> > > >                 per_cpu(watchdog_hardlockup_warned, cpu) = false;
> > > >         }
> > > >  }
> > >
> > > I haven't tested it, but that actually looks like a pretty nice final
> > > result to me. Mayank: What do you think? You'd have to figure out how
> > > to rework your two patches to incorporate Petr's ideas.
> > >
> 
> Thanks for your suggestion, this is pretty close, but we cannot call
> watchdog_hardlockup_update_reset(cpu) in a general else block. If we
> did, the hrtimer_interrupts_missed count would be reset on every check
> that isn't a hardlockup; even when no progress was actually made and
> we would never hit the watchdog_threshold.

Great catch!

> I’ve moved the update/reset inside the "progress detected" path within
> is_hardlockup() instead:
> 
> static bool is_hardlockup(unsigned int cpu)
> {
>         int hrint = atomic_read(&per_cpu(hrtimer_interrupts, cpu));
> 
>         if (per_cpu(hrtimer_interrupts_saved, cpu) != hrint) {
>                 watchdog_hardlockup_update_reset(cpu);
>                 return false;
>         }
> 
>         per_cpu(hrtimer_interrupts_missed, cpu)++;
>         if (per_cpu(hrtimer_interrupts_missed, cpu) <
> watchdog_hardlockup_miss_thresh)
>                 return false;
> 
>         return true;
> }

Looks good to me.

> I still went ahead and flipped the `is_hardlockup` test in the main
> check function to keep the indentation clean

Great.

Best Regards,
Petr