RE: [PATCH] kernel/watchdog: fix spurious hard lockups

[Liang, Kan]

> > > > From: Kan Liang <Kan.liang@xxxxxxxxx>
> > > >
> > > > Some users reported spurious NMI watchdog timeouts.
> > > >
> > > > We now have more and more systems where the Turbo range is wide
> > > enough
> > > > that the NMI watchdog expires faster than the soft watchdog timer
> > > > that updates the interrupt tick the NMI watchdog relies on.
> > >
> > > AFAIR the watchdog doesn't rely on deferred timers so this would
> > > suggest that a standard hrtimer can expire much later than programmed,
> right?
> >
> > The softlockup watchdog relies on hrtimers.
> > The hardlockup watchdog (NMI watchdog) relies on perf subsystem and
> > using unhalted CPU cycles.
> > When the softlockup watchdog expires, it updates the hrtimer_interrupts.
> > When the NMI watchdog expires, it will check the hrtimer_interrupts,
> > and determine if it's a hardlockup.
> > The design was to make the softlockup watchdog runs with 2.5 times the
> > rate of NMI watchdog. So it guarantees that the hrtimer_interrupts is
> > updated before the NMI watchdog expires.
> > That works well if Turbo-Mode is disabled.
> > However, when Turbo-Mode is enabled, unhalted CPU cycles might run
> > much faster than expected, even faster than softlockup watchdog.
> > So the softlockup watchdog will not get a chance to update the
> > hrtimer_interrupts, which will trigger false positives.
> 
> So it is not the hrtimer which doesn't fire but rather the NMI events fire too
> quickly, right?

Yes, the NMI fires too quickly.

Thanks,
Kan