On Mon, May 09, 2022 at 04:03:39PM +0200, Thomas Gleixner wrote:
> On Thu, May 05 2022 at 17:00, Ricardo Neri wrote:
> > +   if (is_hpet_hld_interrupt(hdata)) {
> > +           /*
> > +            * Kick the timer first. If the HPET channel is periodic, it
> > +            * helps to reduce the delta between the expected TSC value and
> > +            * its actual value the next time the HPET channel fires.
> > +            */
> > +           kick_timer(hdata, !(hdata->has_periodic));
> > +
> > +           if (cpumask_weight(hld_data->monitored_cpumask) > 1) {
> > +                   /*
> > +                    * Since we cannot know the source of an NMI, the best
> > +                    * we can do is to use a flag to indicate to all online
> > +                    * CPUs that they will get an NMI and that the source of
> > +                    * that NMI is the hardlockup detector. Offline CPUs
> > +                    * also receive the NMI but they ignore it.
> > +                    *
> > +                    * Even though we are in NMI context, we have concluded
> > +                    * that the NMI came from the HPET channel assigned to
> > +                    * the detector, an event that is infrequent and only
> > +                    * occurs in the handling CPU. There should not be races
> > +                    * with other NMIs.
> > +                    */
> > +                   cpumask_copy(hld_data->inspect_cpumask,
> > +                                cpu_online_mask);
> > +
> > +                   /* If we are here, IPI shorthands are enabled. */
> > +                   apic->send_IPI_allbutself(NMI_VECTOR);
> 
> So if the monitored cpumask is a subset of online CPUs, which is the
> case when isolation features are enabled, then you still send NMIs to
> those isolated CPUs. I'm sure the isolation folks will be enthused.

Yes, I acknowledged this limitation in the cover letter. I should also update
Documentation/admin-guide/lockup-watchdogs.rst.

This patchset proposes the HPET NMI watchdog as an opt-in feature.

Perhaps the limitation might be mitigated by adding a check for non-housekeeping
and non-monitored CPUs in exc_nmi(). However, that will not eliminate the
problem of isolated CPUs also getting the NMI.

Thanks and BR,
Ricardo

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