Re: [PATCH v6 22/29] x86/watchdog/hardlockup: Add an HPET-based hardlockup detector

From: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Andi Kleen <ak@linux.intel.com>,
	linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
	Ricardo Neri <ricardo.neri@intel.com>,
	Stephane Eranian <eranian@google.com>,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	Tony Luck <tony.luck@intel.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH v6 22/29] x86/watchdog/hardlockup: Add an HPET-based hardlockup detector
Date: Fri, 13 May 2022 15:16:50 -0700	[thread overview]
Message-ID: <20220513221650.GA8691@ranerica-svr.sc.intel.com> (raw)
In-Reply-To: <877d6uref8.ffs@tglx>

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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