linux-kernel - Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

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Date:   Wed, 21 Jun 2017 10:07:34 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Kan Liang <kan.liang@...el.com>, linux-kernel@...r.kernel.org,
        dzickus@...hat.com, mingo@...nel.org, akpm@...ux-foundation.org,
        babu.moger@...cle.com, atomlin@...hat.com, prarit@...hat.com,
        torvalds@...ux-foundation.org, peterz@...radead.org,
        eranian@...gle.com, acme@...hat.com, stable@...r.kernel.org
Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

On Wed, Jun 21, 2017 at 05:12:06PM +0200, Thomas Gleixner wrote:
> On Wed, 21 Jun 2017, kan.liang@...el.com wrote:
> >  
> >  #ifdef CONFIG_HARDLOCKUP_DETECTOR
> > +/*
> > + * The NMI watchdog relies on PERF_COUNT_HW_CPU_CYCLES event, which
> > + * can tick faster than the measured CPU Frequency due to Turbo mode.
> > + * That can lead to spurious timeouts.
> > + * To workaround the issue, extending the period by 3 times.
> > + */
> >  u64 hw_nmi_get_sample_period(int watchdog_thresh)
> >  {
> > -	return (u64)(cpu_khz) * 1000 * watchdog_thresh;
> > +	return (u64)(cpu_khz) * 1000 * watchdog_thresh * 3;
> 
> The maximum turbo frequency of any given machine can be retrieved.

Not reliably, e.g. not in virtualization. Also it would require
model specific checks, so as soon as you have a new model and an
old kernel it could still randomly fail.

-Andi