Date: Wed, 21 Jun 2017 10:07:34 -0700
From: Andi Kleen <ak@linux.intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Kan Liang <kan.liang@intel.com>, linux-kernel@vger.kernel.org,
        dzickus@redhat.com, mingo@kernel.org, akpm@linux-foundation.org,
        babu.moger@oracle.com, atomlin@redhat.com, prarit@redhat.com,
        torvalds@linux-foundation.org, peterz@infradead.org,
        eranian@google.com, acme@redhat.com, stable@vger.kernel.org
Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups
Message-ID: <20170621170734.GF23705@tassilo.jf.intel.com>
References: <20170621144118.5939-1-kan.liang@intel.com>
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On Wed, Jun 21, 2017 at 05:12:06PM +0200, Thomas Gleixner wrote:
> On Wed, 21 Jun 2017, kan.liang@intel.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