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

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Date:   Wed, 21 Jun 2017 09:47:47 -0400
From:   Don Zickus <dzickus@...hat.com>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "babu.moger@...cle.com" <babu.moger@...cle.com>,
        "atomlin@...hat.com" <atomlin@...hat.com>,
        "prarit@...hat.com" <prarit@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] kernel/watchdog: fix spurious hard lockups

On Wed, Jun 21, 2017 at 12:40:28PM +0000, Liang, Kan wrote:
> 
> > >
> > > The right fix for mainline can be found here.
> > > perf/x86/intel: enable CPU ref_cycles for GP counter perf/x86/intel,
> > > watchdog: Switch NMI watchdog to ref cycles on x86
> > > https://patchwork.kernel.org/patch/9779087/
> > > https://patchwork.kernel.org/patch/9779089/
> > 
> > Presumably the "right fix" will later be altered to revert this one-line
> > workaround?
> 
> The "right fix" itself will not touch the watchdog rate. I will modify the
> changelog to notify the people who want to do the backport.
> 
> As my understanding, it's not harmful even if we don't revert the
> workaround. It can still detect the hardlockup, only takes
> a tiny bit longer.

It depends on you perspective of harmful. :-) There are folks that would
like that sampling rate to be more accurate, so they can detect problems
soon than later.  You just took an input of 'watchdog_thresh' and blindly
multiplied it by 3, which can confuse an end user who thought they setup a 5
second threshold but instead it turned into a 15 second one. :-(

Cheers,
Don