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

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Date:   Tue, 27 Jun 2017 17:09:22 -0400
From:   Don Zickus <dzickus@...hat.com>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.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>,
        "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 V2] kernel/watchdog: fix spurious hard lockups

On Tue, Jun 27, 2017 at 08:49:19PM +0000, Liang, Kan wrote:
> 
> > On Mon, Jun 26, 2017 at 04:19:27PM -0400, Don Zickus wrote:
> > > On Fri, Jun 23, 2017 at 11:50:25PM +0200, Thomas Gleixner wrote:
> > > > On Fri, 23 Jun 2017, Don Zickus wrote:
> > > > > Hmm, all this work for a temp fix.  Kan, how much longer until the
> > > > > real fix of having perf count the right cycles?
> > > >
> > > > Quite a while. The approach is wilfully breaking the user space ABI,
> > > > which is not going to happen.
> > > >
> > > > And there is a simpler solution as well, as I said here:
> > > >
> > > >
> > > > http://lkml.kernel.org/r/alpine.DEB.2.20.1706221730520.1885@nanos
> > >
> > > Hi Thomas,
> > >
> > > So, you are saying instead of slowing down the perf counter, speed up
> > > the hrtimer to sample more frequently like so:
> > >
> > > diff --git a/kernel/watchdog.c b/kernel/watchdog.c index
> > > 03e0b69..8ff49de 100644
> > > --- a/kernel/watchdog.c
> > > +++ b/kernel/watchdog.c
> > > @@ -160,7 +160,7 @@ static void set_sample_period(void)
> > >  	 * and hard thresholds) to increment before the
> > >  	 * hardlockup detector generates a warning
> > >  	 */
> > > -	sample_period = get_softlockup_thresh() * ((u64)NSEC_PER_SEC / 5);
> > > +	sample_period = get_softlockup_thresh() * ((u64)NSEC_PER_SEC /
> > 10);
> > >  }
> > 
> > Hi Kan,
> > 
> > Will the above patch work for you?
> > 
> 
> I haven't heard back any test result yet.
> 
> The above patch looks good to me.
> But I'm not sure if /10 is enough. We may need /15.
> Anyway, I think we will test /10 first.
> 
> Which workaround do you prefer, the above one or the one checking timestamp?

Let's go with this one, it is simpler.

Cheers,
Don