Date: Mon, 26 Jun 2017 22:30:57 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Don Zickus <dzickus@redhat.com>
cc: Kan Liang <kan.liang@intel.com>, linux-kernel@vger.kernel.org,
        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,
        ak@linux.intel.com, stable@vger.kernel.org
Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups
In-Reply-To: <20170626201927.3ak7fk3yvdzbb4ay@redhat.com>
Message-ID: <alpine.DEB.2.20.1706262226020.2063@nanos>
References: <20170621144118.5939-1-kan.liang@intel.com> <alpine.DEB.2.20.1706212235550.2152@nanos> <20170622154450.2lua7fdmigcixldw@redhat.com> <alpine.DEB.2.20.1706230952190.2647@nanos> <20170623162907.l6inpxgztwwkeaoi@redhat.com> <alpine.DEB.2.20.1706232348020.2234@nanos>
 <20170626201927.3ak7fk3yvdzbb4ay@redhat.com>
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On Mon, 26 Jun 2017, 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);
>  }
> 
>  /* Commands for resetting the watchdog */
> 
> 
> That is another way of doing it.  It just hits all the arches.  It does seem
> cleaner as the watchdog_thresh value still retains it correct meaning.  Are
> the laptop folks going to yell at me some more for waking their systems up
> more? :-)

Yes, that's bound to happen. You might make them less angry if you wake the
softlockup thread only on every second hrtimer expiry, i.e. keeping the
current wakeup rate.  But I can't promise that this will significantly
lower their wrath. :)

Thanks,

	tglx