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

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Date:   Mon, 26 Jun 2017 16:19:28 -0400
From:   Don Zickus <dzickus@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Kan Liang <kan.liang@...el.com>, linux-kernel@...r.kernel.org,
        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, ak@...ux.intel.com,
        stable@...r.kernel.org
Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

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? :-)

Cheers,
Don