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SubjectRe: [PATCH V2] kernel/watchdog: fix spurious hard lockups
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

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