linux-kernel - Re: [PATCH v2] clocksource: Warn if too many missing ticks are detected

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Date:   Mon, 24 Sep 2018 11:35:14 -0400
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
        Stephen Boyd <sboyd@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] clocksource: Warn if too many missing ticks are
 detected

On 09/19/2018 03:53 AM, Thomas Gleixner wrote:
> On Tue, 18 Sep 2018, Waiman Long wrote:
>
>> The clocksource watchdog, when running, is scheduled on all the CPUs in
>> the system sequentially on a round-robin fashion with a period of 0.5s.
>> A bug in the 4.18 kernel is causing missing ticks when nohz_full
>> is specified. Under some circumstances, this causes the watchdog to
>> incorrectly state that the TSC is unstable because of counter overflow
>> in the hpet watchdog clock source after a few minutes delay.
>>
>> That particular bug is fixed by the 4.19 commit 7059b36636beab ("sched:
>> idle: Avoid retaining the tick when it has been stopped"). To make it
>> easier to catch this kind of bug in the future, a check is added to see
>> if there is too much delay in the invocation of the watchdog callback
>> and print a warning once if it happens.
> Second thoughts on this. Putting the check into the clocksource watchdog is
> the wrong place as it's just checking at a place where the symptom
> shows. What about putting it right to the source, i.e. in the timer wheel
> as it does not depend on the clocksource watchdog being active. The
> clocksource watchdog triggering is just one of the symptoms, but in general
> timers being massively late is not a good thing.
>
> Thanks,
>
> 	tglx
>
It looks like there are more complications to deal with in order to do
the proper check. I will leave the check in the watchdog for the time
being and see how thing works out.

-Longman