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

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Message-ID: <20180919091731.GY24124@hirez.programming.kicks-ass.net>
Date:   Wed, 19 Sep 2018 11:17:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Waiman Long <longman@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        linux-kernel@...r.kernel.org, Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH v2] clocksource: Warn if too many missing ticks are
 detected

On Wed, Sep 19, 2018 at 09:53:47AM +0200, 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.

Just make sure to think of the virt case; virt can cause all kind of
'fun' lateness.