linux-kernel - Re: [PATCH clocksource 1/3] clocksource: Reject bogus watchdog clocksource measurements

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Date:   Mon, 21 Nov 2022 08:55:15 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>, <john.stultz@...aro.org>,
        <sboyd@...nel.org>, <corbet@....net>, <Mark.Rutland@....com>,
        <maz@...nel.org>, <kernel-team@...a.com>, <neeraju@...eaurora.org>,
        <ak@...ux.intel.com>, <zhengjun.xing@...el.com>,
        Chris Mason <clm@...a.com>, John Stultz <jstultz@...gle.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH clocksource 1/3] clocksource: Reject bogus watchdog
 clocksource measurements

On Thu, Nov 17, 2022 at 03:09:10PM -0800, Paul E. McKenney wrote:
> On Thu, Nov 17, 2022 at 10:57:34PM +0100, Thomas Gleixner wrote:
> > Paul!
> > 
> > On Mon, Nov 14 2022 at 15:28, Paul E. McKenney wrote:
> > >  
> > > +		/* Check for bogus measurements. */
> > > +		wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
> > > +		if (wd_nsec < (wdi >> 2)) {
> > > +			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced only %lld ns during %d-jiffy time interval, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> > > +			continue;
> > > +		}
> > > +		if (wd_nsec > (wdi << 2)) {
> > > +			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> > > +			continue;
> > > +		}
> > 
> > This is really getting ridiculous.
> 
> I have absolutely no argument with this statement, and going back a
> long time.  ;-)
> 
> But the set of systems that caused me to send this turned out to have
> real divergence between HPET and TSC, and 40 milliseconds per second of
> divergence at that.  So not only do you hate this series, but it is also
> the case that this series doesn't help with the problem at hand.

The drift is about 4% which is quite big. It seems that this is
either problem of HPET/TSC's hardware/firmware, or the problem of
frequency calibration for HPET/TSC. TSC calibration is complex,
as it could be done from different methods depending on hardware
and firmware, could you share the kernel boot log related with
tsc/hpet and clocksource? 

Also if your platform has acpi PM_TIMER, you may try "nohpet"
to use PM_TIMER instead of HPET and check if there is also big
drift between TSC and PM_TIMER.

Thanks,
Feng