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

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Date:   Mon, 21 Nov 2022 10:14:49 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Feng Tang <feng.tang@...el.com>
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 Mon, Nov 21, 2022 at 08:55:15AM +0800, Feng Tang wrote:
> 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.

The kernel is built with CONFIG_X86_PM_TIMER=y, so I was guessing
that there is an ACPI PM_TIMER.  Except that when I booted
without your "Disable clocksource watchdog for TSC on qualified
platforms" patch, I get the following:

[   44.303035] clocksource: timekeeping watchdog on CPU3: Marking clocksource 'tsc-early' as unstable because the skew is too large:
[   44.347034] clocksource:                       'refined-jiffies' wd_nsec: 503923392 wd_now: fffb73f8 wd_last: fffb7200 mask: ffffffff
[   44.374034] clocksource:                       'tsc-early' cs_nsec: 588042081 cs_now: 66c486d157 cs_last: 6682125e5e mask: ffffffffffffffff
[   44.403034] clocksource:                       No current clocksource.
[   44.418034] tsc: Marking TSC unstable due to clocksource watchdog

If PM_TIMER was involved, I would expect 'acpi_pm' instead of
refined-jiffies.  Or am I misinterpreting the output and/or code?

Either way, would it make sense to add CLOCK_SOURCE_MUST_VERIFY to
clocksource_hpet.flags?

I am sending the full console output off-list.  Hey, you asked for it!  ;-)

							Thanx, Paul