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SubjectRe: [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

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