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Message-ID: <20221121152107.GI4001@paulmck-ThinkPad-P17-Gen-1>
Date: Mon, 21 Nov 2022 07:21:07 -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?
Thank you for looking into this!
Knucklehead here failed to save away the dmesg. Let me see what
I can drum up.
> 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.
Let me see about giving this a go, and again, thank you!
Thanx, Paul
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