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

[Paul E. McKenney]

On Wed, Nov 23, 2022 at 10:36:04AM +0800, Feng Tang wrote:
> On Tue, Nov 22, 2022 at 02:07:12PM -0800, Paul E. McKenney wrote:
> [...] 
> > > > If PM_TIMER was involved, I would expect 'acpi_pm' instead of
> > > > refined-jiffies.  Or am I misinterpreting the output and/or code?
> > > 
> > > It's about timing. On a typical server platform, the clocksources
> > > init order could be:
> > >   refined-jiffies --> hpet --> tsc-early --> acpi_pm --> tsc 
> > > 
> > > >From your log, TSC('tsc-early') is disabled before 'acpi_pm' get
> > > initialized, so 'acpi_pm' timer (if exist) had no chance to watchdog
> > > the tsc.
> > > 
> > > > Either way, would it make sense to add CLOCK_SOURCE_MUST_VERIFY to
> > > > clocksource_hpet.flags?
> > > 
> > > Maybe try below patch, which will skip watchdog for 'tsc-early',
> > > while giving 'acpi_pm' timer a chance to watchdog 'tsc'.
> > > 
> > > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > > index cafacb2e58cc..9840f0131764 100644
> > > --- a/arch/x86/kernel/tsc.c
> > > +++ b/arch/x86/kernel/tsc.c
> > > @@ -1131,8 +1131,7 @@ static struct clocksource clocksource_tsc_early = {
> > >  	.uncertainty_margin	= 32 * NSEC_PER_MSEC,
> > >  	.read			= read_tsc,
> > >  	.mask			= CLOCKSOURCE_MASK(64),
> > > -	.flags			= CLOCK_SOURCE_IS_CONTINUOUS |
> > > -				  CLOCK_SOURCE_MUST_VERIFY,
> > > +	.flags			= CLOCK_SOURCE_IS_CONTINUOUS,
> > >  	.vdso_clock_mode	= VDSO_CLOCKMODE_TSC,
> > >  	.enable			= tsc_cs_enable,
> > >  	.resume			= tsc_resume,
> > 
> > Your mainline patch b50db7095fe0 ("x86/tsc: Disable clocksource watchdog
> > for TSC on qualified platorms") mitigates the issue so we are good for
> > the immediate future, at least assuming reliable TSC.
> > 
> > But it also disables checking against HPET, hence my question about
> > marking clocksource_hpet.flags with CLOCK_SOURCE_MUST_VERIFY at boot time
> > on systems whose CPUs have constant_tsc, nonstop_tsc, and tsc_adjust.
> 
> IIUC, this will make TSC to watchdog HPET every 500 ms. We have got
> report that the 500ms watchdog timer had big impact on some parallel
> workload on big servers, that was another factor for us to seek
> stopping the timer.

Another approach would be to slow it down.  Given the tighter bounds
on skew, it could be done every (say) 10 seconds while allowing
2 milliseconds skew instead of the current 100 microseconds.

> Is this about the concern of possible TSC frequency calibration
> issue, as the 40 ms per second drift between HPET and TSC? With 
> b50db7095fe0 backported, we also have another patch to force TSC
> calibration for those platforms which get the TSC freq directly
> from CPUID or MSR and don't have such info in dmesg:
>  "tsc: Refined TSC clocksource calibration: 2693.509 MHz" 
> 
> https://lore.kernel.org/lkml/20220509144110.9242-1-feng.tang@xxxxxxxxx/
> 
> We did met tsc calibration issue due to some firmware issue, and
> this can help to catch it. You can try it if you think it's relevant.

I am giving this a go, thank you!

						Thanx, Paul

> Thanks,
> Feng
> 
> > Would that make sense?
> > 
> > 							Thanx, Paul
> > 
> > > > I am sending the full console output off-list.  Hey, you asked for it!  ;-)
> > > 
> > > Thanks for sharing!
> > > 
> > > Thanks,
> > > Feng
> > > 
> > > > 
> > > > 							Thanx, Paul