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

[Feng Tang]

On Tue, Nov 29, 2022 at 11:29:15AM -0800, Paul E. McKenney wrote:
[...]
> > > > 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.
> > 
> > Yes, this can reduce the OS noise much. One problem is if we make it
> > a general interface, there is some clocksource whose warp time is
> > less than 10 seconds, like ACPI PM_TIMER (3-4 seconds), and I don't
> > know if other ARCHs have similar cases.
> 
> Maybe a simpler approach is for systems with such high sensitivity to
> OS noise to simply disable the clocksource watchdog.  ;-)

That's what the reported did, test with and without "tsc=reliable"
parameter :)

And AFAIK, many customers with big server farms hate to add more
cmdline parameters when we suggested so.

> > > > 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!
> > 
> > Thanks for spending time testing it!
> 
> And here are the results from setting tsc_force_recalibrate to 1:
> 
> $ dmesg | grep -E 'calibrat|clocksource'
> [    5.272939] clocksource: refined-jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 1910969940391419 ns
> [   16.830644] clocksource: hpet: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 76450417870 ns
> [   17.938020] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x36a8d32ce31, max_idle_ns: 881590731004 ns
> [   24.548583] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 1911260446275000 ns
> [   49.762432] clocksource: Switched to clocksource tsc-early
> [   50.076769] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff, max_idle_ns: 2085701024 ns
> [   55.615946] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x36a8d32ce31, max_idle_ns: 881590731004 ns
> [   55.640270] clocksource: Switched to clocksource tsc
> [   56.694371] tsc: Warning: TSC freq calibrated by CPUID/MSR differs from what is calibrated by HW timer, please check with vendor!!
> [   56.724550] tsc: Previous calibrated TSC freq:        1896.000 MHz
> [   56.737646] tsc: TSC freq recalibrated by [HPET]:     1975.000 MHz

Looks like there is really something wrong here. I assume the first
number '1896 MHz' is got from CPUID(0x15)'s math calculation.

I thinks 2 more things could be try:

* add "nohpet" to the cmdline, so the tsc_force_recalibrate should use
  ACPI PM_TIMER to do the calibration, say a third-party check.
* If the system don't have auto-adjusted time setting like NTP, I
  guess the system time will have obvious drift comparing to a normal
  clock or a mobile phone time, as the deviation is about 4%, which
  is 2.4 minutes per hour.

> Apologies for the delay, but reconfigurations put the system off the
> net for some time.

No problem at all, it's your holiday time! Thanks for trying this!

Thanks,
Feng

> 							Thanx, Paul