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

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Nov 30, 2022 at 09:38:00AM +0800, Feng Tang wrote:
> 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.

It can be surprisingly hard.  It is sometimes easier to patch the kernel
to change the default.  Part of the problem is getting the right set
of command-line parameters associated with the right versions of the
kernel in the not-uncommon case where different portions of the server
farm are running different versions of the kernel.

> > > > > 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@intel.com/
> > > > > 
> > > > > 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.

How about this from earlier in that same console?

[    0.000000] efi: EFI v2.80 by American Megatrends
[    0.000000] efi: ACPI=0x6f569000 ACPI 2.0=0x6f569014 TPMFinalLog=0x6f56b000 SMBIOS=0x753e1000 SMBIOS 3.0=0x753e0000 MEMATTR=0x62176018 ESRT=0x64bd1018 TPMEventLog=0x58737018
[    0.000000] [Firmware Bug]: TPM Final Events table missing or invalid
[    0.000000] SMBIOS 3.5.0 present.
[    0.000000] DMI: Quanta Grand Teton 1F0TUBZ0007/Grand Teton MB, BIOS F0T_1A15 08/25/2022
[    0.000000] tsc: Detected 1900.000 MHz processor
[    0.000000] tsc: Detected 1896.000 MHz TSC

> 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.

OK, getting things teed up for TSC recalibration and nohpet.

> * 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.

No ntpd, but there is a chronyd.

I will let you know what happens with HPET disabled and TSC recalibration
enabled.

							Thanx, Paul