Messages in this thread | | | Date | Tue, 29 Nov 2022 11:29:15 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH clocksource 1/3] clocksource: Reject bogus watchdog clocksource measurements |
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On Mon, Nov 28, 2022 at 10:15:43AM +0800, Feng Tang wrote: > On Wed, Nov 23, 2022 at 01:23:48PM -0800, Paul E. McKenney wrote: > > 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. > > 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. ;-)
> > > 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
Apologies for the delay, but reconfigurations put the system off the net for some time.
Thanx, Paul
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