Messages in this thread | | | Date | Wed, 19 Sep 2018 00:06:51 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] clocksource: Warn if too many missing ticks are detected |
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On Tue, 18 Sep 2018, Waiman Long wrote: > On 09/18/2018 05:07 PM, Thomas Gleixner wrote: > > On Tue, 18 Sep 2018, Waiman Long wrote: > >> --- a/kernel/time/clocksource.c > >> +++ b/kernel/time/clocksource.c > >> @@ -140,6 +140,7 @@ static void inline clocksource_watchdog_unlock(unsigned long *flags) > >> * Interval: 0.5sec Threshold: 0.0625s > >> */ > >> #define WATCHDOG_INTERVAL (HZ >> 1) > >> +#define WATCHDOG_INTERNVAL_NS (NSEC_PER_SEC >> 1) > >> #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4) > >> > >> static void clocksource_watchdog_work(struct work_struct *work) > >> @@ -242,6 +243,18 @@ static void clocksource_watchdog(struct timer_list *unused) > >> wd_nsec = clocksource_cyc2ns(delta, watchdog->mult, > >> watchdog->shift); > >> > >> + /* > >> + * When the timer tick is incorrectly stopped on a CPU with > >> + * pending events, for example, it is possible that the > >> + * clocksource watchdog will stop running for a sufficiently > >> + * long enough time to cause overflow in the delta > >> + * computation leading to incorrect report of unstable clock > >> + * source. So print a warning if there is unusually large > >> + * delay (> 0.5s) in the invocation of the watchdog. That > >> + * can indicate a hidden bug in the timer tick code. > >> + */ > >> + WARN_ON_ONCE(!wd_nsec || wd_nsec > 2*WATCHDOG_INTERNVAL_NS); > > But this is using the watchdog delta to check. If that wrapped the > > detection is broken. > > > > I'd rather use watchdog_timer.expires and check against jiffies. That tells > > you how late the timer callback actually is and does not suffer any > > wraparound issues. > > The clocksource_delta() function will deal with wrap-around in the > counter value. It is only when the counter advances more than 0x80000000 > for 32-bit hpet counter mask that a value of 0 will be returned. That is > why I have a !wd_nsec check there. There is a small chance when the > warparound is just within the 1 second window that the test fails. In > this case, the following kind of warning will certainly be triggered:
Ok.
> [ 578.890937] clocksource: timekeeping watchdog on CPU21: Marking > clocksource 'tsc' as unstable because the skew is too large: > [ 578.890938] clocksource: 'hpet' wd_now: > ee332105 wd_last: 544f80e7 mask: ffffffff > [ 578.890939] clocksource: 'tsc' cs_now: > 4b6e6ccb5d609 cs_last: 4b679a469d09e mask: ffffffffffffffff > [ 578.890954] tsc: Marking TSC unstable due to clocksource watchdog > [ 578.890963] TSC found unstable after boot, most likely due to broken > BIOS. Use 'tsc=unstable'. > [ 578.890965] sched_clock: Marking unstable (578920214163, > -28725675)<-(579047174801, -156217937) > [ 578.891056] clocksource: Switched to clocksource hpet > > Another reason that I used wd_nsec is because the data has already been > computed.
Sure, but that's hardly a hotpath.
> I am perfectly fine to use the watchdog_timer.expires as suggested, though.
I really prefer that because it's just more robust and obvious. No head scratching about correctness required.
Thanks,
tglx
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