Re: [PATCH] clocksource: Warn if too many missing ticks are detected

[Thomas Gleixner <tglx@linutronix.de>]

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