Messages in this thread | | | Subject | Re: [PATCH] clocksource: Warn if too many missing ticks are detected | From | Waiman Long <> | Date | Tue, 18 Sep 2018 17:54:16 -0400 |
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On 09/18/2018 05:07 PM, Thomas Gleixner wrote: > Waiman, > > On Tue, 18 Sep 2018, Waiman Long wrote: > >> The clocksource watchdog, when running, is scheduled on all the CPUs in >> the system sequentially on a round-robin fashion with a period of 0.5s. >> A bug in the 4.18 kernel is causing missing ticks when nohz_full >> is specified. Under some circumstances, this causes the watchdog to >> incorrectly state that the TSC is unstable because of counter overflow >> in the hpet watchdog clock source after a few minutes delay. >> >> That particular bug is fixed by the 4.19 commit 7059b36636beab ("sched: >> idle: Avoid retaining the tick when it has been stopped"). To make it >> easier to catch this kind of bug in the future, a check is added to see >> if there is too much delay in the watchdog invocation and print a >> warning once if it happens. > I like the idea. > >> Signed-off-by: Waiman Long <longman@redhat.com> >> --- >> kernel/time/clocksource.c | 13 +++++++++++++ >> 1 file changed, 13 insertions(+) >> >> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c >> index 0e6e97a..2ea5db0 100644 >> --- 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:
[ 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.
I am perfectly fine to use the watchdog_timer.expires as suggested, though.
Cheers, Longman
watchdog_timer.expires
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