Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH] clocksource: Warn if too many missing ticks are detected | Date | Tue, 18 Sep 2018 14:36:14 -0400 |
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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.
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); + delta = clocksource_delta(csnow, cs->cs_last, cs->mask); cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift); wdlast = cs->wd_last; /* save these in case we print them */ -- 1.8.3.1
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