Messages in this thread | | | Date | Tue, 18 Sep 2018 23:07:21 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] clocksource: Warn if too many missing ticks are detected |
| |
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.
Thanks,
tglx
| |