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

[Thomas Gleixner <tglx@linutronix.de>]

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