Re: [PATCH clocksource] Reject bogus watchdog clocksource measurements

[Feng Tang <feng.tang@intel.com>]

On Wed, Oct 19, 2022 at 04:09:04PM -0700, Paul E. McKenney wrote:
> One remaining clocksource-skew issue involves extreme CPU overcommit,
> which can cause the clocksource watchdog measurements to be delayed by
> tens of seconds.  This in turn means that a clock-skew criterion that
> is appropriate for a 500-millisecond interval will instead give lots of
> false positives.

I remembered I saw logs that the watchdog were delayed to dozens of
or hundreds of seconds. 

Thanks for the fix which makes sense to me! with some nits below.

> Therefore, check for the watchdog clocksource reporting much larger or
> much less than the time specified by WATCHDOG_INTERVAL.  In these cases,
> print a pr_warn() warning and refrain from marking the clocksource under
> test as being unstable.
> 
> Reported-by: Chris Mason <clm@meta.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: John Stultz <jstultz@google.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Stephen Boyd <sboyd@kernel.org>
> Cc: Feng Tang <feng.tang@intel.com>
> Cc: Waiman Long <longman@redhat.com>
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 8058bec87acee..dcaf38c062161 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -386,7 +386,7 @@ EXPORT_SYMBOL_GPL(clocksource_verify_percpu);
>  
>  static void clocksource_watchdog(struct timer_list *unused)
>  {
> -	u64 csnow, wdnow, cslast, wdlast, delta;
> +	u64 csnow, wdnow, cslast, wdlast, delta, wdi;
>  	int next_cpu, reset_pending;
>  	int64_t wd_nsec, cs_nsec;
>  	struct clocksource *cs;
> @@ -440,6 +440,17 @@ static void clocksource_watchdog(struct timer_list *unused)
>  		if (atomic_read(&watchdog_reset_pending))
>  			continue;
>  
> +		/* Check for bogus measurements. */
> +		wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
> +		if (wd_nsec < (wdi >> 2)) {
> +			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced only %lld ns during %d-jiffy time interval, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> +			continue;
> +		}

If this happens (500ms timer happens only after less than 125ms),
there is some severe problem with timer/interrupt system. 

> +		if (wd_nsec > (wdi << 2)) {
> +			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> +			continue;
> +		}

I agree with Waiman that some rate limiting may be needed. As there
were reports of hundreds of seconds of delay, 2 seconds delay could
easily happen if a system is too busy or misbehave to trigger this
problem.

Thanks,
Feng