linux-kernel - Re: [PATCH clocksource 2/2] clocksource: Exponential backoff for load-induced bogus watchdog reads

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Message-ID: <61470eb8-fc3c-7f95-881e-03da1805b5ac@redhat.com>
Date:   Thu, 3 Nov 2022 20:20:27 -0400
From:   Waiman Long <longman@...hat.com>
To:     paulmck@...nel.org
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        john.stultz@...aro.org, sboyd@...nel.org, corbet@....net,
        Mark.Rutland@....com, maz@...nel.org, kernel-team@...a.com,
        neeraju@...eaurora.org, ak@...ux.intel.com, feng.tang@...el.com,
        zhengjun.xing@...el.com, John Stultz <jstultz@...gle.com>
Subject: Re: [PATCH clocksource 2/2] clocksource: Exponential backoff for
 load-induced bogus watchdog reads

On 11/3/22 16:49, Paul E. McKenney wrote:
> commit da44b8af99222ff8761a98ca8c00837a7d607d28
> Author: Paul E. McKenney<paulmck@...nel.org>
> Date:   Fri Oct 28 10:38:58 2022 -0700
>
>      clocksource: Exponential backoff for load-induced bogus watchdog reads
>      
>      The clocksource watchdog will reject measurements that are excessively
>      delayed, that is, by more than 1.5 seconds beyond the intended 0.5-second
>      watchdog interval.  On an extremely busy system, this can result in a
>      console message being printed every two seconds.  This is excessively
>      noisy for a non-error condition.
>      
>      Therefore, apply exponential backoff to these messages.  This exponential
>      backoff is capped at 1024 times the watchdog interval, which comes to
>      not quite one message per ten minutes.
>      
>      Please note that the bogus watchdog reads that occur when the watchdog
>      interval is less than 0.125 seconds are still printed unconditionally
>      because these likely correspond to a serious error condition in the
>      timer code or hardware.
>      
>      [ paulmck: Apply Feng Tang feedback. ]
>      [ paulmck: Apply Waiman Long feedback. ]
>      
>      Reported-by: Waiman Long<longman@...hat.com>
>      Reported-by: Feng Tang<feng.tang@...el.com>
>      Signed-off-by: Paul E. McKenney<paulmck@...nel.org>
>      Cc: John Stultz<jstultz@...gle.com>
>      Cc: Thomas Gleixner<tglx@...utronix.de>
>      Cc: Stephen Boyd<sboyd@...nel.org>
>      Cc: Feng Tang<feng.tang@...el.com>
>      Cc: Waiman Long<longman@...hat.com>
>
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index 1d42d4b173271..23b73f2293d6d 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -125,6 +125,9 @@ struct clocksource {
>   	struct list_head	wd_list;
>   	u64			cs_last;
>   	u64			wd_last;
> +	u64			wd_last_bogus;
> +	int			wd_bogus_shift;
> +	unsigned long		wd_bogus_count;
>   #endif
>   	struct module		*owner;
>   };
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 3f5317faf891f..1eefb56505350 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -442,14 +442,33 @@ static void clocksource_watchdog(struct timer_list *unused)
>   
>   		/* Check for bogus measurements. */
>   		wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
> -		if (wd_nsec < (wdi >> 2)) {
> -			/* This usually indicates broken timer code or hardware. */
> -			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);
> +		if (wd_nsec > (wdi << 2) || cs_nsec > (wdi << 2)) {
> +			bool needwarn = false;
> +			u64 wd_lb;
> +
> +			cs->wd_bogus_count++;
> +			if (!cs->wd_bogus_shift) {
> +				needwarn = true;
> +			} else {
> +				delta = clocksource_delta(wdnow, cs->wd_last_bogus, watchdog->mask);
> +				wd_lb = clocksource_cyc2ns(delta, watchdog->mult, watchdog->shift);
> +				if ((1 << cs->wd_bogus_shift) * wdi <= wd_lb)
> +					needwarn = true;
> +			}
> +			if (needwarn) {
> +				/* This can happen on busy systems, which can delay the watchdog. */
> +				pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval (%lu additional), probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL, cs->wd_bogus_count);

Just one question, does "%lu additional" means the number of bogus count 
that doesn't meet the needwarn requirement and hence skipped. If so, I 
think you have to use "cs->wd_bogus_cnt - 1". Other than that, the 
change looks good to me.

Cheers,
Longman