linux-kernel - Re: [PATCH clocksource] Reject bogus watchdog clocksource measurements

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Date:   Wed, 19 Oct 2022 23:01:50 -0400
From:   Waiman Long <longman@...hat.com>
To:     paulmck@...nel.org, linux-kernel@...r.kernel.org
Cc:     clm@...a.com, jstultz@...gle.com, tglx@...utronix.de,
        sboyd@...nel.org, feng.tang@...el.com
Subject: Re: [PATCH clocksource] Reject bogus watchdog clocksource
 measurements

On 10/19/22 19:09, 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.

CPU overcommit means it is running in a VM. Right? Unfortunately, there 
is not a consistent cross-arch way to check for running under a 
hypervisor or we may want to add such a test if available. However, 
CLOCKSOURCE_WATCHDOG is only enabled in x86 and mips. Maybe we can add a 
helper function to do that.


>
> 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@...a.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/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 (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;
> +		}
> +
>   		/* Check the deviation from the watchdog clocksource. */
>   		md = cs->uncertainty_margin + watchdog->uncertainty_margin;
>   		if (abs(cs_nsec - wd_nsec) > md) {

In the worst case, there will be a warning every half second or so. 
Should we rate limit the number of these warnings in some way?

Cheers,
Longman