linux-kernel - Re: [PATCH clocksource 1/2] clocksource: Add comments to classify bogus measurements

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Message-ID: <Y2R1ZyJhgH5AZLvM@feng-clx>
Date:   Fri, 4 Nov 2022 10:13:59 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     "Paul E. McKenney" <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>,
        <zhengjun.xing@...el.com>, John Stultz <jstultz@...gle.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH clocksource 1/2] clocksource: Add comments to classify
 bogus measurements

On Wed, Nov 02, 2022 at 11:40:08AM -0700, Paul E. McKenney wrote:
> An extremely busy system can delay the clocksource watchdog, so that
> the corresponding too-long bogus-measurement error does not necessarily
> imply an error in the system.  However, a too-short bogus-measurement
> error likely indicates a bug in hardware, firmware or software.
> 
> Therefore, add comments clarifying these bogus-measurement pr_warn()s.

Looks good to me.

Reviewed-by: Feng Tang <feng.tang@...el.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>
> ---
>  kernel/time/clocksource.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index dcaf38c062161..3f5317faf891f 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -443,10 +443,12 @@ 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);
>  			continue;
>  		}
>  		if (wd_nsec > (wdi << 2)) {
> +			/* 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, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
>  			continue;
>  		}
> -- 
> 2.31.1.189.g2e36527f23
>