linux-kernel - Re: [PATCH v3 02/17] tick-sched: Warn when next tick seems to be in the past

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Message-ID: <20221025221151.GA1313991@lothringen>
Date:   Wed, 26 Oct 2022 00:11:51 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-pm@...r.kernel.org, Arjan van de Ven <arjan@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v3 02/17] tick-sched: Warn when next tick seems to be in
 the past

On Tue, Oct 25, 2022 at 03:58:35PM +0200, Anna-Maria Behnsen wrote:
> When the next tick is in the past, the delta between basemono and the next
> tick gets negativ. But the next tick should never be in the past. The
> negative effect of a wrong next tick might be a stop of the tick and timers
> might expire late.
> 
> To prevent expensive debugging when changing underlying code, add a
> WARN_ON_ONCE into this code path.
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

> ---
>  kernel/time/tick-sched.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index b0e3c9205946..7ffdc7ba19b4 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -826,6 +826,8 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
>  	 * If the tick is due in the next period, keep it ticking or
>  	 * force prod the timer.
>  	 */
> +	WARN_ON_ONCE(basemono > next_tick);
> +
>  	delta = next_tick - basemono;
>  	if (delta <= (u64)TICK_NSEC) {
>  		/*
> -- 
> 2.30.2
>