linux-kernel - Re: [PATCH v9 03/32] tick-sched: Warn when next tick seems to be in the past

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Date: Wed, 20 Dec 2023 14:27:43 +0100
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 <jstultz@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <edumazet@...gle.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Rik van Riel <riel@...riel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Giovanni Gherdovich <ggherdovich@...e.cz>,
	Lukasz Luba <lukasz.luba@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v9 03/32] tick-sched: Warn when next tick seems to be in
 the past

Le Fri, Dec 01, 2023 at 10:26:25AM +0100, Anna-Maria Behnsen a écrit :
> 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. To prevent complete misbehaviour, also
> reset next_tick to basemono in this case.
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
> v9: Add reset of next_tick to basemono
> ---
>  kernel/time/tick-sched.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 89517cfb6510..b1b591de781e 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -839,6 +839,10 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
>  		ts->next_timer = next_tick;
>  	}
>  
> +	/* Make sure next_tick is never before basemono! */
> +	if (WARN_ON_ONCE(basemono > next_tick))
> +		next_tick = basemono;
> +

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

And some food for thoughts:

1) Is it possible for hrtimer_get_next_event() to return values in the past?
2) Is hrtimer_get_next_event() unconditionally locking &cpu_base->lock even
   high resolution is active? Can we avoid that?

Thanks.



>  	/*
>  	 * If the tick is due in the next period, keep it ticking or
>  	 * force prod the timer.
> -- 
> 2.39.2
>