linux-kernel - Re: [PATCH v9 11/32] timers: Rework idle logic

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Message-ID: <ZYLzlBPpS5zT7OHq@localhost.localdomain>
Date: Wed, 20 Dec 2023 15:00:52 +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 11/32] timers: Rework idle logic

Le Fri, Dec 01, 2023 at 10:26:33AM +0100, Anna-Maria Behnsen a écrit :
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> To improve readability of the code, split base->idle calculation and
> expires calculation into separate parts. While at it, update the comment
> about timer base idle marking.
> 
> Thereby the following subtle change happens if the next event is just one
> jiffy ahead and the tick was already stopped: Originally base->is_idle
> remains true in this situation. Now base->is_idle turns to false. This may
> spare an IPI if a timer is enqueued remotely to an idle CPU that is going
> to tick on the next jiffy.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
> ---
> v9: Re-ordering to not hurt the eyes and update comment
> v4: Change condition to force 0 delta and update commit message (Frederic)
> ---
>  kernel/time/timer.c | 31 ++++++++++++++++---------------
>  1 file changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index fee42dda8237..0826018d9873 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1943,22 +1943,23 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
>  	 */
>  	__forward_timer_base(base, basej);
>  
> -	if (time_before_eq(nextevt, basej)) {
> -		expires = basem;
> -		base->is_idle = false;
> -	} else {
> -		if (base->timers_pending)
> -			expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
> -		/*
> -		 * If we expect to sleep more than a tick, mark the base idle.
> -		 * Also the tick is stopped so any added timer must forward
> -		 * the base clk itself to keep granularity small. This idle
> -		 * logic is only maintained for the BASE_STD base, deferrable
> -		 * timers may still see large granularity skew (by design).
> -		 */
> -		if ((expires - basem) > TICK_NSEC)
> -			base->is_idle = true;
> +	if (base->timers_pending) {
> +		/* If we missed a tick already, force 0 delta */
> +		if (time_before(nextevt, basej))
> +			nextevt = basej;
> +		expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
>  	}
> +
> +	/*
> +	 * Base is idle if the next event is more than a tick away.
> +	 *
> +	 * If the base is marked idle then any timer add operation must forward
> +	 * the base clk itself to keep granularity small. This idle logic is
> +	 * only maintained for the BASE_STD base, deferrable timers may still
> +	 * see large granularity skew (by design).
> +	 */
> +	base->is_idle = time_after(nextevt, basej + 1);
> +

Much better, thanks! :-)