linux-kernel - Re: [PATCH v10 20/20] timers: Always queue timers on the local CPU

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Date: Thu, 1 Feb 2024 18:36:04 +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>,
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH v10 20/20] timers: Always queue timers on the local CPU

Le Mon, Jan 15, 2024 at 03:37:43PM +0100, Anna-Maria Behnsen a écrit :
> The timer pull model is in place so we can remove the heuristics which try
> to guess the best target CPU at enqueue/modification time.
> 
> All non pinned timers are queued on the local CPU in the separate storage
> and eventually pulled at expiry time to a remote CPU.
> 
> Originally-by: Richard Cochran (linutronix GmbH) <richardcochran@...il.com>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>

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

Just one detail below:

> @@ -590,10 +590,13 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
>  
>  	/*
>  	 * We might have to IPI the remote CPU if the base is idle and the
> -	 * timer is not deferrable. If the other CPU is on the way to idle
> -	 * then it can't set base->is_idle as we hold the base lock:
> +	 * timer is pinned. If it is a non pinned timer, it is only queued
> +	 * on the remote CPU, when timer was running during queueing. Then
> +	 * everything is handled by remote CPU anyway. If the other CPU is
> +	 * on the way to idle then it can't set base->is_idle as we hold
> +	 * the base lock:
>  	 */
> -	if (base->is_idle)
> +	if (base->is_idle && timer->flags & TIMER_PINNED)

Is the TIMER_PINNED test necessary? If base->is_idle, then the timer
is now guaranteed to be TIMER_PINNED, right?

Thanks.