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Date: Fri, 2 Feb 2024 12:57:44 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	John Stultz <jstultz@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Eric Dumazet <edumazet@google.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Arjan van de Ven <arjan@infradead.org>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Rik van Riel <riel@surriel.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sebastian Siewior <bigeasy@linutronix.de>,
	Giovanni Gherdovich <ggherdovich@suse.cz>,
	Lukasz Luba <lukasz.luba@arm.com>,
	"Gautham R . Shenoy" <gautham.shenoy@amd.com>,
	Srinivas Pandruvada <srinivas.pandruvada@intel.com>,
	K Prateek Nayak <kprateek.nayak@amd.com>,
	Richard Cochran <richardcochran@gmail.com>
Subject: Re: [PATCH v10 20/20] timers: Always queue timers on the local CPU
Message-ID: <ZbzYuKUd2F37nnnB@lothringen>
References: <20240115143743.27827-1-anna-maria@linutronix.de>
 <20240115143743.27827-21-anna-maria@linutronix.de>
 <ZbvWhOOblt3YPaSV@localhost.localdomain>
 <87plxflx4x.fsf@somnus>
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On Thu, Feb 01, 2024 at 09:58:38PM +0100, Anna-Maria Behnsen wrote:
> Frederic Weisbecker <frederic@kernel.org> writes:
> 
> > Le Mon, Jan 15, 2024 at 03:37:43PM +0100, Anna-Maria Behnsen a ?crit :
> >> 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@gmail.com>
> >> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
> >
> > Reviewed-by: Frederic Weisbecker <frederic@kernel.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?
> >
> 
> Yes, you are right. Should I drop it? To clarify it, I could add a
> 
>   WARN_ON_ONCE(!timer->flags & TIMER_PINNED)

Yep, that looks good!

Thanks.

> 
> instead.
> 
> Thanks,
> 
> 	Anna-Maria
>