linux-kernel - Re: [PATCH v11 03/20] timers: Move marking timer bases idle into tick_nohz_stop

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Message-ID: <ZdZevnP0p1y476jn@pavilion.home>
Date: Wed, 21 Feb 2024 21:36:14 +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>,
	Christian Loehle <christian.loehle@....com>
Subject: Re: [PATCH v11 03/20] timers: Move marking timer bases idle into
 tick_nohz_stop_tick()

Le Wed, Feb 21, 2024 at 10:05:31AM +0100, Anna-Maria Behnsen a écrit :
> The timer base is marked idle when get_next_timer_interrupt() is
> executed. But the decision whether the tick will be stopped and whether the
> system is able to go idle is done later. When the timer bases is marked
> idle and a new first timer is enqueued remote an IPI is raised. Even if it
> is not required because the tick is not stopped and the timer base is
> evaluated again at the next tick.
> 
> To prevent this, the timer base is marked idle in tick_nohz_stop_tick() and
> get_next_timer_interrupt() is streamlined by only looking for the next timer
> interrupt. All other work is postponed to timer_base_try_to_set_idle() which is
> called by tick_nohz_stop_tick(). timer_base_try_to_set_idle() never resets
> timer_base::is_idle state. This is done when the tick is restarted via
> tick_nohz_restart_sched_tick().
> 
> With this, tick_sched::tick_stopped and timer_base::is_idle are always in
> sync. So there is no longer the need to execute timer_clear_idle() in
> tick_nohz_idle_retain_tick(). This was required before, as
> tick_nohz_next_event() set timer_base::is_idle even if the tick would not be
> stopped. So timer_clear_idle() is only executed, when timer base is idle. So the
> check whether timer base is idle, is now no longer required as well.
> 
> While at it fix some nearby whitespace damage as well.
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>

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