linux-kernel - Re: [PATCH v10 03/20] timers: Move marking timer bases idle into tick_nohz_stop

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Date: Mon, 22 Jan 2024 22:49:23 +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 v10 03/20] timers: Move marking timer bases idle into
 tick_nohz_stop_tick()

Le Mon, Jan 22, 2024 at 12:45:03PM +0100, Anna-Maria Behnsen a écrit :
> Frederic Weisbecker <frederic@...nel.org> writes:
> 
> > Le Mon, Jan 15, 2024 at 03:37:26PM +0100, Anna-Maria Behnsen a écrit :
> >> @@ -889,12 +884,41 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
> >>  static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> >>  {
> >>  	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
> >> +	unsigned long basejiff = ts->last_jiffies;
> >>  	u64 basemono = ts->timer_expires_base;
> >> -	u64 expires = ts->timer_expires;
> >> +	bool timer_idle;
> >> +	u64 expires;
> >>  
> >>  	/* Make sure we won't be trying to stop it twice in a row. */
> >>  	ts->timer_expires_base = 0;
> >>  
> >> +	/*
> >> +	 * Now the tick should be stopped definitely - so the timer base needs
> >> +	 * to be marked idle as well to not miss a newly queued timer.
> >> +	 */
> >> +	expires = timer_base_try_to_set_idle(basejiff, basemono, &timer_idle);
> >> +	if (!timer_idle) {
> >> +		/*
> >> +		 * Do not clear tick_stopped here when it was already set - it
> >
> > Can that really happen? Looking at __get_next_timer_interrupt(), you're making a
> > behavioural change: if base->is_idle was previously set and the next timer is
> > now below/equal a jiffy, base->is_idle is not going to be cleared by
> > __get_next_timer_interrupt().
> >
> > Therefore you shouldn't observe ts->tick_stopped && !timer_idle
> >
> > But I'm assuming that behavioural change wasn't intended?
> 
> It was intended to keep tick_stopped and base->is_idle in sync. So when
> tick_stopped is set also base->is_idle needs to be set and dropping it
> before tick_stopped is dropped will break the plan to keep it in sync.

Ok that sounds good.

Thanks!