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

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Message-ID: <Zaf6E61X_l1Bx6Mu@localhost.localdomain>
Date: Wed, 17 Jan 2024 17:02:27 +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 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?

> +		 * will be retained on the next idle iteration when the tick
> +		 * expired earlier than expected.

I'm a bit confused by this sentence.

> +		 */
> +		expires = basemono + TICK_NSEC;

Do you need this line?

> @@ -1147,11 +1175,6 @@ void tick_nohz_idle_stop_tick(void)
>  void tick_nohz_idle_retain_tick(void)
>  {
>  	tick_nohz_retain_tick(this_cpu_ptr(&tick_cpu_sched));

Looks like the content of tick_nohz_retain_tick() can move here now.

> -	/*
> -	 * Undo the effect of get_next_timer_interrupt() called from
> -	 * tick_nohz_next_event().
> -	 */
> -	timer_clear_idle();
>  }

Thanks.