linux-kernel - Re: [PATCH v10 04/20] timers: Optimization for timer_base_try_to_set

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Date: Wed, 17 Jan 2024 17:45:31 +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 04/20] timers: Optimization for
 timer_base_try_to_set_idle()

Le Mon, Jan 15, 2024 at 03:37:27PM +0100, Anna-Maria Behnsen a écrit :
> When tick is stopped also the timer base is_idle flag is set. When
> reentering the timer_base_try_to_set_idle() with the tick stopped, there is
> no need to check whether the timer base needs to be set idle again. When a
> timer was enqueued in the meantime, this is already handled by the
> tick_nohz_next_event() call which was executed before
> tick_nohz_stop_tick().
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> ---
>  kernel/time/tick-sched.c |  2 +-
>  kernel/time/timer.c      | 11 ++++++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index c6223afc801f..27f1a2ae7f39 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -886,7 +886,7 @@ 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;
> -	bool timer_idle;
> +	bool timer_idle = ts->tick_stopped;
>  	u64 expires;
>  
>  	/* Make sure we won't be trying to stop it twice in a row. */
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 3a668060692e..2f69a485a070 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1999,13 +1999,18 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
>   * timer_base_try_to_set_idle() - Try to set the idle state of the timer bases
>   * @basej:	base time jiffies
>   * @basem:	base time clock monotonic
> - * @idle:	pointer to store the value of timer_base->is_idle
> + * @idle:	pointer to store the value of timer_base->is_idle on return;
> + *		*idle contains the information whether tick was already stopped
>   *
> - * Returns the tick aligned clock monotonic time of the next pending
> - * timer or KTIME_MAX if no timer is pending.
> + * Returns the tick aligned clock monotonic time of the next pending timer or
> + * KTIME_MAX if no timer is pending. When tick was already stopped KTIME_MAX is
> + * returned as well.
>   */
>  u64 timer_base_try_to_set_idle(unsigned long basej, u64 basem, bool *idle)
>  {
> +	if (*idle)
> +		return KTIME_MAX;

Ok now I see the reason behind the behavioural change.

So either:

* We remove the old behaviour consisting in clearing base->is_idle if the new
  next timer is within a jiffy while the tick is stopped. But then the changelog
  from the previous patch should state that and comments must be clarified.

or:

* We restore the old behaviour, making things a bit more complicated I guess.

Thanks.