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

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Date: Mon, 22 Jan 2024 12:48:21 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
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()

Frederic Weisbecker <frederic@...nel.org> writes:

> 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.
>

I would like to take 'either'. I thought the changelog already mentioned
it. But maybe I have to make it more explicit. I'll go and rework the
comments once more.

Thanks,

	Anna-Maria