linux-kernel - Re: [PATCH v10a] timers: Move marking timer bases idle into tick_nohz_stop

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Message-ID: <87o7cb40sx.fsf@somnus>
Date: Tue, 20 Feb 2024 16:23:26 +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 v10a] timers: Move marking timer bases idle into
 tick_nohz_stop_tick()

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

> On Tue, Feb 20, 2024 at 03:00:57PM +0100, Anna-Maria Behnsen wrote:
>> Frederic Weisbecker <frederic@...nel.org> writes:
>> 
>> > Le Tue, Feb 20, 2024 at 01:02:18PM +0100, Anna-Maria Behnsen a écrit :
>> >> Frederic Weisbecker <frederic@...nel.org> writes:
>> >> 
>> >> > Le Tue, Feb 20, 2024 at 11:48:19AM +0100, Anna-Maria Behnsen a écrit :
>> >> >> Frederic Weisbecker <frederic@...nel.org> writes:
>> >> >> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> >> >> index 01fb50c1b17e..b93f0e6f273f 100644
>> >> >> --- a/kernel/time/tick-sched.c
>> >> >> +++ b/kernel/time/tick-sched.c
>> >> >> @@ -895,21 +895,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>> >> >>  	/* Make sure we won't be trying to stop it twice in a row. */
>> >> >>  	ts->timer_expires_base = 0;
>> >> >>  
>> >> >> -	/*
>> >> >> -	 * If this CPU is the one which updates jiffies, then give up
>> >> >> -	 * the assignment and let it be taken by the CPU which runs
>> >> >> -	 * the tick timer next, which might be this CPU as well. If we
>> >> >> -	 * don't drop this here, the jiffies might be stale and
>> >> >> -	 * do_timer() never gets invoked. Keep track of the fact that it
>> >> >> -	 * was the one which had the do_timer() duty last.
>> >> >> -	 */
>> >> >> -	if (cpu == tick_do_timer_cpu) {
>> >> >> -		tick_do_timer_cpu = TICK_DO_TIMER_NONE;
>> >> >> -		ts->do_timer_last = 1;
>> >> >> -	} else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
>> >> >> -		ts->do_timer_last = 0;
>> >> >> -	}
>> >> >> -
>> >> >>  	/* Skip reprogram of event if it's not changed */
>> >> >>  	if (ts->tick_stopped && (expires == ts->next_tick)) {
>> >> >>  		/* Sanity check: make sure clockevent is actually programmed */
>> >> >
>> >> > That should work but then you lose the optimization that resets
>> >> > ts->do_timer_last even if the next timer hasn't changed.
>> >> >
>> >> 
>> >> Beside of this optimization thing, I see onther problem. But I'm not
>> >> sure, if I understood it correctly: When the CPU drops the
>> >> tick_do_timer_cpu assignment and stops the tick, it is possible, that
>> >> this CPU nevertheless executes tick_sched_do_timer() and then reassigns
>> >> to tick_do_timer_cpu?
>> >
>> > Yes but in this case a timer interrupt has executed and ts->next_tick
>> > is cleared, so the above skip reprogramm branch is not taken.
>> >
>> 
>> Yes... So I need to change it without dropping the
>> optimization. Otherwise someone might complain about it.
>> 
>> Two possible solutions:
>> 
>> a) split out this if/else thing for dropping the tick_do_timer_cpu
>>    assignment into a separate function and call it:
>>    - before the return in the skip reprogramm branch
>>    - and after the if clause which contains stopping the tick (where it
>>      is executed in the current proposal)
>> 
>> b) Take my current proposal and add before the return in the skip
>>    reprogramm branch the following lines:
>> 
>>    if (tick_do_timer_cpu != TICK_DO_TIMER_NONE)
>>    	ts->do_timer_last = 0;
>> 
>>    as the first part of the tick_do_timer_cpu/last logic shouldn't be
>>    required (because then also ts->next_tick is already cleared).
>> 
>> What do you prefere? Or do you prefere something else?
>
> Wouldn't the following work? If timer_idle is false, then the tick isn't
> even stopped and there is nothing to do? So you can early return.
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index fdd57f1af1d7..1b2984acafbd 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -924,6 +924,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>  		expires = ts->timer_expires;
>  	}
>  
> +	if (!timer_idle)
> +		return;
> +
>  	/*
>  	 * If this CPU is the one which updates jiffies, then give up
>  	 * the assignment and let it be taken by the CPU which runs

Yes... And then I can drop the if (!timer_idle) thing inside
!ts->tick_stopped branch.