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

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Date: Tue, 20 Feb 2024 16:10:40 +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 v10a] timers: Move marking timer bases idle into
 tick_nohz_stop_tick()

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