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

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Message-ID: <ZdScXhIS_G1cjaWG@localhost.localdomain>
Date: Tue, 20 Feb 2024 13:34:38 +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()

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.

Thanks.

> 
> Then it is mandatory that we have this drop the assignment also in the
> path when the tick is already stopped. Otherwise the problem described
> in the comment could happen with stale jiffies, no?
> 
> Thanks
> 
> > Thanks.
> >
> >
> >
> >> @@ -938,6 +923,21 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> >>  		trace_tick_stop(1, TICK_DEP_MASK_NONE);
> >>  	}
> >>  
> >> +	/*
> >> +	 * 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;
> >> +	}
> >> +
> >>  	ts->next_tick = expires;
> >>  
> >>  	/*