linux-kernel - Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

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Date: Mon, 5 Feb 2024 21:30:02 +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 18/20] timers: Implement the hierarchical pull model

Le Mon, Feb 05, 2024 at 02:29:34PM +0100, Anna-Maria Behnsen a écrit :
> Anna-Maria Behnsen <anna-maria@...utronix.de> writes:
> 
> > Frederic Weisbecker <frederic@...nel.org> writes:
> >
> >> Le Thu, Feb 01, 2024 at 05:15:37PM +0100, Anna-Maria Behnsen a écrit :
> >>> Frederic Weisbecker <frederic@...nel.org> writes:
> >>> 
> >>> > Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
> >>> > Heh, I was about to say that it's impossible that timer_base_is_idle()
> >>> > at this stage but actually if we run in nohz_full...
> >>> >
> >>> > It happens so that nohz_full is deactivated until rcutree_online_cpu()
> >>> > which calls tick_dep_clear() but it's a pure coincidence that might
> >>> > disappear one day. So yes, let's keep it that way.
> >>> 
> >>> I instrumented the code (with NOHZ FULL and NOHZ_IDLE) to make sure the
> >>> timer migration hierarchy state 'idle' is in sync with the timer base
> >>> 'idle'. And this was one part where it was possible that it runs out of
> >>> sync as I remember correctly. But if I understood you correctly, this
> >>> shouldn't happen at the moment?
> >>
> >> Well, it's not supposed to :-)
> >
> > Hmm, let me double check this and run the tests on the instrumented
> > version...
> 
> I added a prinkt() to verify what I think I remember. I was able to see
> the prints. So it seems, that the coincidence that nohz_full is
> deactivated until rcutree_online_cpu() already disappeared.

Nice, then I guess it can become a WARN_ON.

Thanks.

> 
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -1672,6 +1672,8 @@ static int tmigr_cpu_online(unsigned int
>  	tmc->idle = timer_base_is_idle();
>  	if (!tmc->idle)
>  		__tmigr_cpu_activate(tmc);
> +	else
> +		printk("TIMER BASE IS IDLE\n");
>  	tmc->online = true;
>  	raw_spin_unlock_irq(&tmc->lock);
>  	return 0;
> 
> Thanks,
> 
>         Anna-Maria
>