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

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Date: Mon, 05 Feb 2024 14:29:34 +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 18/20] timers: Implement the hierarchical pull model

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.

--- 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