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

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Date: Tue, 30 Jan 2024 16:29:59 +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 Sun, Jan 28, 2024 at 04:58:55PM +0100, Anna-Maria Behnsen a écrit :
> Frederic Weisbecker <frederic@...nel.org> writes:
> >> +
> >> +			if (tmigr_check_lonely(topgroup))
> >> +				return READ_ONCE(topgroup->next_expiry);
> 
> When I hand in tevt->global as a parameter, I'll need to compare the
> first expiry of the toplevel group and the tevt->global value and return
> the earlier expiry. Only a single child is active in top level, so it
> might be that this CPU is the last active CPU in hierarchy.
> 
> I didn't check all the way to the top whether all groups are
> 'lonely'. So when the top level group has only a single active child, it
> is also possible that the child of the top level group has two active
> children... Then a return of KTIME_MAX would be also a more precise
> forecast.
> 
> This quick check is there to keep the overhead minimal when checking
> whether it might be possible to go idle. So I don't know, if we should
> add this additional check per level (which is pretty simple when using
> group->parent for walking the hierarchy). What do you think?

Not sure. Maybe if the tree never exceeds 3 levels (does it?) it's ok to
do the walk?

Thanks.