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

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Date: Tue, 30 Jan 2024 17:45:06 +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

Frederic Weisbecker <frederic@...nel.org> writes:

> 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?

Levels depend on the number of CPUs and NUMA nodes

Take 4096 CPUs, all evenly distributed across 4 NUMA nodes, then you
will end up with 6 levels.

Thanks,

	Anna-Maria