Received-SPF: pass (google.com: domain of linux-kernel+bounces-44973-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
From: Anna-Maria Behnsen <anna-maria@linutronix.de>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
 John Stultz <jstultz@google.com>, Thomas Gleixner <tglx@linutronix.de>,
 Eric Dumazet <edumazet@google.com>, "Rafael J . Wysocki"
 <rafael.j.wysocki@intel.com>, Arjan van de Ven <arjan@infradead.org>,
 "Paul E . McKenney" <paulmck@kernel.org>, Rik van Riel <riel@surriel.com>,
 Steven Rostedt <rostedt@goodmis.org>, Sebastian Siewior
 <bigeasy@linutronix.de>, Giovanni Gherdovich <ggherdovich@suse.cz>, Lukasz
 Luba <lukasz.luba@arm.com>, "Gautham R . Shenoy" <gautham.shenoy@amd.com>,
 Srinivas Pandruvada <srinivas.pandruvada@intel.com>, K Prateek Nayak
 <kprateek.nayak@amd.com>
Subject: Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model
In-Reply-To: <ZbkV9wVxFhQxujb3@localhost.localdomain>
References: <20240115143743.27827-1-anna-maria@linutronix.de>
 <20240115143743.27827-19-anna-maria@linutronix.de>
 <ZbJwol4QoSgz0QlH@lothringen> <877cjtcuv4.fsf@somnus>
 <ZbkV9wVxFhQxujb3@localhost.localdomain>
Date: Tue, 30 Jan 2024 17:45:06 +0100
Message-ID: <87zfwm93e5.fsf@somnus>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Frederic Weisbecker <frederic@kernel.org> writes:

> Le Sun, Jan 28, 2024 at 04:58:55PM +0100, Anna-Maria Behnsen a =C3=A9crit=
 :
>> Frederic Weisbecker <frederic@kernel.org> writes:
>> >> +
>> >> +			if (tmigr_check_lonely(topgroup))
>> >> +				return READ_ONCE(topgroup->next_expiry);
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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