linux-kernel - Re: [PATCH v2] sched/deadline: only set free

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Message-ID: <20250903075436.GN3245006@noisy.programming.kicks-ass.net>
Date: Wed, 3 Sep 2025 09:54:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Doug Berger <opendmb@...il.com>, Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org,
	Florian Fainelli <florian.fainelli@...adcom.com>
Subject: Re: [PATCH v2] sched/deadline: only set free_cpus for online
 runqueues

On Mon, Aug 18, 2025 at 02:58:19PM +0200, Juri Lelli wrote:
> Hello,
> 
> On 14/08/25 18:22, Doug Berger wrote:
> > Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus
> > to reflect rd->online") introduced the cpudl_set/clear_freecpu
> > functions to allow the cpu_dl::free_cpus mask to be manipulated
> > by the deadline scheduler class rq_on/offline callbacks so the
> > mask would also reflect this state.
> > 
> > Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask
> > from cpudl_find()") removed the check of the cpu_active_mask to
> > save some processing on the premise that the cpudl::free_cpus
> > mask already reflected the runqueue online state.
> > 
> > Unfortunately, there are cases where it is possible for the
> > cpudl_clear function to set the free_cpus bit for a CPU when the
> > deadline runqueue is offline. When this occurs while a CPU is
> > connected to the default root domain the flag may retain the bad
> > state after the CPU has been unplugged. Later, a different CPU
> > that is transitioning through the default root domain may push a
> > deadline task to the powered down CPU when cpudl_find sees its
> > free_cpus bit is set. If this happens the task will not have the
> > opportunity to run.
> > 
> > One example is outlined here:
> > https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com
> > 
> > Another occurs when the last deadline task is migrated from a
> > CPU that has an offlined runqueue. The dequeue_task member of
> > the deadline scheduler class will eventually call cpudl_clear
> > and set the free_cpus bit for the CPU.
> > 
> > This commit modifies the cpudl_clear function to be aware of the
> > online state of the deadline runqueue so that the free_cpus mask
> > can be updated appropriately.
> > 
> > It is no longer necessary to manage the mask outside of the
> > cpudl_set/clear functions so the cpudl_set/clear_freecpu
> > functions are removed. In addition, since the free_cpus mask is
> > now only updated under the cpudl lock the code was changed to
> > use the non-atomic __cpumask functions.
> > 
> > Signed-off-by: Doug Berger <opendmb@...il.com>
> > ---
> 
> This looks now good to me.
> 
> Acked-by: Juri Lelli <juri.lelli@...hat.com>

So I just had a look at said patch because Juri here poked me; and I
came away with the feeling that cpudl_clear() is now a misnomen, seeing
how it is called from rq_online_dl().

Would cpudl_update() be a better name?