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

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Message-ID: <aKMja4BvgQ5vFaNN@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 18 Aug 2025 14:58:19 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Doug Berger <opendmb@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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

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>

Thanks,
Juri