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

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Message-ID: <8c46a270-0e97-4c50-b5f3-ed58ee0c2d11@gmail.com>
Date: Tue, 2 Sep 2025 16:47:53 -0700
From: Florian Fainelli <f.fainelli@...il.com>
To: Juri Lelli <juri.lelli@...hat.com>, 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

On 8/18/25 05:58, 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>

Thanks Juri, can we apply it?
-- 
Florian