linux-kernel - Re: [PATCH v8 06/19] cpuset: Don't use the cpu_possible

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Message-ID: <877dj9ees8.mognet@arm.com>
Date:   Fri, 04 Jun 2021 18:11:03 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        kernel-team@...roid.com, Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH v8 06/19] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1

On 02/06/21 17:47, Will Deacon wrote:
> @@ -3322,9 +3322,13 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>
>  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
>  {
> +	const struct cpumask *cs_mask;
> +	const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
> +
>       rcu_read_lock();
> -	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> -		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
> +	cs_mask = task_cs(tsk)->cpus_allowed;
> +	if (is_in_v2_mode() && cpumask_subset(cs_mask, possible_mask))
> +		do_set_cpus_allowed(tsk, cs_mask);

Since the task will still go through the is_cpu_allowed() loop in
select_fallback_rq() after this, is the subset check actually required
here?

It would have more merit if cpuset_cpus_allowed_fallback() returned whether
it actually changed the allowed mask or not, in which case we could branch
either to the is_cpu_allowed() loop (as we do unconditionally now), or to
the 'state == possible' switch case.

>       rcu_read_unlock();
>
>       /*
> --
> 2.32.0.rc0.204.g9fa02ecfa5-goog