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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org
Cc:     linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        kernel-team@android.com
Subject: Re: [PATCH v8 05/19] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection
In-Reply-To: <20210602164719.31777-6-will@kernel.org>
References: <20210602164719.31777-1-will@kernel.org> <20210602164719.31777-6-will@kernel.org>
Date:   Fri, 04 Jun 2021 18:10:46 +0100
Message-ID: <878s3peesp.mognet@arm.com>
MIME-Version: 1.0
Content-Type: text/plain
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On 02/06/21 17:47, Will Deacon wrote:
> Asymmetric systems may not offer the same level of userspace ISA support
> across all CPUs, meaning that some applications cannot be executed by
> some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
> not feature support for 32-bit applications on both clusters.
>
> On such a system, we must take care not to migrate a task to an
> unsupported CPU when forcefully moving tasks in select_fallback_rq()
> in response to a CPU hot-unplug operation.
>
> Introduce a task_cpu_possible_mask() hook which, given a task argument,
> allows an architecture to return a cpumask of CPUs that are capable of
> executing that task. The default implementation returns the
> cpu_possible_mask, since sane machines do not suffer from per-cpu ISA
> limitations that affect scheduling. The new mask is used when selecting
> the fallback runqueue as a last resort before forcing a migration to the
> first active CPU.
>

Nit: Some uses of this mask (cpu_is_allowed(), __set_cpus_allowed_ptr())
don't apply to kthreads. This makes sense for the 32-bit@EL0 faff, but it
wouldn't hurt to point this out somewhere IMO.

Also, that's an odd place for the definitions, but IIRC there isn't a much
better choice.

Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>