linux-kernel - Re: [PATCH v8 05/19] sched: Introduce task_cpu_possible

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Message-ID: <20210607170403.GA7650@willie-the-truck>
Date:   Mon, 7 Jun 2021 18:04:03 +0100
From:   Will Deacon <will@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.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
Subject: Re: [PATCH v8 05/19] sched: Introduce task_cpu_possible_mask() to
 limit fallback rq selection

On Fri, Jun 04, 2021 at 06:10:46PM +0100, Valentin Schneider wrote:
> 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.

That's a good point: even after these patches, we still assume the kernel
(and therefore kthreads) can run on all CPUs. I'll expand the comment.

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

Short of adding a new header just for this, I couldn't find anything, no.

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

Thanks!

Will