Re: [PATCH v8 05/19] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection

From: Will Deacon <will@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	linux-kernel@vger.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
Date: Mon, 7 Jun 2021 18:04:03 +0100	[thread overview]
Message-ID: <20210607170403.GA7650@willie-the-truck> (raw)
In-Reply-To: <878s3peesp.mognet@arm.com>

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@arm.com>

Thanks!

Will