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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

  reply	other threads:[~2021-06-07 17:04 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-02 16:47 [PATCH v8 00/19] Add support for 32-bit tasks on asymmetric AArch32 systems Will Deacon
2021-06-02 16:47 ` [PATCH v8 01/19] arm64: cpuinfo: Split AArch32 registers out into a separate struct Will Deacon
2021-06-03 12:38   ` Mark Rutland
2021-06-03 17:24     ` Will Deacon
2021-06-02 16:47 ` [PATCH v8 02/19] arm64: Allow mismatched 32-bit EL0 support Will Deacon
2021-06-03 12:37   ` Mark Rutland
2021-06-03 17:44     ` Will Deacon
2021-06-04  9:38       ` Mark Rutland
2021-06-04 11:05         ` Will Deacon
2021-06-04 12:04           ` Mark Rutland
2021-06-04 13:50             ` Will Deacon
2021-06-02 16:47 ` [PATCH v8 03/19] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched " Will Deacon
2021-06-02 16:47 ` [PATCH v8 04/19] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs Will Deacon
2021-06-02 16:47 ` [PATCH v8 05/19] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection Will Deacon
2021-06-04 17:10   ` Valentin Schneider
2021-06-07 17:04     ` Will Deacon [this message]
2021-06-02 16:47 ` [PATCH v8 06/19] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1 Will Deacon
2021-06-04 17:11   ` Valentin Schneider
2021-06-07 17:20     ` Will Deacon
2021-06-10 10:20       ` Valentin Schneider
2021-06-02 16:47 ` [PATCH v8 07/19] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus() Will Deacon
2021-06-04 17:11   ` Valentin Schneider
2021-06-02 16:47 ` [PATCH v8 08/19] sched: Reject CPU affinity changes based on task_cpu_possible_mask() Will Deacon
2021-06-04 17:11   ` Valentin Schneider
2021-06-07 22:43     ` Will Deacon
2021-06-02 16:47 ` [PATCH v8 09/19] sched: Introduce task_struct::user_cpus_ptr to track requested affinity Will Deacon
2021-06-04 17:12   ` Valentin Schneider
2021-06-02 16:47 ` [PATCH v8 10/19] sched: Split the guts of sched_setaffinity() into a helper function Will Deacon
2021-06-04 17:12   ` Valentin Schneider
2021-06-02 16:47 ` [PATCH v8 11/19] sched: Allow task CPU affinity to be restricted on asymmetric systems Will Deacon
2021-06-04 17:12   ` Valentin Schneider
2021-06-07 22:52     ` Will Deacon
2021-06-10 10:20       ` Valentin Schneider
2021-06-02 16:47 ` [PATCH v8 12/19] sched: Introduce task_cpus_dl_admissible() to check proposed affinity Will Deacon
2021-06-03  9:43   ` Daniel Bristot de Oliveira
2021-06-03  9:52     ` Will Deacon
2021-06-02 16:47 ` [PATCH v8 13/19] arm64: Implement task_cpu_possible_mask() Will Deacon
2021-06-02 16:47 ` [PATCH v8 14/19] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Will Deacon
2021-06-03  9:45   ` Daniel Bristot de Oliveira
2021-06-02 16:47 ` [PATCH v8 15/19] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system Will Deacon
2021-06-03 12:58   ` Mark Rutland
2021-06-03 17:40     ` Will Deacon
2021-06-04  9:49       ` Mark Rutland
2021-06-04 12:14         ` Qais Yousef
2021-06-02 16:47 ` [PATCH v8 16/19] arm64: Advertise CPUs capable of running 32-bit applications in sysfs Will Deacon
2021-06-02 16:47 ` [PATCH v8 17/19] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0 Will Deacon
2021-06-02 16:47 ` [PATCH v8 18/19] arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores Will Deacon
2021-06-02 16:47 ` [PATCH v8 19/19] Documentation: arm64: describe asymmetric 32-bit support Will Deacon

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