linux-kernel - Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed

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Date:   Thu, 19 Nov 2020 14:54:47 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Will Deacon <will@...nel.org>
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>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity


On 19/11/20 13:13, Will Deacon wrote:
> On Thu, Nov 19, 2020 at 12:47:34PM +0000, Valentin Schneider wrote:
>>
>> On 13/11/20 09:37, 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.
>> >
>> > Although userspace can carefully manage the affinity masks for such
>> > tasks, one place where it is particularly problematic is execve()
>> > because the CPU on which the execve() is occurring may be incompatible
>> > with the new application image. In such a situation, it is desirable to
>> > restrict the affinity mask of the task and ensure that the new image is
>> > entered on a compatible CPU.
>>
>> > From userspace's point of view, this looks the same as if the
>> > incompatible CPUs have been hotplugged off in its affinity mask.
>>
>> {pedantic reading warning}
>>
>> Hotplugged CPUs *can* be set in a task's affinity mask, though interact
>> weirdly with cpusets [1]. Having it be the same as hotplug would mean
>> keeping incompatible CPUs allowed in the affinity mask, but preventing them
>> from being picked via e.g. is_cpu_allowed().
>
> Sure, but I was talking about what userspace sees, and I don't think it ever
> sees CPUs that have been hotplugged off, right? That is, sched_getaffinity()
> masks its result with the active_mask.
>

Right, this wasn't pedantic reading, but reading between the lines!