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

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Date:   Thu, 19 Nov 2020 12:47:34 +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 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().

I was actually hoping this could be a feasible approach, i.e. have an
extra CPU active mask filter for any task:

  cpu_active_mask & arch_cpu_allowed_mask(p)

rather than fiddle with task affinity. Sadly this would also require fixing
up pretty much any place that uses cpu_active_mask(), and probably places
that use p->cpus_ptr as well. RT / DL balancing comes to mind, because that
uses either a task's affinity or a CPU's root domain (which reflects the
cpu_active_mask) to figure out where to push a task.

So while I'm wary of hacking up affinity, I fear it might be the lesser
evil :(

[1]: https://lore.kernel.org/lkml/1251528473.590671.1579196495905.JavaMail.zimbra@efficios.com/

> In preparation for restricting the affinity mask for compat tasks on
> arm64 systems without uniform support for 32-bit applications, introduce
> a restrict_cpus_allowed_ptr(), which allows the current affinity mask
> for a task to be shrunk to the intersection of a parameter mask.
>
> Signed-off-by: Will Deacon <will@...nel.org>