linux-kernel - Re: [PATCH v4 09/14] cpuset: Don't use the cpu_possible

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Message-ID: <20201201223056.GB28496@willie-the-truck>
Date:   Tue, 1 Dec 2020 22:30:56 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Qais Yousef <qais.yousef@....com>,
        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>,
        Suren Baghdasaryan <surenb@...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 v4 09/14] cpuset: Don't use the cpu_possible_mask as a
 last resort for cgroup v1

On Tue, Dec 01, 2020 at 03:56:49PM +0000, Quentin Perret wrote:
> On Tuesday 01 Dec 2020 at 14:11:21 (+0000), Qais Yousef wrote:
> > For cpusets, if hotunplug results in an empty cpuset, then all tasks are moved
> > to the nearest ancestor if I read the code correctly. In our case, only 32bit
> > tasks have to move out to retain this behavior. Since now for the first time we
> > have tasks that can't run on all cpus.
> > 
> > Which by the way might be the right behavior for 64bit tasks execing 32bit
> > binary in a 64bit only cpuset. I suggested SIGKILL'ing them but maybe moving
> > them to the nearest ancestor too is more aligned with the behavior above.
> 
> Hmm, I guess that means putting all 32-bit-execd-from-64-bit tasks in
> the root group in Android. I'll try and check the implications, but that
> might be just fine... Sounds like a sensible behaviour to me anyways.

I'll look into this -- anything we can do to avoid forcefully resetting the
affinity mask to the arch_task_cpu_possible_mask() is worth considering.

Will