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Date:   Tue, 1 Dec 2020 22:30:56 +0000
From:   Will Deacon <will@kernel.org>
To:     Quentin Perret <qperret@google.com>
Cc:     Qais Yousef <qais.yousef@arm.com>,
        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>,
        Suren Baghdasaryan <surenb@google.com>,
        Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        kernel-team@android.com
Subject: Re: [PATCH v4 09/14] cpuset: Don't use the cpu_possible_mask as a
 last resort for cgroup v1
Message-ID: <20201201223056.GB28496@willie-the-truck>
References: <20201124155039.13804-1-will@kernel.org>
 <20201124155039.13804-10-will@kernel.org>
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 <20201130170531.qo67rai5lftskmk2@e107158-lin.cambridge.arm.com>
 <20201130173610.GA1715200@google.com>
 <20201201115842.t77abecneuesd5ih@e107158-lin.cambridge.arm.com>
 <20201201123748.GA1896574@google.com>
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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