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

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Date:   Mon, 30 Nov 2020 17:36:10 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Will Deacon <will@...nel.org>,
        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 Monday 30 Nov 2020 at 17:05:31 (+0000), Qais Yousef wrote:
> I create 3 cpusets: 64bit, 32bit and mix. As the name indicates, 64bit contains
> all 64bit-only cpus, 32bit contains 32bit-capable ones and mix has a mixture of
> both.
> 
> If I try to move my test binary to 64bit cpuset, it moves there and I see the
> WARN_ON_ONCE() triggered. The task has attached to the new cpuset but
> set_allowed_cpus_ptr() has failed and we end up with whatever affinity we had
> previously. Breaking cpusets effectively.

Right, and so does exec'ing from a 64 bit task into 32 bit executable
from within a 64 bit-only cpuset :( . And there is nothing we can really
do about it, we cannot fail the exec because we need this to work for
existing apps, and there is no way the Android framework can know
upfront.

So the only thing we can do really is WARN() and proceed to ignore the
cpuset, which is what this series does :/. It's not exactly pretty but I
don't think we can do much better than that TBH, and it's the same thing
for the example you brought up. Failing cpuset_can_attach() will not
help, we can only WARN and proceed ...

Now, Android should be fine with that I think. We only need the kernel
to implement a safe fallback mechanism when userspace gives
contradictory commands, because we know there are edge cases userspace
_cannot_ deal with correctly, but this fallback doesn't need to be
highly optimized (at least for Android), but I'm happy to hear what
others think.

Thanks,
Quentin