linux-kernel - Re: [PATCH v3 11/14] sched: Reject CPU affinity changes based on arch_cpu_allowed

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Date:   Thu, 19 Nov 2020 16:44:13 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
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>,
        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 11/14] sched: Reject CPU affinity changes based on
 arch_cpu_allowed_mask()

On Thu, Nov 19, 2020 at 02:30:12PM +0000, Quentin Perret wrote:
> On Thursday 19 Nov 2020 at 11:07:24 (+0000), Will Deacon wrote:
> > Yeah, the cpuset code ignores the return value of set_cpus_allowed_ptr() in
> > update_tasks_cpumask() so the failure won't be propagated, but then again I
> > think that might be the right thing to do. Nothing prevents 32-bit and
> > 64-bit tasks from co-existing in the same cpuseti afaict, so forcing the
> > 64-bit tasks onto the 32-bit-capable cores feels much worse than the
> > approach taken here imo.
> 
> Ack. And thinking about it more, failing the cgroup operation wouldn't
> guarantee that the task's affinity and the cpuset are aligned anyway. We
> could still exec into a 32 bit task from within a 64 bit-only cpuset.
> 
> > The interesting case is what happens if the cpuset for a 32-bit task is
> > changed to contain only the 64-bit-only cores. I think that's a userspace
> > bug
> 
> Maybe, but I think Android will do exactly that in some cases :/
> 
> > but the fallback rq selection should avert disaster.
> 
> I thought _this_ patch was 'fixing' this case by making the cpuset
> operation pretty much a nop on the task affinity? The fallback rq stuff
> is all about hotplug no?

Yeah, sorry, I wasn't clear. This patch postpones disaster until hotplug
off time, when cpuset_cpus_allowed_fallback() will fail and
select_fallback_rq() will have to step in.

Will