linux-kernel - Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED

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Date:   Tue, 18 May 2021 11:28:34 +0100
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>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, kernel-team@...roid.com
Subject: Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into
 SCHED_DEADLINE

[Dropping Li Zefan as his mail is bouncing]

On Tue, May 18, 2021 at 10:20:38AM +0000, Quentin Perret wrote:
> On Tuesday 18 May 2021 at 10:47:17 (+0100), Will Deacon wrote:
> > On asymmetric systems where the affinity of a task is restricted to
> > contain only the CPUs capable of running it, admission to the deadline
> > scheduler is likely to fail because the span of the sched domain
> > contains incompatible CPUs. Although this is arguably the right thing to
> > do, it is inconsistent with the case where the affinity of a task is
> > restricted after already having been admitted to the deadline scheduler.
> > 
> > For example, on an arm64 system where not all CPUs support 32-bit
> > applications, a 64-bit deadline task can exec() a 32-bit image and have
> > its affinity forcefully restricted.
> 
> So I guess the alternative would be to fail exec-ing into 32bit from a
> 64bit DL task, and then drop this patch?
> 
> The nice thing about your approach is that existing applications won't
> really notice a difference (execve would still 'work'), but on the cons
> side it breaks admission control, which is sad.

Right, with your suggestion here we would forbid any 32-bit deadline tasks
on an asymmetric system, even if you'd gone to the extraordinary effort
to cater for that (e.g. by having a separate root domain).

> I don't expect this weird execve-to-32bit pattern from DL to be that
> common in practice (at the very least not in Android), so maybe we could
> start with the stricter version (fail the execve), and wait to see if
> folks complain? Making things stricter later will be harder.
> 
> Thoughts?

I don't have strong opinions on this, but I _do_ want the admission via
sched_setattr() to be consistent with execve(). What you're suggesting
ticks that box, but how many applications are prepared to handle a failed
execve()? I suspect it will be fatal.

Probably also worth pointing out that the approach here will at least
warn in the execve() case when the affinity is overridden for a deadline
task.

Will