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

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Message-ID: <YKObZ1GcfVIVWRWt@google.com>
Date:   Tue, 18 May 2021 10:48:07 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Will Deacon <will@...nel.org>
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

On Tuesday 18 May 2021 at 11:28:34 (+0100), Will Deacon wrote:
> 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.

Yep, probably.

> 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.

Right so I think either way will be imperfect, so I agree with the
above.

Maybe one thing though is that, IIRC, userspace _can_ disable admission
control if it wants to. In this case I'd have no problem with allowing
this weird behaviour when admission control is off -- the kernel won't
provide any guarantees. But if it's left on, then it's a different
story.

So what about we say, if admission control is off, we allow execve() and
sched_setattr() with appropriate warnings as you suggest, but if
admission control is on then we fail both?

We might still see random failures in the wild if admission control is
left enabled on those devices but then I think these could qualify as
a device misconfiguration, not as a kernel bug.

Thoughts?

Thanks,
Quentin