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

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Date:   Thu, 20 May 2021 14:38:55 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Quentin Perret <qperret@...gle.com>, Will Deacon <will@...nel.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        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>,
        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 5/20/21 12:33 PM, Quentin Perret wrote:
> On Thursday 20 May 2021 at 11:16:41 (+0100), Will Deacon wrote:
>> Ok, thanks for the insight. In which case, I'll go with what we discussed:
>> require admission control to be disabled for sched_setattr() but allow
>> execve() to a 32-bit task from a 64-bit deadline task with a warning (this
>> is probably similar to CPU hotplug?).
> 
> Still not sure that we can let execve go through ... It will break AC
> all the same, so it should probably fail as well if AC is on IMO
> 

If the cpumask of the 32-bit task is != of the 64-bit task that is executing it,
the admission control needs to be re-executed, and it could fail. So I see this
operation equivalent to sched_setaffinity(). This will likely be true for future
schedulers that will allow arbitrary affinities (AC should run on affinity
change, and could fail).

I would vote with Juri: "I'd go with fail hard if AC is on, let it
pass if AC is off (supposedly the user knows what to do)," (also hope nobody
complains until we add better support for affinity, and use this as a motivation
to get back on this front).

-- Daniel