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

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Date:   Fri, 21 May 2021 13:26:10 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        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 20/05/2021 20:03, Will Deacon wrote:
> Hi Dietmar,
> 
> On Thu, May 20, 2021 at 07:55:27PM +0200, Dietmar Eggemann wrote:
>> On 20/05/2021 18:00, Daniel Bristot de Oliveira wrote:
>>> On 5/20/21 5:06 PM, Dietmar Eggemann wrote:
>>>> (1) # chrt -d -T 5000000 -P 16666666 0 ./32bit_app
>>>>
>>>> (2) # ./32bit_app &
>>>>
>>>>     # chrt -d -T 5000000 -P 16666666 -p 0 pid_of(32bit_app)
>>>>
>>>>
>>>> Wouldn't the behaviour of (1) and (2) be different w/o this patch?
>>>>
>>>> In (1) __sched_setscheduler() happens before execve so it operates on
>>>> p->cpus_ptr equal span.
>>>>
>>>> In (2) span != p->cpus_ptr so DL AC will fail.
>>>>
>>>
>>> As far as I got, the case (1) would be spitted in two steps:
>>>
>>>  - __sched_setscheduler() will work, then
>>>  - execv() would fail because (span != p->cpus_ptr)
>>>
>>> So... at the end, both (1) and (2) would result in a failure...
>>>
>>> am I missing something?
>>
>> Not sure. Reading this thread I was under the assumption that the only
>> change would be the drop of this patch. But I assume there is also this
>> 'if DL AC is on then let sched_setattr() fail for this 32bit task'.
>>
>> IMHO, the current patch-stack w/o this patch should let (1) succeed with
>> DL AC.
> 
> That's what I'm proposing, yes, but others (including Daniel) prefer to
> fail the execve(). See my other reply just now for a summary [1].

[...]

Thanks, Will ... Now I understand. Or at least I think I do ;-)

> [1] https://lore.kernel.org/lkml/20210520180138.GA10523@willie-the-truck/T/#u