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

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Message-ID: <8bc24850-3a14-5dd2-fbc2-bf745616949f@arm.com>
Date:   Thu, 20 May 2021 19:55:27 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        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 20/05/2021 18:00, Daniel Bristot de Oliveira wrote:
> On 5/20/21 5:06 PM, Dietmar Eggemann wrote:
>> On 20/05/2021 14:38, Daniel Bristot de Oliveira wrote:
>>> 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
>>
>> (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.