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

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Date:   Thu, 20 May 2021 18:00:29 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Dietmar Eggemann <dietmar.eggemann@....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 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?

-- Daniel