[PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE

[Dietmar Eggemann]

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.