[PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity

[Valentin Schneider]

On 19/11/20 11:05, Will Deacon wrote:
> On Thu, Nov 19, 2020 at 09:18:20AM +0000, Quentin Perret wrote:
>> > @@ -1937,20 +1931,69 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>> >             * OK, since we're going to drop the lock immediately
>> >             * afterwards anyway.
>> >             */
>> > -		rq = move_queued_task(rq, &rf, p, dest_cpu);
>> > +		rq = move_queued_task(rq, rf, p, dest_cpu);
>> >    }
>> >  out:
>> > -	task_rq_unlock(rq, p, &rf);
>> > +	task_rq_unlock(rq, p, rf);
>>
>> And that's a little odd to have here no? Can we move it back on the
>> caller's side?
>
> I don't think so, unfortunately. __set_cpus_allowed_ptr_locked() can trigger
> migration, so it can drop the rq lock as part of that and end up relocking a
> new rq, which it also unlocks before returning. Doing the unlock in the
> caller is therfore even weirder, because you'd have to return the lock
> pointer or something horrible like that.
>
> I did add a comment about this right before the function and it's an
> internal function to the scheduler so I think it's ok.
>

An alternative here would be to add a new SCA_RESTRICT flag for
__set_cpus_allowed_ptr() (see migrate_disable() faff in
tip/sched/core). Not fond of either approaches, but the flag thing would
avoid this "quirk".

> Will