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

[Will Deacon]

On Thu, Nov 19, 2020 at 11:27:55AM +0000, Valentin Schneider wrote:
> 
> 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".

I tried this when I read about the migrate_disable() stuff on lwn, but I
didn't really find it any better to work with tbh. It also doesn't help
with the locking that Quentin was mentioning, does it? (i.e. you still
have to allocate).

Will