Re: [PATCH v7 01/22] sched: Favour predetermined active CPU as migration destination

[Will Deacon]

On Wed, May 26, 2021 at 12:14:20PM +0100, Valentin Schneider wrote:
> On 25/05/21 16:14, Will Deacon wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 5226cc26a095..1702a60d178d 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1869,6 +1869,7 @@ static struct rq *move_queued_task(struct rq *rq, struct rq_flags *rf,
> >  struct migration_arg {
> >       struct task_struct		*task;
> >       int				dest_cpu;
> > +	const struct cpumask		*dest_mask;
> >       struct set_affinity_pending	*pending;
> >  };
> >
> > @@ -1917,6 +1918,7 @@ static int migration_cpu_stop(void *data)
> >       struct set_affinity_pending *pending = arg->pending;
> >       struct task_struct *p = arg->task;
> >       int dest_cpu = arg->dest_cpu;
> > +	const struct cpumask *dest_mask = arg->dest_mask;
> >       struct rq *rq = this_rq();
> >       bool complete = false;
> >       struct rq_flags rf;
> > @@ -1956,12 +1958,8 @@ static int migration_cpu_stop(void *data)
> >                       complete = true;
> >               }
> >
> > -		if (dest_cpu < 0) {
> > -			if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
> > -				goto out;
> > -
> > -			dest_cpu = cpumask_any_distribute(&p->cpus_mask);
> > -		}
> > +		if (dest_mask && (cpumask_test_cpu(task_cpu(p), dest_mask)))
> > +			goto out;
> >
> 
> IIRC the reason we deferred the pick to migration_cpu_stop() was because of
> those insane races involving multiple SCA calls the likes of:
> 
>   p->cpus_mask = [0, 1]; p on CPU0
> 
>   CPUx                           CPUy                   CPU0
> 
>   SCA(p, [2])
>     __do_set_cpus_allowed();
>     queue migration_cpu_stop()
>                                  SCA(p, [3])
>                                    __do_set_cpus_allowed();
>                                                         migration_cpu_stop()
> 
> The stopper needs to use the latest cpumask set by the second SCA despite
> having an arg->pending set up by the first SCA. Doesn't this break here?

Yes, well spotted. I was so caught up with the hotplug race that I didn't
even consider a straightforward SCA race. Hurumph.

> I'm not sure I've paged back in all of the subtleties laying in ambush
> here, but what about the below?

I can't break it, but I'm also not very familiar with this code. Please can
you post it as a proper patch so that I drop this from my series?

Thanks,

Will