Re: [External] Re: [PATCH 1/2] sched/numa: Stop an exhastive search if an idle core is found

[Hao Jia]

On 2022/10/25 Mel Gorman wrote:
> On Tue, Oct 25, 2022 at 07:10:22PM +0800, Hao Jia wrote:
> > On 2022/10/25 Mel Gorman wrote:
> > > On Tue, Oct 25, 2022 at 11:16:29AM +0800, Hao Jia wrote:
> > > > > Remove the change in the first hunk and call break in the second hunk
> > > > > after updating ns->idle_cpu.
> > > >
> > > > Yes, thanks for your review.
> > > > If I understand correctly, some things might look like this.
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index e4a0b8bd941c..dfcb620bfe50 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -1792,7 +1792,7 @@ static void update_numa_stats(struct task_numa_env
> > > > *env,
> > > >                   ns->nr_running += rq->cfs.h_nr_running;
> > > >                   ns->compute_capacity += capacity_of(cpu);
> > > >
> > > > -               if (find_idle && !rq->nr_running && idle_cpu(cpu)) {
> > > > +               if (find_idle && idle_core < 0 && !rq->nr_running &&
> > > > idle_cpu(cpu)) {
> > > >                           if (READ_ONCE(rq->numa_migrate_on) ||
> > > >                               !cpumask_test_cpu(cpu, env->p->cpus_ptr))
> > > >                                   continue;
> > >
> > > I meant more like the below but today I wondered why did I not do this in
> > > the first place? The answer is because it's wrong and broken in concept.
> > >
> > > The full loop is needed to calculate approximate NUMA stats at a
> > > point in time. For example, the src and dst nr_running is needed by
> > > task_numa_find_cpu. The search for an idle CPU or core in update_numa_stats
> > > is simply taking advantage of the fact we are scanning anyway to keep
> > > track of an idle CPU or core to avoid a second search as per ff7db0bf24db
> > > ("sched/numa: Prefer using an idle CPU as a migration target instead of
> > > comparing tasks")
> > >
> > > The patch I had in mind is below but that said, for both your version and
> > > my initial suggestion
> > >
> > > Naked-by: Mel Gorman <mgorman@xxxxxxx>
> > >
> > > For the record, this is what I was suggesting initially because it's more
> > > efficient but it's wrong, don't do it.
> >
> > Thanks for the detailed explanation, maybe my commit message misled you.
>
> Yes, I did end up confusing myself. The title and changelog referred to
> stopping a search which made me think of terms of "this whole loop can
> terminate early" which it can't but it *can* stop checking for a new idle
> core. If an idle core has been found, it follows that an idle CPU has also
> been found. While numa_idle_core checks this explicitly, your patch avoids
> an unnecessary cpumask_test_cpu so it has value.

Thank you for your review, I will change the commit message and send 
patch v2.


> > Yes, we can't stop the whole loop of scanning the CPU because we have a lot
> > of NUMA information to count.
> >
> > But we can stop looking for the next idle core or idle cpu after finding an
> > idle core.
> >
> > So, please review the previous code.
>
> You're right and sorry for the noise.
>
> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Thanks!

>
Thanks,
Hao