Messages in this thread | | | Date | Tue, 25 Oct 2022 14:32:26 +0100 | From | Mel Gorman <> | Subject | Re: [External] Re: [PATCH 1/2] sched/numa: Stop an exhastive search if an idle core is found |
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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@suse.de> > > > > 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.
> 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@techsingularity.net>
-- Mel Gorman SUSE Labs
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