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

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e4a0b8bd941c..7f1f6a1736a5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1800,7 +1800,12 @@ static void update_numa_stats(struct task_numa_env *env,
if (ns->idle_cpu == -1)
ns->idle_cpu = cpu;

+ /* If we find an idle core, stop searching. */
idle_core = numa_idle_core(idle_core, cpu);
+ if (idle_core >= 0) {
+ ns->idle_cpu = idle_core;
+ break;
+ }
}
}
rcu_read_unlock();
@@ -1808,9 +1813,6 @@ static void update_numa_stats(struct task_numa_env *env,
ns->weight = cpumask_weight(cpumask_of_node(nid));

ns->node_type = numa_classify(env->imbalance_pct, ns);
-
- if (idle_core >= 0)
- ns->idle_cpu = idle_core;
}

static void task_numa_assign(struct task_numa_env *env,
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