Messages in this thread Patch in this message | | | Date | Tue, 25 Oct 2022 10:32:26 +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 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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