Re: [PATCH 2/4] sched,numa: weigh nearby nodes for task placement on complex NUMA topologies

[Peter Zijlstra]

On Thu, May 08, 2014 at 01:23:29PM -0400, r...@redhat.com wrote:
> @@ -930,7 +987,7 @@ static inline unsigned long group_faults_cpu(struct 
> numa_group *group, int nid)
>   */
>  static inline unsigned long task_weight(struct task_struct *p, int nid)
>  {
> -     unsigned long total_faults;
> +     unsigned long total_faults, score;
>  
>       if (!p->numa_faults_memory)
>               return 0;
> @@ -940,15 +997,32 @@ static inline unsigned long task_weight(struct 
> task_struct *p, int nid)
>       if (!total_faults)
>               return 0;
>  
> -     return 1000 * task_faults(p, nid) / total_faults;
> +     score = 1000 * task_faults(p, nid);
> +     score += nearby_nodes_score(p, nid, true);
> +
> +     score /= total_faults;
> +
> +     return score;
>  }
>  
>  static inline unsigned long group_weight(struct task_struct *p, int nid)
>  {
> -     if (!p->numa_group || !p->numa_group->total_faults)
> +     unsigned long total_faults, score;
> +
> +     if (!p->numa_group)
> +             return 0;
> +
> +     total_faults = p->numa_group->total_faults;
> +
> +     if (!total_faults)
>               return 0;
>  
> -     return 1000 * group_faults(p, nid) / p->numa_group->total_faults;
> +     score = 1000 * group_faults(p, nid);
> +     score += nearby_nodes_score(p, nid, false);
> +
> +     score /= total_faults;
> +
> +     return score;
>  }

OK, and that's just sad..

See task_numa_placement(), which does:

        for_each_online_node(nid) {
                weight = task_weight(p, nid) + group_weight(p, nid);
                if (weight > max_weight) {
                        max_weight = weight;
                        max_nid = nid;
                }
        }

So not only is that loop now O(nr_nodes^2), the inner loops doubly
iterates all nodes.

Also, {task,group}_weight() functions were like cheap-ish (/me mumbles
something about people using !2^n scaling factors for no sane reason).
And they're used all over with that in mind.

But look what you did to migrate_improves_locality(), that will now
iterate all nodes _4_ times, and its called for every single task we try
and migrate during load balance, while holding rq->lock.

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