Re: [PATCH v2 11/19] sched/numa: Restrict migrating in parallel to the same node.

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 20, 2018 at 10:32:52PM +0530, Srikar Dronamraju wrote:
> Since task migration under numa balancing can happen in parallel, more
> than one task might choose to move to the same node at the same time.
> This can cause load imbalances at the node level.
> 
> The problem is more likely if there are more cores per node or more
> nodes in system.
> 
> Use a per-node variable to indicate if task migration
> to the node under numa balance is currently active.
> This per-node variable will not track swapping of tasks.


> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 50c7727..87fb20e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1478,11 +1478,22 @@ struct task_numa_env {
>  static void task_numa_assign(struct task_numa_env *env,
>  			     struct task_struct *p, long imp)
>  {
> +	pg_data_t *pgdat = NODE_DATA(cpu_to_node(env->dst_cpu));
>  	struct rq *rq = cpu_rq(env->dst_cpu);
>  
>  	if (xchg(&rq->numa_migrate_on, 1))
>  		return;
>  
> +	if (!env->best_task && env->best_cpu != -1)
> +		WRITE_ONCE(pgdat->active_node_migrate, 0);
> +
> +	if (!p) {
> +		if (xchg(&pgdat->active_node_migrate, 1)) {
> +			WRITE_ONCE(rq->numa_migrate_on, 0);
> +			return;
> +		}
> +	}
> +
>  	if (env->best_cpu != -1) {
>  		rq = cpu_rq(env->best_cpu);
>  		WRITE_ONCE(rq->numa_migrate_on, 0);


Urgh, that's prertty magical code. And it doesn't even have a comment.

For isntance, I cannot tell why we clear that active_node_migrate thing
right there.