Re: [RFC PATCH] sched/fair: Make tg->load_avg per node

[Chen Yu]

On 2023-03-27 at 13:39:55 +0800, Aaron Lu wrote:
> 
> The reason why only cpus of one node has bigger overhead is: task_group
> is allocated on demand from a slab and whichever cpu happens to do the
> allocation, the allocated tg will be located on that node and accessing
> to tg->load_avg will have a lower cost for cpus on the same node and
> a higer cost for cpus of the remote node.
[...]
>  static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  {
>  	long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
> +	int node = cpu_to_node(cfs_rq->rq->cpu);
>  
>  	/*
>  	 * No need to update load_avg for root_task_group as it is not used.
> @@ -3616,7 +3617,7 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  		return;
>  
>  	if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
> -		atomic_long_add(delta, &cfs_rq->tg->load_avg);
> +		atomic_long_add(delta, &cfs_rq->tg->node_info[node]->load_avg);
When entered enqueue_entity(cfs_rq, se) -> update_tg_load_avg(cfs_rq)
the cfs_rq->rq->cpu is not necessary the current cpu, so the node returned by
cpu_to_node(cfs_rq->rq->cpu) is not necessary the current node as the current
CPU, would atomic_add still introduce cross-node overhead due to remote access
to cfs_rq->tg->node_info[node]->load_avg in this case, or do I miss something?

thanks,
Chenyu