linux-kernel - Re: [RFC PATCH] sched/fair: Make tg->load

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Message-ID: <ZENcN3suKMCaHm1w@chenyu5-mobl1>
Date:   Sat, 22 Apr 2023 12:01:59 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Aaron Lu <aaron.lu@...el.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        "Mel Gorman" <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Nitin Tekchandani <nitin.tekchandani@...el.com>,
        Waiman Long <longman@...hat.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] sched/fair: Make tg->load_avg per node

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