linux-kernel - Re: [PATCH v8 2/4] sched: Rewrite runnable load and utilization average tracking

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Message-ID: <20150618230554.GA3436@intel.com>
Date:	Fri, 19 Jun 2015 07:05:54 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, pjt@...gle.com, bsegall@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, len.brown@...el.com,
	rafael.j.wysocki@...el.com, fengguang.wu@...el.com,
	srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH v8 2/4] sched: Rewrite runnable load and utilization
 average tracking

On Fri, Jun 19, 2015 at 02:00:38PM +0800, Boqun Feng wrote:
> However, update_cfs_rq_load_avg() only updates cfs_rq->avg, the change
> won't be contributed or aggregated to cfs_rq's parent in the
> for_each_leaf_cfs_rq loop, therefore that's actually not a bottom-up
> update.
> 
> To fix this, I think we can add a update_cfs_shares(cfs_rq) after
> update_cfs_rq_load_avg(). Like:
> 
>  	for_each_leaf_cfs_rq(rq, cfs_rq) {
> -		/*
> -		 * Note: We may want to consider periodically releasing
> -		 * rq->lock about these updates so that creating many task
> -		 * groups does not result in continually extending hold time.
> -		 */
> -		__update_blocked_averages_cpu(cfs_rq->tg, rq->cpu);
> +		/* throttled entities do not contribute to load */
> +		if (throttled_hierarchy(cfs_rq))
> +			continue;
> +
> +		update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq);
> +		update_cfs_share(cfs_rq);
>  	}
> 
> However, I think update_cfs_share isn't cheap, because it may do a
> bottom-up update once called. So how about just update the root cfs_rq?
> Like:
> 
> -	/*
> -	 * Iterates the task_group tree in a bottom up fashion, see
> -	 * list_add_leaf_cfs_rq() for details.
> -	 */
> -	for_each_leaf_cfs_rq(rq, cfs_rq) {
> -		/*
> -		 * Note: We may want to consider periodically releasing
> -		 * rq->lock about these updates so that creating many task
> -		 * groups does not result in continually extending hold time.
> -		 */
> -		__update_blocked_averages_cpu(cfs_rq->tg, rq->cpu);
> -	}
> +	update_cfs_rq_load_avg(rq_clock_task(rq), rq->cfs_rq);

Hi Boqun,

Did I get you right:

This rewrite patch does not NEED to aggregate entity's load to cfs_rq,
but rather directly update the cfs_rq's load (both runnable and blocked),
so there is NO NEED to iterate all of the cfs_rqs.

So simply updating the top cfs_rq is already equivalent to the stock.

It is better if we iterate the cfs_rq to update the actually weight
(update_cfs_share), because the weight may have already changed, which
would in turn change the load. But update_cfs_share is not cheap.

Right?

Thanks,
Yuyang
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