linux-kernel - Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

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Date:	Mon, 7 Jul 2014 12:07:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	rafael.j.wysocki@...el.com, arjan.van.de.ven@...el.com,
	len.brown@...el.com, alan.cox@...el.com, mark.gross@...el.com,
	pjt@...gle.com, fengguang.wu@...el.com
Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average
 tracking

On Wed, Jul 02, 2014 at 10:30:56AM +0800, Yuyang Du wrote:
> The idea of per entity runnable load average (aggregated to cfs_rq and task_group load)
> was proposed by Paul Turner, and it is still followed by this rewrite. But this rewrite
> is made due to the following ends:
> 
> (1). cfs_rq's load average (namely runnable_load_avg and blocked_load_avg) is updated
> incrementally by one entity at one time, which means the cfs_rq load average is only
> partially updated or asynchronous accross its entities (the entity in question is up
> to date and contributes to the cfs_rq, but all other entities are effectively lagging
> behind).
> 
> (2). cfs_rq load average is different between top rq->cfs_rq and task_group's per CPU
> cfs_rqs in whether or not blocked_load_average contributes to the load.
> 
> (3). How task_group's load is tracked is very confusing and complex.
> 
> Therefore, this rewrite tackles these by:
> 
> (1). Combine runnable and blocked load averages for cfs_rq. And track cfs_rq's load average
> as a whole (contributed by all runnabled and blocked entities on this cfs_rq).
> 
> (2). Only track task load average. Do not track task_group's per CPU entity average, but
> track that entity's own cfs_rq's aggregated average.
> 
> This rewrite resutls in significantly reduced codes and expected consistency and clarity.
> Also, if draw the lines of previous cfs_rq runnable_load_avg and blocked_load_avg and the
> new rewritten load_avg, then compare those lines, you can see the new load_avg is much
> more continuous (no abrupt jumping ups and downs) and decayed/updated more quickly and
> synchronously.

This patch is too big.. and I can't figure out wth you've done. The
Changelog also doesn't seem to help much.

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