linux-kernel - Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average

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Date:	Fri, 1 Aug 2014 03:16:38 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	peterz@...radead.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, pjt@...gle.com, bsegall@...gle.com,
	arjan.van.de.ven@...el.com, len.brown@...el.com,
	rafael.j.wysocki@...el.com, alan.cox@...el.com,
	mark.gross@...el.com, fengguang.wu@...el.com,
	morten.rasmussen@....com
Subject: Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average

Hi Vincent,

On Thu, Jul 31, 2014 at 11:56:13AM +0200, Vincent Guittot wrote:
> 
> load_sum is now the average runnable time before being weighted

So when weight changes, load_avg will completely use new weight. I have
some cents:

1) Task does not change weight much, so it is practically ok

2) Group entity does change weight much, and very likely back and forth,
   so I really think keeping the intact history will make everything
   more predictable/stable, prevent thrashing, etc.

3) If you do the same for cfs_rq->load.weight, then we simply abandoned
   blocked entities, and all states won't compute. So we then need to
   maintain blocked load average again, and we just can't do cfs_rq load
   average as a whole anymore, but must update at the granularity of an
   entity...

Anyway, it does not seem to me you really need to change load_sum, no? So
could you please not change it?

> The sum of usage_sum of the tasks that are on a rq, is used to detect
> the overload of a rq.

I think you only need usage_sum for task and rq, but not cfs_rq. Others
are ok.

> Does something like the patch below to be applied of top of your patchset, seem
> reasonable add-on?
> 

If you only add running statistics, I am all good, and indeed reasonable if
you can make good use of it. I am not at all against adding anything or
adding running average or unweighted anything...

Thanks,
Yuyang
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