linux-kernel - Re: [PATCH v8 1/4] sched: Remove rq's runnable avg

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Date:	Mon, 22 Jun 2015 06:26:01 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	"mingo@...nel.org" <mingo@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"bsegall@...gle.com" <bsegall@...gle.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
	"fengguang.wu@...el.com" <fengguang.wu@...el.com>,
	"boqun.feng@...il.com" <boqun.feng@...il.com>,
	"srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v8 1/4] sched: Remove rq's runnable avg

Hi Dietmar,

On Fri, Jun 19, 2015 at 07:27:24PM +0100, Dietmar Eggemann wrote:
> Hi Yuyang,
> 
> On 15/06/15 20:26, Yuyang Du wrote:
> > The current rq->avg is not used at all since its merge into kernel,
> > and the code is in the scheduler's hot path, so remove it.
> 
> are you sure that this is the case? I was always under the impression
> that w/ CONFIG_FAIR_GROUP_SCHED=y rq->avg (runnable_avg_sum, avg_period)
> is used to calculate contrib in __update_tg_runnable_avg() for the root
> group (cfs_rq->tg->css.id = 1).
> 
> On tg's w/ cfs_rq->tg->css.id > 1, se->avg (runnable_avg_sum,
> avg_period) is used instead but we simply don't have a tg related se for
> the root group. IMHO, that's why we have this rq::avg.
 
Yes, I agree. But the root group's avg is not useful anyway. If it is,
we sure should keep it.

> I understand that w/ the second patch in your series you don't need
> rq::avg any more.

And the rq->avg can be replaced by the root cfs_rq's util_avg, too.

Thanks,
Yuyang
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