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

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Date:	Mon, 22 Jun 2015 19:18:05 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Yuyang Du <yuyang.du@...el.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

On 21/06/15 23:26, Yuyang Du wrote:
> 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.

You're right. Both consumers of tg->load_avg and tg->runnable_avg
(__update_group_entity_contrib() and calc_tg_weight()) are never called
w/ cfs_rq->tg->css.id < 2 .

I did some tests w/ task groups on my ARM platform. The system survives
an appropriate BUG_ON in both functions just fine.

Maybe you can put a little bit more information why rq::avg isn't needed
even in the CONFIG_FAIR_GROUP_SCHED=y case into the patch header?

I assume getting rid of extra code in the current per-entity
load-tracking code is a good thing. And this patch is probably less
complicated to understand than your second one :-)

> 
>> 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.

Given the fact that it is not used this wouldn't be necessary.

[...]

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