linux-kernel - Re: [PATCH v10 6/7] sched: Provide runnable_load_avg back to cfs

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Date:	Tue, 21 Jul 2015 18:18:46 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Yuyang Du <yuyang.du@...el.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, umgwanakikbuti@...il.com,
	len.brown@...el.com, rafael.j.wysocki@...el.com,
	arjan@...ux.intel.com, fengguang.wu@...el.com
Subject: Re: [PATCH v10 6/7] sched: Provide runnable_load_avg back to cfs_rq

On Tue, Jul 21, 2015 at 08:44:01AM +0800, Yuyang Du wrote:
> On Tue, Jul 21, 2015 at 09:08:07AM +0800, Boqun Feng wrote:
> > Hi Yuyang,
> > 
> > On Wed, Jul 15, 2015 at 08:04:41AM +0800, Yuyang Du wrote:
> > > The cfs_rq's load_avg is composed of runnable_load_avg and blocked_load_avg.
> > > Before this series, sometimes the runnable_load_avg is used, and sometimes
> > > the load_avg is used. Completely replacing all uses of runnable_load_avg
> > > with load_avg may be too big a leap, i.e., the blocked_load_avg is concerned
> > > to result in overrated load. Therefore, we get runnable_load_avg back.
> > > 
> > > The new cfs_rq's runnable_load_avg is improved to be updated with all of the
> > > runnable sched_eneities at the same time, so the one sched_entity updated and
> > > the others stale problem is solved.
> > > 
> > 
> > How about tracking cfs_rq's blocked_load_avg instead of
> > runnable_load_avg, because, AFAICS:
> > 
> > cfs_rq->runnable_load_avg = se->avg.load_avg - cfs_rq->blocked_load_avg.
> 
> No, cfs_rq->runnable_load_avg = cfs_rq->avg.load_avg - cfs_rq->blocked_load_avg,
> without rounding errors and the like.
>  

Oh, sorry.. yeah, you're right here.

> > se is the corresponding sched_entity of cfs_rq. And when we need the
> > runnable_load_avg, we just calculate by the expression above.
> > 
> > This can be thought as a lazy way to update runnable_load_avg, and we
> > don't need to modify __update_load_avg any more.
> 
> Not lazy at all, but adding (as of now) useless blocked_load_avg and an
> extra subtraction.

but we can remove runnable_load_avg tracking code in __update_load_avg,
as you do in this patch, right?

> Or did you forget blocked_load_avg also needs to be updated/decayed as
> time elapses?

I know we need to update or decay the blocked_load_avg, but we only need
to update and decay when 1) entity dequeued/enqueued 2) entity migrated
or 3) we need the runnable_load_avg value calcuated by blocked_load_avg,
right?

These are more rare than __update_load_avg called, right?

Regards,
Boqun


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