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

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Date:	Mon, 28 Jul 2014 01:36:16 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>,
	arjan.van.de.ven@...el.com, Len Brown <len.brown@...el.com>,
	rafael.j.wysocki@...el.com, alan.cox@...el.com,
	"Gross, Mark" <mark.gross@...el.com>,
	"fengguang.wu@...el.com" <fengguang.wu@...el.com>
Subject: Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average
 tracking

Hi Vincent,

On Fri, Jul 18, 2014 at 11:43:00AM +0200, Vincent Guittot wrote:
> > @@ -2291,23 +2299,24 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
> >         delta >>= 10;
> >         if (!delta)
> >                 return 0;
> > -       sa->last_runnable_update = now;
> > +       sa->last_update_time = now;
> >
> >         /* delta_w is the amount already accumulated against our next period */
> > -       delta_w = sa->runnable_avg_period % 1024;
> > +       delta_w = sa->period_contrib;
> >         if (delta + delta_w >= 1024) {
> > -               /* period roll-over */
> >                 decayed = 1;
> >
> > +               /* how much left for next period will start over, we don't know yet */
> > +               sa->period_contrib = 0;
> > +
> >                 /*
> >                  * Now that we know we're crossing a period boundary, figure
> >                  * out how much from delta we need to complete the current
> >                  * period and accrue it.
> >                  */
> >                 delta_w = 1024 - delta_w;
> > -               if (runnable)
> > -                       sa->runnable_avg_sum += delta_w;
> > -               sa->runnable_avg_period += delta_w;
> > +               if (w)
> > +                       sa->load_sum += w * delta_w;
> 
> Do you really need to have *w for computing the load_sum ? can't you
> only use it when computing the load_avg ?
> 
> sa->load_avg = div_u64(sa->load_sum * w , LOAD_AVG_MAX)
> 

For task, assuming its load.weight does not change much, yes, we can. But in theory, task's
load.weight can change, and *w in load_sum can take into that change. For group entity
and cfs_rq, its load.weight changes all the time, I don't know how to do it without *w
for load_sum.

Sorry for my irresponsiveness for last week. I was on vacation and unfortunately failed to
connect VPN from where I was.

Thanks,
Yuyang
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