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

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Date:	Tue, 29 Jul 2014 11:12:37 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Yuyang Du <yuyang.du@...el.com>
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

On 27 July 2014 19:36, Yuyang Du <yuyang.du@...el.com> wrote:
> 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

I would even say that the load_avg of a task should not be impacted by
an old priority value. Once, the priority of a task is changed, we
should only take into account this new priority to weight the load_avg
of the task

> 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.

IMHO, we should apply the same policy than the one i mentioned for
task. So the load_avg of an entity or a cfs_rq will not be disturbed
by an old but no more valid weight

Vincent
>
> 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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