linux-kernel - Re: [PATCH 09/14] sched,fair: refactor enqueue/dequeue

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Message-ID: <20190731153027.GV31381@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Jul 2019 17:30:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, pjt@...gle.com,
        dietmar.eggemann@....com, mingo@...hat.com,
        morten.rasmussen@....com, tglx@...utronix.de,
        mgorman@...hsingularity.net, vincent.guittot@...aro.org
Subject: Re: [PATCH 09/14] sched,fair: refactor enqueue/dequeue_entity

On Wed, Jul 31, 2019 at 11:03:01AM -0400, Rik van Riel wrote:

> I think I understand the problem you are pointing out, but if
> update_load_avg() keeps the load average for the runqueue unchanged
> (because that is rate limited to once a jiffy, and has been like that
> for a while), why would calc_group_shares() result in a different
> value than what it returned the last time?
> 
> What am I overlooking?

I'm thinking you're thinking (3):

           tg->weight * grq->avg.load_avg
  shares = ------------------------------
                 tg->load_avg

Where: tg->load_avg ~= \Sum grq->avg.load_avg


Which is the straight forward shares calculation, and purely depends on
the load averages (which haven't been changed etc..)

But what we actually do is (6):

                                    tg->weight * grq->avg.load_avg
  shares = ---------------------------------------------------------------------------
           tg->load_avg - grq->avg.load_avg + max(grq->load.weight, grq->avg.load_avg)

And even if tg->load_avg and grq->avg.load_avg haven't changed,
grq->load.weight most certainly has.