From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752972AbdECJhm (ORCPT <rfc822;w@1wt.eu>);
        Wed, 3 May 2017 05:37:42 -0400
Received: from merlin.infradead.org ([205.233.59.134]:39070 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752027AbdECJhe (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 3 May 2017 05:37:34 -0400
Date: Wed, 3 May 2017 11:37:23 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Mike Galbraith <efault@gmx.de>, Paul Turner <pjt@google.com>,
        Chris Mason <clm@fb.com>, kernel-team@fb.com
Subject: Re: [PATCH 2/2] sched/fair: Always propagate runnable_load_avg
Message-ID: <20170503093723.tnt53ppb23tn6buz@hirez.programming.kicks-ass.net>
References: <20170424201344.GA14169@wtj.duckdns.org>
 <20170424201444.GC14169@wtj.duckdns.org>
 <CAKfTPtDsZ4bRbmd47a3P-jDq4GC8FfM9=b+jpqnLEHqA8L+UtQ@mail.gmail.com>
 <20170426225202.GC11348@wtj.duckdns.org>
 <CAKfTPtAasz-YAUGRKciY_Pab13n+2QGU6-xC=j7ef+jvqGwFtQ@mail.gmail.com>
 <20170428203347.GC19364@htj.duckdns.org>
 <CAKfTPtB3PectZVm_z6NhSszXwG-G=y-J1Y5Rxk48bLxHQ8mx4g@mail.gmail.com>
 <20170502215054.GC5335@htj.duckdns.org>
 <CAKfTPtB9kns1X7zCGYrd98M1ubHq5RfAqiopcCw1-4Q6Y=nYdw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAKfTPtB9kns1X7zCGYrd98M1ubHq5RfAqiopcCw1-4Q6Y=nYdw@mail.gmail.com>
User-Agent: NeoMutt/20170113 (1.7.2)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, May 03, 2017 at 09:34:51AM +0200, Vincent Guittot wrote:

> We use load_avg for calculating a stable share and we want to use it
> more and more.  So breaking it because it's easier doesn't seems to be
> the right way to do IMHO

So afaict we calculate group se->load.weight (aka shares, see
calc_cfs_shares), using cfs_rq->avg.load_avg, which feeds into
tg->load_avg through cfs_rq->tg_load_avg_contrib and
cfs_rq->load.weight.

And cfs_rq->avg.load_avg is computed from cfs_rq->load.weight, which
is \Sum se->load.weight.

OTOH group se->avg.load_avg isn't used much, which is TJ's point.

The only cases where group se->avg.load_avg are relevant to
cfs_rq->avg.load are the cases I listed yesterday, group creation and
group destruction. There we use the group se->avg.load_avg to manage the
boundary conditions.

So with the proposed change to se->avg.load_avg we can have some
(temporary) boundary effect when you destroy a lot of (previously
active) cgroups.


Of course, it could be I overlooked something, in which case, please
tell :-)


That said, I agree it would be nice to entirely get rid of runnable_avg,
but that is a much larger change and would require a lot more work. I
don't immediately see why we can't fix the thing now and then work on
removing runnable_load_avg later.

Of course, we should not regress either, I'll go read up on that part.