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Date: Fri, 24 Jun 2016 15:45:15 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Jirka Hladky <jhladky@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Kamil Kolakowski <kkolakow@redhat.com>,
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        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Benjamin Segall <bsegall@google.com>, Paul Turner <pjt@google.com>,
        Matt Fleming <matt@codeblueprint.co.uk>
Subject: Re: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and
 SPECjvm2008 benchmarks against 4.6 kernel
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On Fri, Jun 24, 2016 at 03:23:37PM +0200, Vincent Guittot wrote:
> > It seemed like a simple and cheap way to increase accuracy, nothing more
> > behind it until the commit you referred to.
> 
> Thanks for the clarification.
> I thought that the difference should always be smaller than 1/64th of
> the cfs_rq->avg.load_avg thanks to update_tg_load_avg

Right, another reason I just remembered is that it ensures:

	tg_weight >= cfs_rq_weight

Because if this is the only task in the entire group and cfs_rq
increased (but did not exceed the 1/64th threshold) you get the group
weight being smaller than the entity weight, which would be weird.