linux-kernel - Re: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and SPECjvm2008 benchmarks against 4.6 kernel

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Message-ID: <20160624134515.GC30154@twins.programming.kicks-ass.net>
Date:	Fri, 24 Jun 2016 15:45:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Jirka Hladky <jhladky@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kamil Kolakowski <kkolakow@...hat.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Yuyang Du <yuyang.du@...el.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and
 SPECjvm2008 benchmarks against 4.6 kernel

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.