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

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Date:	Fri, 24 Jun 2016 15:33:47 +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:
> On 24 June 2016 at 15:09, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Fri, Jun 24, 2016 at 02:44:07PM +0200, Vincent Guittot wrote:
> >
> >> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> > index 22d64b3f5876..d4f6fb2f3057 100644
> >> > --- a/kernel/sched/fair.c
> >> > +++ b/kernel/sched/fair.c
> >> > @@ -2484,7 +2484,7 @@ static inline long calc_tg_weight(struct task_group *tg, struct cfs_rq *cfs_rq)
> >> >          */
> >> >         tg_weight = atomic_long_read(&tg->load_avg);
> >> >         tg_weight -= cfs_rq->tg_load_avg_contrib;
> >> > -       tg_weight += cfs_rq->load.weight;
> >> > +       tg_weight += cfs_rq->avg.load_avg;
> >>
> >> IIUC, you are reverting
> >> commit  fde7d22e01aa (sched/fair: Fix overly small weight for
> >> interactive group entities)
> >
> > Ah!, I hadn't yet done a git-blame on this. Right you are, we should
> > have put a comment there.
> >
> > So the problem here is that since commit:
> >
> >   2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
> >
> > load.weight and avg.load_avg are in different metrics. Which completely
> > wrecked things.
> >
> > The obvious alternative is using:
> >
> >         scale_load_down(cfs_rq->load.weight);
> >
> > Let me go run that through the benchmark.
> 
> Yes, looks to be good alternative

Does indeed also work. Let me go write a Changelog and try and magic it
into sched/urgent.