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 15:09:32 +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 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.

> I have one question regarding the use of cfs_rq->avg.load_avg
> cfs_rq->tg_load_avg_contrib is the sampling of cfs_rq->avg.load_avg so
> I'm curious to understand why you use cfs_rq->avg.load_avg instead of
> keeping cfs_rq->tg_load_avg_contrib. Do you think that the sampling is
> not accurate enough to prevent any significant difference between both
> when we use tg->load_avg ?

I'm not entirely sure I understand your question; is it to the existence
of calc_tg_weight()? That is, why use calc_tg_weight() and not use
tg->load_avg as is?

It seemed like a simple and cheap way to increase accuracy, nothing more
behind it until the commit you referred to.