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 14:44:07 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.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

Hi Peter,

On 24 June 2016 at 14:02, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Jun 24, 2016 at 09:44:41AM +0200, Jirka Hladky wrote:
>> Hi Peter,
>>
>> thanks a lot for looking into it!
>>
>> I have tried to disable autogroups
>>
>> sysctl -w kernel.sched_autogroup_enabled=0
>>
>> and I can confirm that performance is then back at level as in 4.6 kernel.
>
> So unless the heat has made me do really silly things, the below seems
> to cure things. Could you please verify?
>
>
> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> 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)

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 ?


>
>         return tg_weight;
>  }
> @@ -2494,7 +2494,7 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
>         long tg_weight, load, shares;
>
>         tg_weight = calc_tg_weight(tg, cfs_rq);
> -       load = cfs_rq->load.weight;
> +       load = cfs_rq->avg.load_avg;
>
>         shares = (tg->shares * load);
>         if (tg_weight)