linux-kernel - Re: [patch v6 03/21] sched: only count runnable avg on cfs_rq's nr

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Date:	Tue, 2 Apr 2013 16:30:48 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, Paul Turner <pjt@...gle.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>, gregkh@...uxfoundation.org,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch v6 03/21] sched: only count runnable avg on cfs_rq's nr_running

On 30 March 2013 15:34, Alex Shi <alex.shi@...el.com> wrote:
> Old function count the runnable avg on rq's nr_running even there is
> only rt task in rq. That is incorrect, so correct it to cfs_rq's
> nr_running.
>
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2881d42..026e959 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2829,7 +2829,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>         }
>
>         if (!se) {
> -               update_rq_runnable_avg(rq, rq->nr_running);
> +               update_rq_runnable_avg(rq, rq->cfs.nr_running);

A RT task that preempts your CFS task will be accounted in the
runnable_avg fields. So whatever you do, RT task will impact your
runnable_avg statistics. Instead of trying to get only CFS tasks, you
should take into account all tasks activity in the rq.

Vincent
>                 inc_nr_running(rq);
>         }
>         hrtick_update(rq);
> --
> 1.7.12
>
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