linux-kernel - Re: [PATCH v2 3/3] sched/fair: Disable tg load_avg update for root_task

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Date:	Wed, 02 Dec 2015 11:55:44 -0800
From:	bsegall@...gle.com
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Yuyang Du <yuyang.du@...el.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2 3/3] sched/fair: Disable tg load_avg update for root_task_group

Waiman Long <Waiman.Long@....com> writes:

> Currently, the update_tg_load_avg() function attempts to update the
> tg's load_avg value whenever the load changes even for root_task_group
> where the load_avg value will never be used. This patch will disable
> the load_avg update when the given task group is the root_task_group.
>
> Running a Java benchmark with noautogroup and a 4.3 kernel on a
> 16-socket IvyBridge-EX system, the amount of CPU time (as reported by
> perf) consumed by task_tick_fair() which includes update_tg_load_avg()
> decreased from 0.71% to 0.22%, a more than 3X reduction. The Max-jOPs
> results also increased slightly from 983015 to 986449.
>
> Signed-off-by: Waiman Long <Waiman.Long@....com>

Reviewed-by: Ben Segall <bsegall@...gle.com>
> ---
>  kernel/sched/fair.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8f1eccc..4607cb7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2670,6 +2670,12 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq, int force)
>  {
>  	long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
>  
> +	/*
> +	 * No need to update load_avg for root_task_group as it is not used.
> +	 */
> +	if (cfs_rq->tg == &root_task_group)
> +		return;
> +
>  	if (force || abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
>  		atomic_long_add(delta, &cfs_rq->tg->load_avg);
>  		cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg;
--
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