linux-kernel - [tip:sched/core] sched/fair: Disable the task group load_avg update for the root_task

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Date:	Fri, 4 Dec 2015 03:58:01 -0800
From:	tip-bot for Waiman Long <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	mingo@...nel.org, peterz@...radead.org, hpa@...or.com,
	efault@....de, torvalds@...ux-foundation.org, yuyang.du@...el.com,
	pjt@...gle.com, linux-kernel@...r.kernel.org,
	morten.rasmussen@....com, doug.hatch@....com, bsegall@...gle.com,
	tglx@...utronix.de, scott.norton@....com, Waiman.Long@....com
Subject: [tip:sched/core] sched/fair:
  Disable the task group load_avg update for the root_task_group

Commit-ID:  aa0b7ae06387d40a988ce16a189082dee6e570bc
Gitweb:     http://git.kernel.org/tip/aa0b7ae06387d40a988ce16a189082dee6e570bc
Author:     Waiman Long <Waiman.Long@....com>
AuthorDate: Wed, 2 Dec 2015 13:41:50 -0500
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 4 Dec 2015 10:34:49 +0100

sched/fair: Disable the task group load_avg update for the root_task_group

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Ben Segall <bsegall@...gle.com>
Cc: Douglas Hatch <doug.hatch@....com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Morten Rasmussen <morten.rasmussen@....com>
Cc: Paul Turner <pjt@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Scott J Norton <scott.norton@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Yuyang Du <yuyang.du@...el.com>
Link: http://lkml.kernel.org/r/1449081710-20185-4-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4b0e8b8..1093873 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2709,6 +2709,12 @@ 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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