linux-kernel - [tip:sched/core] sched: Reduce contention in update_cfs_rq_blocked

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Date:	Tue, 9 Sep 2014 07:52:38 -0700
From:	tip-bot for Jason Low <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, bsegall@...gle.com, hpa@...or.com,
	mingo@...nel.org, torvalds@...ux-foundation.org, pjt@...gle.com,
	peterz@...radead.org, tim.c.chen@...ux.intel.com,
	yuyang.du@...el.com, jason.low2@...com, Waiman.Long@...com,
	chegu_vinod@...com, tglx@...utronix.de, scott.norton@...com,
	aswin@...com
Subject: [tip:sched/core] sched:
  Reduce contention in update_cfs_rq_blocked_load()

Commit-ID:  8236d907ab3411ad452280faa8b26c1347327380
Gitweb:     http://git.kernel.org/tip/8236d907ab3411ad452280faa8b26c1347327380
Author:     Jason Low <jason.low2@...com>
AuthorDate: Tue, 2 Sep 2014 00:41:24 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 9 Sep 2014 06:47:29 +0200

sched: Reduce contention in update_cfs_rq_blocked_load()

When running workloads on 2+ socket systems, based on perf profiles, the
update_cfs_rq_blocked_load() function often shows up as taking up a
noticeable % of run time.

Much of the contention is in __update_cfs_rq_tg_load_contrib() when we
update the tg load contribution stats.  However, it turns out that in many
cases, they don't need to be updated and "tg_contrib" is 0.

This patch adds a check in __update_cfs_rq_tg_load_contrib() to skip updating
tg load contribution stats when nothing needs to be updated. This reduces the
cacheline contention that would be unnecessary.

Reviewed-by: Ben Segall <bsegall@...gle.com>
Reviewed-by: Waiman Long <Waiman.Long@...com>
Signed-off-by: Jason Low <jason.low2@...com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Cc: Paul Turner <pjt@...gle.com>
Cc: jason.low2@...com
Cc: Yuyang Du <yuyang.du@...el.com>
Cc: Aswin Chandramouleeswaran <aswin@...com>
Cc: Chegu Vinod <chegu_vinod@...com>
Cc: Scott J Norton <scott.norton@...com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: http://lkml.kernel.org/r/1409643684.19197.15.camel@j-VirtualBox
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 50d2025..be9e97b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2382,6 +2382,9 @@ static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
 	tg_contrib = cfs_rq->runnable_load_avg + cfs_rq->blocked_load_avg;
 	tg_contrib -= cfs_rq->tg_load_contrib;
 
+	if (!tg_contrib)
+		return;
+
 	if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
 		atomic_long_add(tg_contrib, &tg->load_avg);
 		cfs_rq->tg_load_contrib += tg_contrib;
--
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