linux-kernel - [tip:sched/core] sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target

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Date:	Sun, 1 Feb 2015 09:52:24 -0800
From:	tip-bot for Tim Chen <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	suruchi.a.kadu@...el.com, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, sbohrer@...advisors.com, peterz@...radead.org,
	mingo@...nel.org, tglx@...utronix.de, doug.nelson@...el.com,
	torvalds@...ux-foundation.org, tim.c.chen@...ux.intel.com,
	hpa@...or.com
Subject: [tip:sched/core] sched/rt:
  Reduce rq lock contention by eliminating locking of non-feasible target

Commit-ID:  80e3d87b2c5582db0ab5e39610ce3707d97ba409
Gitweb:     http://git.kernel.org/tip/80e3d87b2c5582db0ab5e39610ce3707d97ba409
Author:     Tim Chen <tim.c.chen@...ux.intel.com>
AuthorDate: Fri, 12 Dec 2014 15:38:12 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 30 Jan 2015 19:38:49 +0100

sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target

This patch adds checks that prevens futile attempts to move rt tasks
to a CPU with active tasks of equal or higher priority.

This reduces run queue lock contention and improves the performance of
a well known OLTP benchmark by 0.7%.

Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Shawn Bohrer <sbohrer@...advisors.com>
Cc: Suruchi Kadu <suruchi.a.kadu@...el.com>
Cc: Doug Nelson<doug.nelson@...el.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: http://lkml.kernel.org/r/1421430374.2399.27.camel@schen9-desk2.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/rt.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 6725e3c..f4d4b07 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1340,7 +1340,12 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	     curr->prio <= p->prio)) {
 		int target = find_lowest_rq(p);
 
-		if (target != -1)
+		/*
+		 * Don't bother moving it if the destination CPU is
+		 * not running a lower priority task.
+		 */
+		if (target != -1 &&
+		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
 			cpu = target;
 	}
 	rcu_read_unlock();
@@ -1617,6 +1622,16 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 
 		lowest_rq = cpu_rq(cpu);
 
+		if (lowest_rq->rt.highest_prio.curr <= task->prio) {
+			/*
+			 * Target rq has tasks of equal or higher priority,
+			 * retrying does not release any lock and is unlikely
+			 * to yield a different result.
+			 */
+			lowest_rq = NULL;
+			break;
+		}
+
 		/* if the prio of this runqueue changed, try again */
 		if (double_lock_balance(rq, lowest_rq)) {
 			/*
--
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