linux-kernel - [PATCH 3.2 19/24] sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target

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Date:	Tue, 03 Mar 2015 22:11:28 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Tim Chen" <tim.c.chen@...ux.intel.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Suruchi Kadu" <suruchi.a.kadu@...el.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	"Doug Nelson" <doug.nelson@...el.com>,
	"Shawn Bohrer" <sbohrer@...advisors.com>,
	"Ingo Molnar" <mingo@...nel.org>
Subject: [PATCH 3.2 19/24] sched/rt: Reduce rq lock contention by
 eliminating locking of non-feasible target

3.2.68-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Tim Chen <tim.c.chen@...ux.intel.com>

commit 80e3d87b2c5582db0ab5e39610ce3707d97ba409 upstream.

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>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/sched_rt.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1067,7 +1067,12 @@ select_task_rq_rt(struct task_struct *p,
 	    (p->rt.nr_cpus_allowed > 1)) {
 		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();
@@ -1346,6 +1351,16 @@ static struct rq *find_lock_lowest_rq(st
 
 		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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