[PATCH 3.16.y-ckt 181/183] sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target

[Luis Henriques]

3.16.7-ckt8 -stable review patch.  If anyone has any objections, please let me know.

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

From: Tim Chen <tim.c.chen at linux.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 at linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Cc: Shawn Bohrer <sbohrer at rgmadvisors.com>
Cc: Suruchi Kadu <suruchi.a.kadu at intel.com>
Cc: Doug Nelson<doug.nelson at intel.com>
Cc: Steven Rostedt <rostedt at goodmis.org>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Link: http://lkml.kernel.org/r/1421430374.2399.27.camel@schen9-desk2.jf.intel.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 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 a49083192c64..5d720ac96246 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1336,7 +1336,12 @@ 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();
@@ -1608,6 +1613,16 @@ 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)) {
 			/*