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@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Cc: Suruchi Kadu <suruchi.a.kadu@intel.com>
Cc: Doug Nelson<doug.nelson@intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1421430374.2399.27.camel@schen9-desk2.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 16 insertions, 1 deletions

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 6725e3c49660..f4d4b077eba0 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)) {
                         /*