From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 13 Sep 2011 16:42:35 +0200
Subject: sched: Disable TTWU_QUEUE on RT
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 1109242528fd60f75042b111c4454b866fd3285a
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

The queued remote wakeup mechanism can introduce rather large
latencies if the number of migrated tasks is high. Disable it for RT.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 kernel/sched/features.h |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -45,11 +45,16 @@ SCHED_FEAT(LB_BIAS, true)
  */
 SCHED_FEAT(NONTASK_CAPACITY, true)
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+SCHED_FEAT(TTWU_QUEUE, false)
+#else
+
 /*
  * Queue remote wakeups on the target CPU and process them
  * using the scheduler IPI. Reduces rq->lock contention/bounces.
  */
 SCHED_FEAT(TTWU_QUEUE, true)
+#endif
 
 /*
  * When doing wakeups, attempt to limit superfluous scans of the LLC domain.