From: Peter Zijlstra <peterz@infradead.org>
Date: Mon, 9 Mar 2009 12:56:21 +0000 (+0100)
Subject: sched: optimize ttwu vs group scheduling
X-Git-Url: http://git.osdn.net/view?a=commitdiff_plain;h=57310a98a354e84279d7c8af2f48805a62372e53;p=sagit-ice-cold%2Fkernel_xiaomi_msm8998.git

sched: optimize ttwu vs group scheduling

Impact: micro-optimization

We can avoid the sched domain walk on try_to_wake_up() when we know
there are no groups.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1236603381.8389.455.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---

diff --git a/kernel/sched.c b/kernel/sched.c
index e0fa739a441b..af5cd1b2d03e 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -331,6 +331,13 @@ static DEFINE_PER_CPU(struct rt_rq, init_rt_rq) ____cacheline_aligned_in_smp;
  */
 static DEFINE_SPINLOCK(task_group_lock);
 
+#ifdef CONFIG_SMP
+static int root_task_group_empty(void)
+{
+	return list_empty(&root_task_group.children);
+}
+#endif
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
 #ifdef CONFIG_USER_SCHED
 # define INIT_TASK_GROUP_LOAD	(2*NICE_0_LOAD)
@@ -391,6 +398,13 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
 
 #else
 
+#ifdef CONFIG_SMP
+static int root_task_group_empty(void)
+{
+	return 1;
+}
+#endif
+
 static inline void set_task_rq(struct task_struct *p, unsigned int cpu) { }
 static inline struct task_group *task_group(struct task_struct *p)
 {
@@ -2318,7 +2332,7 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync)
 		sync = 0;
 
 #ifdef CONFIG_SMP
-	if (sched_feat(LB_WAKEUP_UPDATE)) {
+	if (sched_feat(LB_WAKEUP_UPDATE) && !root_task_group_empty()) {
 		struct sched_domain *sd;
 
 		this_cpu = raw_smp_processor_id();