[tip:sched/core] sched: optimize ttwu vs group scheduling

[Peter Zijlstra <peterz@infradead.org>]

Commit-ID:  6352997e4525536e8242a913362b52541c0ca67c
Gitweb:     http://git.kernel.org/tip/6352997e4525536e8242a913362b52541c0ca67c
Author:     "Peter Zijlstra" <peterz@infradead.org>
AuthorDate: Mon, 9 Mar 2009 13:56:21 +0100
Commit:     Ingo Molnar <mingo@elte.hu>
CommitDate: Tue, 10 Mar 2009 16:35:12 +0100

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>


---
 kernel/sched.c |   16 +++++++++++++++-
 1 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index e0fa739..a7ecb65 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();