sched: introduce SCHED_FEAT_SYNC_WAKEUPS, turn it off

turn off sync wakeups by default. They are not needed anymore - the
buddy logic should be smart enough to keep the system from
overscheduling.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

diff --git a/kernel/sched.c b/kernel/sched.c
index d8456a9..263e25e 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -627,6 +627,7 @@
 	SCHED_FEAT_START_DEBIT		= 4,
 	SCHED_FEAT_HRTICK		= 8,
 	SCHED_FEAT_DOUBLE_TICK		= 16,
+	SCHED_FEAT_SYNC_WAKEUPS		= 32,
 };
 
 const_debug unsigned int sysctl_sched_features =
@@ -634,7 +635,8 @@
 		SCHED_FEAT_WAKEUP_PREEMPT	* 1 |
 		SCHED_FEAT_START_DEBIT		* 1 |
 		SCHED_FEAT_HRTICK		* 1 |
-		SCHED_FEAT_DOUBLE_TICK		* 0;
+		SCHED_FEAT_DOUBLE_TICK		* 0 |
+		SCHED_FEAT_SYNC_WAKEUPS		* 0;
 
 #define sched_feat(x) (sysctl_sched_features & SCHED_FEAT_##x)
 
@@ -1916,6 +1918,9 @@
 	long old_state;
 	struct rq *rq;
 
+	if (!sched_feat(SYNC_WAKEUPS))
+		sync = 0;
+
 	smp_wmb();
 	rq = task_rq_lock(p, &flags);
 	old_state = p->state;