sched: Complete buddy switches

Add a NEXT_BUDDY feature flag to aid in debugging.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -1501,7 +1501,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
 	 */
 	if (sched_feat(LAST_BUDDY) && likely(se->on_rq && curr != rq->idle))
 		set_last_buddy(se);
-	set_next_buddy(pse);
+	if (sched_feat(NEXT_BUDDY))
+		set_next_buddy(pse);
 
 	/*
 	 * We can come here with TIF_NEED_RESCHED already set from new task

kernel/sched_features.h

@@ -14,5 +14,6 @@ SCHED_FEAT(LB_WAKEUP_UPDATE, 1)
 SCHED_FEAT(ASYM_EFF_LOAD, 1)
 SCHED_FEAT(WAKEUP_SYNC, 0)
 SCHED_FEAT(WAKEUP_OVERLAP, 0)
+SCHED_FEAT(NEXT_BUDDY, 1)
 SCHED_FEAT(LAST_BUDDY, 1)
 SCHED_FEAT(OWNER_SPIN, 1)