sched: Add WF_FORK

Avoid the cache buddies from biasing the time distribution away
from fork()ers. Normally the next buddy will be the preferred
scheduling target, but this makes fork()s prefer to run the new
child, whereas we prefer to run the parent, since that will
generate more work.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -1028,6 +1028,7 @@ struct sched_domain;
  * wake flags
  */
 #define WF_SYNC		0x01		/* waker goes to sleep after wakup */
+#define WF_FORK		0x02		/* child wakeup after fork */
 
 struct sched_class {
 	const struct sched_class *next;

kernel/sched.c

@@ -2602,7 +2602,7 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
 		inc_nr_running(rq);
 	}
 	trace_sched_wakeup_new(rq, p, 1);
-	check_preempt_curr(rq, p, 0);
+	check_preempt_curr(rq, p, WF_FORK);
 #ifdef CONFIG_SMP
 	if (p->sched_class->task_wake_up)
 		p->sched_class->task_wake_up(rq, p);

kernel/sched_fair.c

@@ -1580,7 +1580,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int flags
 	 */
 	if (sched_feat(LAST_BUDDY) && likely(se->on_rq && curr != rq->idle))
 		set_last_buddy(se);
-	if (sched_feat(NEXT_BUDDY))
+	if (sched_feat(NEXT_BUDDY) && !(flags & WF_FORK))
 		set_next_buddy(pse);
 
 	/*