[patch] sched: ensure than a child can't start left of it's parent's vruntime at fork() time

[Mike Galbraith <efault@gmx.de>]

sched: ensure than a child can't start left of it's parent's vruntime at fork() time.

With START_DEBIT we make room for the new task, but with child_runs_first,
the debit lands on the parent when we swap vruntimes.  There's nothing to
prevent the parent's vruntime having been right of min_vruntime at fork()
time, meaning that the child can gain vruntime, thus increasing scheduling
latency for waiters.

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

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index cc97ea4..eb1269c 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -728,11 +728,11 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 
 			vruntime -= thresh;
 		}
-
-		/* ensure we never gain time by being placed backwards. */
-		vruntime = max_vruntime(se->vruntime, vruntime);
 	}
 
+	/* ensure we never gain time by being placed backwards. */
+	vruntime = max_vruntime(se->vruntime, vruntime);
+
 	se->vruntime = vruntime;
 }
 
@@ -1756,6 +1757,8 @@ static void task_new_fair(struct rq *rq, struct task_struct *p)
 	sched_info_queued(p);
 
 	update_curr(cfs_rq);
+	if (curr)
+		se->vruntime = curr->vruntime;	
 	place_entity(cfs_rq, se, 1);
 
 	/* 'curr' will be NULL if the child belongs to a different group */