[patch 07/46] sched: fix update_min_vruntime

[Greg KH]

2.6.28-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>

commit e17036dac189dd034c092a91df56aa740db7146d upstream.

Impact: fix SCHED_IDLE latency problems

OK, so we have 1 running task A (which is obviously curr and the tree is
equally obviously empty).

'A' nicely chugs along, doing its thing, carrying min_vruntime along as it
goes.

Then some whacko speed freak SCHED_IDLE task gets inserted due to SMP
balancing, which is very likely far right, in that case

update_curr
  update_min_vruntime
    cfs_rq->rb_leftmost := true (the crazy task sitting in a tree)
      vruntime = se->vruntime

and voila, min_vruntime is waaay right of where it ought to be.

OK, so why did I write it like that to begin with...

Aah, yes.

Say we've just dequeued current

schedule
  deactivate_task(prev)
    dequeue_entity
      update_min_vruntime

Then we'll set

  vruntime = cfs_rq->min_vruntime;

we find !cfs_rq->curr, but do find someone in the tree. Then we _must_
do vruntime = se->vruntime, because

 vruntime = min_vruntime(vruntime := cfs_rq->min_vruntime, se->vruntime)

will not advance vruntime, and cause lags the other way around (which we
fixed with that initial patch: 1af5f730fc1bf7c62ec9fb2d307206e18bf40a69
(sched: more accurate min_vruntime accounting).

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Tested-by: Mike Galbraith <efault@xxxxxx>
Acked-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 kernel/sched_fair.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -283,7 +283,7 @@ static void update_min_vruntime(struct c
 						   struct sched_entity,
 						   run_node);
 
-		if (vruntime == cfs_rq->min_vruntime)
+		if (!cfs_rq->curr)
 			vruntime = se->vruntime;
 		else
 			vruntime = min_vruntime(vruntime, se->vruntime);

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