[049/165] sched: Fix over-scheduling bug

[Greg KH <gregkh@suse.de>]

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

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

From: Alex,Shi <alex.shi@intel.com>

commit 3c93717cfa51316e4dbb471e7c0f9d243359d5f8 upstream.

Commit e70971591 ("sched: Optimize unused cgroup configuration") introduced
an imbalanced scheduling bug.

If we do not use CGROUP, function update_h_load won't update h_load. When the
system has a large number of tasks far more than logical CPU number, the
incorrect cfs_rq[cpu]->h_load value will cause load_balance() to pull too
many tasks to the local CPU from the busiest CPU. So the busiest CPU keeps
going in a round robin. That will hurt performance.

The issue was found originally by a scientific calculation workload that
developed by Yanmin. With that commit, the workload performance drops
about 40%.

 CPU  before    after

 00   : 2       : 7
 01   : 1       : 7
 02   : 11      : 6
 03   : 12      : 7
 04   : 6       : 6
 05   : 11      : 7
 06   : 10      : 6
 07   : 12      : 7
 08   : 11      : 6
 09   : 12      : 6
 10   : 1       : 6
 11   : 1       : 6
 12   : 6       : 6
 13   : 2       : 6
 14   : 2       : 6
 15   : 1       : 6

Reviewed-by: Yanmin zhang <yanmin.zhang@intel.com>
Signed-off-by: Alex Shi <alex.shi@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1276754893.9452.5442.camel@debian>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 kernel/sched.c |    3 ---
 1 file changed, 3 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1717,9 +1717,6 @@ static void update_shares_locked(struct
 
 static void update_h_load(long cpu)
 {
-	if (root_task_group_empty())
-		return;
-
 	walk_tg_tree(tg_load_down, tg_nop, (void *)cpu);
 }