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From: Greg KH on 30 Jul 2010 14:00
2.6.34-stable review patch. If anyone has any objections, please let us know.

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

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

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

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1675,9 +1675,6 @@ static void update_shares(struct sched_d

static void update_h_load(long cpu)
{
- if (root_task_group_empty())
- return;
-
walk_tg_tree(tg_load_down, tg_nop, (void *)cpu);
}



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