sched: fix mysql+oltp regression

Defer commit 6d299f1b53b84e2665f402d9bcc494800aba6386 to the next release.

Testing of the tip/sched/clock tree revealed a mysql+oltp regression
which bisection eventually traced back to this commit in mainline.

Pertinent test results:  Three run sysbench averages, throughput units
in read/write requests/sec.

clients         1     2     4     8    16    32    64
6e0534f      9646 17876 34774 33868 32230 30767 29441
2.6.26.1     9112 17936 34652 33383 31929 30665 29232
6d299f1      9112 14637 28370 33339 32038 30762 29204

Note: subsequent commits hide the majority of this regression until you
apply the clock fixes, at which time it reemerges at full magnitude.

We cannot see anything bad about the change itself so we defer it to the
next release until this problem is fully analysed.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 12 insertions, 7 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 0fe94ea43f32..fb8994c6d4bb 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1442,18 +1442,23 @@ __load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
         struct task_struct *p = NULL;
         struct sched_entity *se;
 
-        while (next != &cfs_rq->tasks) {
+        if (next == &cfs_rq->tasks)
+                return NULL;
+
+        /* Skip over entities that are not tasks */
+        do {
                 se = list_entry(next, struct sched_entity, group_node);
                 next = next->next;
+        } while (next != &cfs_rq->tasks && !entity_is_task(se));
 
-                /* Skip over entities that are not tasks */
-                if (entity_is_task(se)) {
-                        p = task_of(se);
-                        break;
-                }
-        }
+        if (next == &cfs_rq->tasks)
+                return NULL;
 
         cfs_rq->balance_iterator = next;
+
+        if (entity_is_task(se))
+                p = task_of(se);
+
         return p;
 }