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Subject[PATCH] sched: Fix __load_balance_iterator() for cfq with only one task
sched: Fix __load_balance_iterator() for cfq with only one task.

From: Gautham R Shenoy <ego@in.ibm.com>

The __load_balance_iterator() returns a NULL when there's only one
sched_entity which is a task. It is caused by the following code-path.


/* 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));
if (next == &cfs_rq->tasks)
return NULL;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This will return NULL even when se is a task.
As a side-effect, there was a regression in sched_mc behavior since 2.6.25,
since iter_move_one_task() when it calls load_balance_start_fair(),
would not get any tasks to move!

Fix this by checking if the last entity was a task or not.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
---
kernel/sched_fair.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index fb8994c..f1c96e3 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1451,7 +1451,7 @@ __load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
next = next->next;
} while (next != &cfs_rq->tasks && !entity_is_task(se));

- if (next == &cfs_rq->tasks)
+ if (next == &cfs_rq->tasks && !entity_is_task(se))
return NULL;

cfs_rq->balance_iterator = next;
--
Thanks and Regards
gautham

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