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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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