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    Subject[PATCH -v8 2/7] sched: limit the scope of clear_buddies
    The clear_buddies function does not seem to play well with the concept
    of hierarchical runqueues. In the following tree, task groups are
    represented by 'G', tasks by 'T', next by 'n' and last by 'l'.

    (nl)
    / \
    G(nl) G
    / \ \
    T(l) T(n) T

    This situation can arise when a task is woken up T(n), and the previously
    running task T(l) is marked last.

    When clear_buddies is called from either T(l) or T(n), the next and last
    buddies of the group G(nl) will be cleared. This is not the desired
    result, since we would like to be able to find the other type of buddy
    in many cases.

    This especially a worry when implementing yield_task_fair through the
    buddy system.

    The fix is simple: only clear the buddy type that the task itself
    is indicated to be. As an added bonus, we stop walking up the tree
    when the buddy has already been cleared or pointed elsewhere.

    Signed-off-by: Rik van Riel <riel@redhat.coM>
    ---
    kernel/sched_fair.c | 30 +++++++++++++++++++++++-------
    1 files changed, 23 insertions(+), 7 deletions(-)
    diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
    index f4ee445..0321473 100644
    --- a/kernel/sched_fair.c
    +++ b/kernel/sched_fair.c
    @@ -784,19 +784,35 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
    __enqueue_entity(cfs_rq, se);
    }

    -static void __clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
    +static void __clear_buddies_last(struct sched_entity *se)
    {
    - if (!se || cfs_rq->last == se)
    - cfs_rq->last = NULL;
    + for_each_sched_entity(se) {
    + struct cfs_rq *cfs_rq = cfs_rq_of(se);
    + if (cfs_rq->last == se)
    + cfs_rq->last = NULL;
    + else
    + break;
    + }
    +}

    - if (!se || cfs_rq->next == se)
    - cfs_rq->next = NULL;
    +static void __clear_buddies_next(struct sched_entity *se)
    +{
    + for_each_sched_entity(se) {
    + struct cfs_rq *cfs_rq = cfs_rq_of(se);
    + if (cfs_rq->next == se)
    + cfs_rq->next = NULL;
    + else
    + break;
    + }
    }

    static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
    {
    - for_each_sched_entity(se)
    - __clear_buddies(cfs_rq_of(se), se);
    + if (cfs_rq->last == se)
    + __clear_buddies_last(se);
    +
    + if (cfs_rq->next == se)
    + __clear_buddies_next(se);
    }

    static void


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