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    Subject[PATCH v2] sched: Accelerate "pick_next_entity" under special condition
    From: wangyun <wangyun@linux.vnet.ibm.com>

    In original code, we get the next entity in this way:

    if(condition1)
    result=value1;
    if(condition2)
    result=value2;
    if(condition3)
    result=value3;
    return result;

    So if condition3 is true, we will get value3, but still
    need to check condition1 and condition2, this will waste
    our time.

    This patch will change the way like:

    if(condition3) {
    result=value3;
    goto out;
    }
    if(condition2) {
    result=value2;
    goto out;
    }
    if(condition1) {
    result=value1;
    goto out;
    }

    out:
    return result;

    So we can avoid check condition2 and condition1 when
    condition3 is true now.

    v2:
    1. do not use ugly macro any more.
    2. add more description.

    Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
    ---
    kernel/sched/fair.c | 26 +++++++++++++++-----------
    1 files changed, 15 insertions(+), 11 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 84adb2d..e8a72b2 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -1308,29 +1308,33 @@ static struct sched_entity
    *pick_next_entity(struct cfs_rq *cfs_rq)
    struct sched_entity *left = se;

    /*
    - * Avoid running the skip buddy, if running something else can
    - * be done without getting too unfair.
    + * Someone really wants this to run. If it's not unfair, run it.
    */
    - if (cfs_rq->skip == se) {
    - struct sched_entity *second = __pick_next_entity(se);
    - if (second && wakeup_preempt_entity(second, left) < 1)
    - se = second;
    + if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
    + se = cfs_rq->next;
    + goto out;
    }

    /*
    * Prefer last buddy, try to return the CPU to a preempted task.
    */
    - if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
    + if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
    se = cfs_rq->last;
    + goto out;
    + }

    /*
    - * Someone really wants this to run. If it's not unfair, run it.
    + * Avoid running the skip buddy, if running something else can
    + * be done without getting too unfair.
    */
    - if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
    - se = cfs_rq->next;
    + if (cfs_rq->skip == se) {
    + struct sched_entity *second = __pick_next_entity(se);
    + if (second && wakeup_preempt_entity(second, left) < 1)
    + se = second;
    + }

    +out:
    clear_buddies(cfs_rq, se);
    -
    return se;
    }

    --
    1.7.4.1


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