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    Subject[tip:sched/urgent] sched: Move update_curr() in check_preempt_wakeup() to avoid redundant call
    Commit-ID:  a65ac745e47e91f9d98dbf07f22ed0492e34d998
    Gitweb: http://git.kernel.org/tip/a65ac745e47e91f9d98dbf07f22ed0492e34d998
    Author: Jupyung Lee <jupyung@gmail.com>
    AuthorDate: Tue, 17 Nov 2009 18:51:40 +0900
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Wed, 9 Dec 2009 10:03:06 +0100

    sched: Move update_curr() in check_preempt_wakeup() to avoid redundant call

    If a RT task is woken up while a non-RT task is running,
    check_preempt_wakeup() is called to check whether the new task can
    preempt the old task. The function returns quickly without going deeper
    because it is apparent that a RT task can always preempt a non-RT task.

    In this situation, check_preempt_wakeup() always calls update_curr() to
    update vruntime value of the currently running task. However, the
    function call is unnecessary and redundant at that moment because (1) a
    non-RT task can always be preempted by a RT task regardless of its
    vruntime value, and (2) update_curr() will be called shortly when the
    context switch between two occurs.

    By moving update_curr() in check_preempt_wakeup(), we can avoid
    redundant call to update_curr(), slightly reducing the time taken to
    wake up RT tasks.

    Signed-off-by: Jupyung Lee <jupyung@gmail.com>
    [ Place update_curr() right before the wake_preempt_entity() call, which
    is the only thing that relies on the updated vruntime ]
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <1258451500-6714-1-git-send-email-jupyung@gmail.com>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/sched_fair.c | 4 ++--
    1 files changed, 2 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
    index 44ec80c..4dec185 100644
    --- a/kernel/sched_fair.c
    +++ b/kernel/sched_fair.c
    @@ -1651,8 +1651,6 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
    int sync = wake_flags & WF_SYNC;
    int scale = cfs_rq->nr_running >= sched_nr_latency;

    - update_curr(cfs_rq);
    -
    if (unlikely(rt_prio(p->prio))) {
    resched_task(curr);
    return;
    @@ -1710,6 +1708,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_

    BUG_ON(!pse);

    + update_curr(cfs_rq);
    +
    if (wakeup_preempt_entity(se, pse) == 1) {
    resched_task(curr);
    /*

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