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    SubjectRe: [PATCH] sched: fix rt_nr_migratory corruption raised in push_rt_task()
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    On Mon, 2011-05-30 at 21:39 +0800, Hillf Danton wrote:
    > When pushing, if a pushable task could not be pushed, it is dequeued with no
    > updating the rt_nr_migratory element of RT run-queue, then rt_nr_migratory is
    > corrupted, which is fixed by removing the dequeue operation.

    Hm. I think you're right that this dequeue_pushable_task() call can be
    removed, at least I didn't see it's reason for existing. I'd word it a
    bit differently though, and not return 1 unless we really did push.

    From: Hillf Danton <dhillf@gmail.com>

    sched, rt: fix rq->rt.pushable_tasks bug in push_rt_task()

    Do not call dequeue_pushable_task() when failing to push an eligible
    task, as it remains pushable, merely not at this particular moment.

    Signed-off-by: Hillf Danton <dhillf@gmail.com>
    ---
    kernel/sched_rt.c | 15 ++++++++-------
    1 file changed, 8 insertions(+), 7 deletions(-)

    Index: linux-2.6.40.git/kernel/sched_rt.c
    ===================================================================
    --- linux-2.6.40.git.orig/kernel/sched_rt.c
    +++ linux-2.6.40.git/kernel/sched_rt.c
    @@ -1378,6 +1378,7 @@ static int push_rt_task(struct rq *rq)
    {
    struct task_struct *next_task;
    struct rq *lowest_rq;
    + int ret = 0;

    if (!rq->rt.overloaded)
    return 0;
    @@ -1410,7 +1411,7 @@ retry:
    if (!lowest_rq) {
    struct task_struct *task;
    /*
    - * find lock_lowest_rq releases rq->lock
    + * find_lock_lowest_rq releases rq->lock
    * so it is possible that next_task has migrated.
    *
    * We need to make sure that the task is still on the same
    @@ -1420,12 +1421,11 @@ retry:
    task = pick_next_pushable_task(rq);
    if (task_cpu(next_task) == rq->cpu && task == next_task) {
    /*
    - * If we get here, the task hasn't moved at all, but
    - * it has failed to push. We will not try again,
    - * since the other cpus will pull from us when they
    - * are ready.
    + * The task hasn't migrated, and is still the next
    + * eligible task, but we failed to find a run-queue
    + * to push it to. Do not retry in this case, since
    + * other cpus will pull from us when ready.
    */
    - dequeue_pushable_task(rq, next_task);
    goto out;
    }

    @@ -1444,6 +1444,7 @@ retry:
    deactivate_task(rq, next_task, 0);
    set_task_cpu(next_task, lowest_rq->cpu);
    activate_task(lowest_rq, next_task, 0);
    + ret = 1;

    resched_task(lowest_rq->curr);

    @@ -1452,7 +1453,7 @@ retry:
    out:
    put_task_struct(next_task);

    - return 1;
    + return ret;
    }

    static void push_rt_tasks(struct rq *rq)



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