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    SubjectRe: [PATCH 1/2] sched: push rt tasks only if newly activated tasks have been added
    Hi Gregory,

    consider the following 2-cpu system: cpu0 and cpu1.

    cpu0: is idle --> in such a state, it never pulls RT tasks on its own.

    T0 and T1 are RT tasks


    cpu1: T0 is running

    T1 is of the same prio as T0 (shouldn't really matter but to get the
    same result it would require altering the flow of events slightly)

    T1's affinity allows it to be run only on cpu1.
    T0 can run on both.

    try_to_wake_up() is called for T1.
    --> select_task_rq_rt() => gives cpu1
    --> task_wake_up_rt()
    ---> push_rt_tasks() -> rq->rt.pushed = 1

    now, neither T1 (due to its affinity), nor T0 (it's running) can be
    pushed away to cpu0.

    [ btw., (1) I'd expect that this task_wake_up_rt() thing should be
    redundant, logically-wise... I'll check once more and comment later
    (2) any example when (p->prio >= rq->rt.highest_prio) is not true in
    task_wake_up_rt() ?

    as a result, rq->rt.pushed == 1.

    Now, post_schedule_rt() won't call push_rt_tasks().

    T0 and T1 are both running for some time on cpu1 (possibly
    context-switching if they are both of SCHED_RR type).

    Then they both block, _first_ T1 and then T0.

    After some interval of time, they wake up (let's say they are
    periodic) in the following order: _first_ T0 and then T1.

    rq->rt.pushed becomes 0 and here we are back to square#0. The whole
    story repeats again.

    cpu0 is idle so it won't pull T0. Both T0 and T1 are competing for the
    same cpu. Not good.

    am I missing smth?

    Best regards,
    Dmitry Adamushko

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