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    SubjectRe: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention
    On 12/16, Peter Zijlstra wrote:
    >
    > It does the state and on_rq checks first, if we find on_rq,

    The problem is, somehow we should check both on_rq and state
    at the same time,

    > +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    > {
    > - int cpu, orig_cpu, this_cpu, success = 0;
    > + int cpu, load, ret = 0;
    > unsigned long flags;
    > - unsigned long en_flags = ENQUEUE_WAKEUP;
    > - struct rq *rq;
    >
    > - this_cpu = get_cpu();
    > + smp_mb();

    Yes, we need the full mb(). without subsequent spin_lock(), wmb()
    can't act as a smp_store_load_barrier() (which we don't have).

    > + if (p->se.on_rq && ttwu_force(p, state, wake_flags))
    > + return 1;

    ----- WINDOW -----

    > + for (;;) {
    > + unsigned int task_state = p->state;
    > +
    > + if (!(task_state & state))
    > + goto out;
    > +
    > + load = task_contributes_to_load(p);
    > +
    > + if (cmpxchg(&p->state, task_state, TASK_WAKING) == task_state)
    > + break;

    Suppose that we have a task T sleeping in TASK_INTERRUPTIBLE state,
    and this cpu does try_to_wake_up(TASK_INTERRUPTIBLE). on_rq == false.
    try_to_wake_up() starts the "for (;;)" loop.

    However, in the WINDOW above, it is possible that somebody else wakes
    it up, and then this task changes its state to TASK_INTERRUPTIBLE again.

    Then we set ->state = TASK_WAKING, but this (still running) T restores
    TASK_RUNNING after us.

    Oleg.



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