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    SubjectRe: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention
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    On Thu, 2010-12-16 at 19:42 +0100, Oleg Nesterov wrote:
    > On 12/16, Peter Zijlstra wrote:
    > >
    > > +static int ttwu_force(struct task_struct *p, int wake_flags)
    > > +{
    > > + struct rq *rq;
    > > + int ret = 0;
    > > +
    > > + /*
    > > + * Since we've already set TASK_WAKING this task's CPU cannot
    > > + * change from under us.
    >
    > I think it can. Yes, we've set TASK_WAKING. But, at least the task
    > itself can change its state back to TASK_RUNNING without calling
    > schedule. Say, __wait_event()-like code.

    Oh crud, you're right, that's going to make all this cmpxchg stuff lots
    more interesting :/

    > > +static int
    > > +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    > > {
    > > - int cpu, orig_cpu, this_cpu, success = 0;
    > > + int cpu = task_cpu(p);
    > > unsigned long flags;
    > > - unsigned long en_flags = ENQUEUE_WAKEUP;
    > > - struct rq *rq;
    > > + int success = 0;
    > > + int load;
    > >
    > > - this_cpu = get_cpu();
    > > -
    > > - smp_wmb();
    > > - rq = task_rq_lock(p, &flags);
    > > - if (!(p->state & state))
    > > - goto out;
    > > + local_irq_save(flags);
    > > + for (;;) {
    > > + unsigned int task_state = p->state;
    > >
    > > - cpu = task_cpu(p);
    > > + if (!(task_state & state))
    > > + goto out;
    >
    > Well, this surely breaks the code like
    >
    > CONDITION = true;
    > wake_up_process(p);
    >
    > At least we need mb() before we check task_state the first time.

    You're right (wmb, at least), I left that out because I had the cmpxchg
    in there that provides a mb, but didn't notice I read the state before
    that.. /me goes put the smp_wmb() back.


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