Messages in this thread | | | Date | Fri, 17 Dec 2010 17:54:14 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention |
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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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