Re: [PATCH][RFC]: mutex: adaptive spin

Lai Jiangshan
2009-01-07T04:01:10Z

Peter Zijlstra wrote:
> +void mutex_spin_or_schedule(struct mutex_waiter *waiter, long state, unsigned long *flags)
> +{
> + struct mutex *lock = waiter->lock;
> + struct task_struct *task = waiter->task;
> + struct task_struct *owner = lock->owner;
> + struct rq *rq;
> +
> + if (!owner)
> + goto do_schedule;
> +
> + rq = task_rq(owner);
> +
> + if (rq->curr != owner) {
> +do_schedule:
> + __set_task_state(task, state);
> + spin_unlock_mutex(&lock->wait_lock, *flags);
> + schedule();
> + } else {
> + spin_unlock_mutex(&lock->wait_lock, *flags);
> + for (;;) {
> + /* Stop spinning when there's a pending signal. */
> + if (signal_pending_state(state, task))
> + break;
> +
> + /* Owner changed, bail to revalidate state */
> + if (lock->owner != owner)
> + break;
> +
> + /* Owner stopped running, bail to revalidate state */
> + if (rq->curr != owner)
> + break;
> +

2 questions from my immature thought:

1) Do we need keep gcc from optimizing when we access lock->owner
and rq->curr in the loop?

2) "if (rq->curr != owner)" need become smarter.
schedule()
{
select_next
rq->curr = next;
contex_swith
}
we also spin when owner is select_next-ing in schedule().
but select_next is not fast enough.

Lai.