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

Peter Zijlstra
2009-01-07T06:32:50Z

On Wed, 2009-01-07 at 11:57 +0800, Lai Jiangshan wrote:
> 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?

cpu_relax() is a compiler barrier iirc.

> 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.

I'm not sure what you're saying here..