On Thu, May 22, 2014 at 05:04:51PM +0200, Peter Zijlstra wrote:
> On Thu, May 22, 2014 at 03:40:45PM +0100, Mel Gorman wrote:
>
> > > +static bool __wake_up_common(wait_queue_head_t *q, unsigned int mode,
> > > int nr_exclusive, int wake_flags, void *key)
> > > {
> > > wait_queue_t *curr, *next;
> > > + bool woke = false;
> > >
> > > list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
> > > unsigned flags = curr->flags;
> > >
> > > + if (curr->func(curr, mode, wake_flags, key)) {
> > > + woke = true;
> > > + if ((flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
> > > + break;
> > > + }
> > > }
> > > +
> > > + return woke;
> >
> > Ok, thinking about this more I'm less sure.
> >
> > There are cases where the curr->func returns false even though there is a
> > task that needs to run -- task was already running or preparing to run. We
> > potentially end up clearing PG_waiters while there are still tasks on the
> > waitqueue. As __finish_wait checks if the waitqueue is empty and the last
> > waiter clears the bit I think there is nothing to gain by trying to do the
> > same job in __wake_up_page_bit.
>
> Hmm, I think you're right, we need the test result from
> wake_bit_function(), unpolluted by the ttwu return value.
Which would be a bit too special cased and not a clear win. I at least
added a comment to explain what is going on here.
/*
* Unlike __wake_up_bit it is necessary to check waitqueue_active
* under the wqh->lock to avoid races with parallel additions that
* could result in lost wakeups.
*/
spin_lock_irqsave(&wqh->lock, flags);
if (waitqueue_active(wqh)) {
/*
* Try waking a task on the queue. Responsibility for clearing
* the PG_waiters bit is left to the last waiter on the
* waitqueue as PageWaiters is called outside wqh->lock and
* we cannot miss wakeups. Due to hashqueue collisions, there
* may be colliding pages that still have PG_waiters set but
* the impact means there will be at least one unnecessary
* lookup of the page waitqueue on the next unlock_page or
* end of writeback.
*/
__wake_up_common(wqh, TASK_NORMAL, 1, 0, &key);
} else {
/* No potential waiters, safe to clear PG_waiters */
ClearPageWaiters(page);
}
spin_unlock_irqrestore(&wqh->lock, flags);
--
Mel Gorman
SUSE Labs
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