Re: [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue lookups in unlock_page fastpath v7

[Peter Zijlstra]

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.

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