Re: [PATCH 17/17] mm: filemap: Avoid unnecessary barries and waitqueue lookup in unlock_page fastpath

[Mel Gorman <mgorman@suse.de>]

On Mon, May 05, 2014 at 12:50:54PM +0200, Jan Kara wrote:
> On Thu 01-05-14 09:44:48, Mel Gorman wrote:
> > From: Nick Piggin <npiggin@suse.de>
> > 
> > This patch introduces a new page flag for 64-bit capable machines,
> > PG_waiters, to signal there are processes waiting on PG_lock and uses it to
> > avoid memory barriers and waitqueue hash lookup in the unlock_page fastpath.
> > 
> > This adds a few branches to the fast path but avoids bouncing a dirty
> > cache line between CPUs. 32-bit machines always take the slow path but the
> > primary motivation for this patch is large machines so I do not think that
> > is a concern.
> ...
> >  /* 
> > diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> > index 7d50f79..fb83fe0 100644
> > --- a/kernel/sched/wait.c
> > +++ b/kernel/sched/wait.c
> > @@ -304,8 +304,7 @@ int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *arg)
> >  		= container_of(wait, struct wait_bit_queue, wait);
> >  
> >  	if (wait_bit->key.flags != key->flags ||
> > -			wait_bit->key.bit_nr != key->bit_nr ||
> > -			test_bit(key->bit_nr, key->flags))
> > +			wait_bit->key.bit_nr != key->bit_nr)
> >  		return 0;
> >  	else
> >  		return autoremove_wake_function(wait, mode, sync, key);
>   This change seems to be really unrelated? And it would deserve a comment
> on its own I'd think so maybe split that in a separate patch?
> 

Without it processes can sleep forever on the lock bit and hang due to
races between when the PG_waiters is set and cleared. I'll investigate
if this can be done a better way.

> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index c60ed0f..93e4385 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > +int  __wait_on_page_locked_killable(struct page *page)
> > +{
> > +	int ret = 0;
> > +	wait_queue_head_t *wq = page_waitqueue(page);
> > +	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
> > +
> > +	if (!test_bit(PG_locked, &page->flags))
> > +		return 0;
> > +	do {
> > +		prepare_to_wait(wq, &wait.wait, TASK_KILLABLE);
> > +		if (!PageWaiters(page))
> > +			SetPageWaiters(page);
> > +		if (likely(PageLocked(page)))
> > +			ret = sleep_on_page_killable(page);
> > +		finish_wait(wq, &wait.wait);
> > +	} while (PageLocked(page) && !ret);
>   So I'm somewhat wondering why this is the only page waiting variant that
> does finish_wait() inside the loop. Everyone else does it outside the while
> loop which seems sufficient to me even in this case...
> 

No reason. The finish_wait can always be outside. I'll fix it up.
Thanks.

-- 
Mel Gorman
SUSE Labs