linux-kernel - Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock

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Date:	Wed, 21 May 2014 18:08:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue
 lookups in unlock_page fastpath v5

On Wed, May 21, 2014 at 04:33:57PM +0100, Mel Gorman wrote:
> > > +__prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait,
> > > +			struct page *page, int state, bool exclusive)
> > >  {
> > >  	unsigned long flags;
> > >  
> > > +	if (page && !PageWaiters(page))
> > > +		SetPageWaiters(page);
> > > +	if (list_empty(&wait->task_list)) {
> > > +		if (exclusive) {
> > > +			wait->flags |= WQ_FLAG_EXCLUSIVE;
> > > +			__add_wait_queue_tail(q, wait);
> > > +		} else {
> > 
> > I'm fairly sure we've just initialized the wait thing to 0, so clearing
> > the bit would be superfluous.
> > 
> 
> I assume you mean the clearing of WQ_FLAG_EXCLUSIVE. It may or may not be
> superflous. If it's an on-stack wait_queue_t initialised with DEFINE_WAIT()
> then it's redundant. If it's a wait_queue_t that is being reused and
> sometimes used for exclusive waits and other times for non-exclusive
> waits then it's required. The API allows this to happen so I see no harm
> is clearing the flag like the old code did. Am I missing your point?

Yeah, I'm not aware of any other users except the on-stack kind, but
you're right.

Maybe we should stick an object_is_on_stack() test in there to see if
anything falls out, something for a rainy afternoon perhaps..

> > > +void __wake_up_page_bit(wait_queue_head_t *wqh, struct page *page, void *word, int bit)
> > > +{
> > > +	struct wait_bit_key key = __WAIT_BIT_KEY_INITIALIZER(word, bit);
> > > +	unsigned long flags;
> > > +
> > > +	spin_lock_irqsave(&wqh->lock, flags);
> > > +	if (waitqueue_active(wqh))
> > > +		__wake_up_common(wqh, TASK_NORMAL, 1, 0, &key);
> > > +	else
> > > +		ClearPageWaiters(page);
> > > +	spin_unlock_irqrestore(&wqh->lock, flags);
> > > +}
> > 
> > Seeing how word is always going to be &page->flags, might it make sense
> > to remove that argument?
> > 
> 
> The wait_queue was defined on-stack with DEFINE_WAIT_BIT which uses
> wake_bit_function() as a wakeup function and that thing consumes both the
> page->flags and the bit number it's interested in. This is used for both
> PG_writeback and PG_locked so assumptions cannot really be made about
> the value.

Well, both PG_flags come from the same &page->flags word, right? But
yeah, if we ever decide to grow the page frame with another flags word
we'd be in trouble :-)

In any case I don't feel too strongly about either of these points.
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