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

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Date:	Thu, 15 May 2014 17:34:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 v4

On 05/15, Peter Zijlstra wrote:
>
> So I suppose I'm failing to see the problem with something like:

Yeeees, I was thinking about something like this too ;)

> static inline void lock_page(struct page *page)
> {
> 	if (!trylock_page(page))
> 		__lock_page(page);
> }
>
> static inline void unlock_page(struct page *page)
> {
> 	clear_bit_unlock(&page->flags, PG_locked);
> 	if (PageWaiters(page))
> 		__unlock_page();
> }

but in this case we need mb() before PageWaiters(), I guess.

> void __lock_page(struct page *page)
> {
> 	struct wait_queue_head_t *wqh = page_waitqueue(page);
> 	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
>
> 	spin_lock_irq(&wqh->lock);
> 	if (!PageWaiters(page))
> 		SetPageWaiters(page);
>
> 	wait.flags |= WQ_FLAG_EXCLUSIVE;
> 	preempt_disable();

why?

> 	do {
> 		if (list_empty(&wait->task_list))
> 			__add_wait_queue_tail(wqh, &wait);
>
> 		set_current_state(TASK_UNINTERRUPTIBLE);
>
> 		if (test_bit(wait.key.bit_nr, wait.key.flags)) {
> 			spin_unlock_irq(&wqh->lock);
> 			schedule_preempt_disabled();
> 			spin_lock_irq(&wqh->lock);

OK, probably to avoid the preemption before schedule(). Still can't
undestand why this makes sense, but in this case it would be better
to do disable/enable under "if (test_bit())" ?

Of course, this needs more work for lock_page_killable(), but this
should be simple.

Oleg.

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