linux-kernel - Re: [PATCH 17/17] mm: filemap: Avoid unnecessary barries and waitqueue lookup in unlock

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Date:	Mon, 5 May 2014 12:50:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.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>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 17/17] mm: filemap: Avoid unnecessary barries and
 waitqueue lookup in unlock_page fastpath

On Thu 01-05-14 09:44:48, Mel Gorman wrote:
> From: Nick Piggin <npiggin@...e.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?

> 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...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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