linux-kernel - Re: [PATCH 2/4] mm: pagevec: Defer deciding what LRU to add a page to until pagevec drain time

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Date:	Wed, 15 May 2013 15:53:30 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Alexey Lyahkov <alexey.lyashkov@...il.com>,
	Andrew Perepechko <anserper@...ru>,
	Robin Dong <sanbai@...bao.com>, Theodore Tso <tytso@....edu>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Bernd Schubert <bernd.schubert@...tmail.fm>,
	David Howells <dhowells@...hat.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux-ext4 <linux-ext4@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 2/4] mm: pagevec: Defer deciding what LRU to add a page
 to until pagevec drain time

On Mon, 13 May 2013 11:21:20 +0100 Mel Gorman <mgorman@...e.de> wrote:

> mark_page_accessed cannot activate an inactive page that is located on
> an inactive LRU pagevec. Hints from filesystems may be ignored as a
> result. In preparation for fixing that problem, this patch removes the
> per-LRU pagevecs and leaves just one pagevec. The final LRU the page is
> added to is deferred until the pagevec is drained.
> 
> This means that fewer pagevecs are available and potentially there is
> greater contention on the LRU lock. However, this only applies in the case
> where there is an almost perfect mix of file, anon, active and inactive
> pages being added to the LRU. In practice I expect that we are adding
> stream of pages of a particular time and that the changes in contention
> will barely be measurable.
> 
> ...
>
> index c612a6a..0911579 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -39,7 +39,7 @@
>  /* How many pages do we try to swap or page in/out together? */
>  int page_cluster;
>  
> -static DEFINE_PER_CPU(struct pagevec[NR_LRU_LISTS], lru_add_pvecs);
> +static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
>  static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
>  static DEFINE_PER_CPU(struct pagevec, lru_deactivate_pvecs);
>  
> @@ -460,13 +460,18 @@ EXPORT_SYMBOL(mark_page_accessed);
>   */

The comment preceding __lru_cache_add() needs an update.

>  void __lru_cache_add(struct page *page, enum lru_list lru)
>  {
> -	struct pagevec *pvec = &get_cpu_var(lru_add_pvecs)[lru];
> +	struct pagevec *pvec = &get_cpu_var(lru_add_pvec);
> +
> +	if (is_active_lru(lru))
> +		SetPageActive(page);
> +	else
> +		ClearPageActive(page);

The whole use of `enum lru_list' here has always made my cry, but I'll weep
about that in [4/4].

>  	page_cache_get(page);
>  	if (!pagevec_space(pvec))
>  		__pagevec_lru_add(pvec, lru);
>  	pagevec_add(pvec, page);
> -	put_cpu_var(lru_add_pvecs);
> +	put_cpu_var(lru_add_pvec);
>  }
>
> ...
>
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