Re: [PATCH 1/3] mm: pagevec: Defer deciding what LRU to add a page to until pagevec drain time

[Jan Kara <jack@suse.cz>]

On Mon 29-04-13 17:31:57, Mel Gorman 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.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  mm/swap.c | 37 +++++++++++++++++--------------------
>  1 file changed, 17 insertions(+), 20 deletions(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 8a529a0..80fbc37 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -36,7 +36,7 @@
>  /* 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);
>  
> @@ -456,13 +456,18 @@ EXPORT_SYMBOL(mark_page_accessed);
>   */
>  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);
>  
>  	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);
>  }
>  EXPORT_SYMBOL(__lru_cache_add);
>  
> @@ -475,13 +480,11 @@ void lru_cache_add_lru(struct page *page, enum lru_list lru)
>  {
>  	if (PageActive(page)) {
>  		VM_BUG_ON(PageUnevictable(page));
> -		ClearPageActive(page);
>  	} else if (PageUnevictable(page)) {
>  		VM_BUG_ON(PageActive(page));
> -		ClearPageUnevictable(page);
>  	}
>  
> -	VM_BUG_ON(PageLRU(page) || PageActive(page) || PageUnevictable(page));
> +	VM_BUG_ON(PageLRU(page));
>  	__lru_cache_add(page, lru);
>  }
>  
> @@ -582,15 +585,10 @@ static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec,
>   */
>  void lru_add_drain_cpu(int cpu)
>  {
> -	struct pagevec *pvecs = per_cpu(lru_add_pvecs, cpu);
> -	struct pagevec *pvec;
> -	int lru;
> +	struct pagevec *pvec = &per_cpu(lru_add_pvec, cpu);
>  
> -	for_each_lru(lru) {
> -		pvec = &pvecs[lru - LRU_BASE];
> -		if (pagevec_count(pvec))
> -			__pagevec_lru_add(pvec, lru);
> -	}
> +	if (pagevec_count(pvec))
> +		__pagevec_lru_add(pvec, NR_LRU_LISTS);
>  
>  	pvec = &per_cpu(lru_rotate_pvecs, cpu);
>  	if (pagevec_count(pvec)) {
> @@ -789,17 +787,16 @@ void lru_add_page_tail(struct page *page, struct page *page_tail,
>  static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
>  				 void *arg)
>  {
> -	enum lru_list lru = (enum lru_list)arg;
> -	int file = is_file_lru(lru);
> -	int active = is_active_lru(lru);
> +	enum lru_list requested_lru = (enum lru_list)arg;
> +	int file = page_is_file_cache(page);
> +	int active = PageActive(page);
> +	enum lru_list lru = page_lru(page);
>  
> -	VM_BUG_ON(PageActive(page));
> +	WARN_ON_ONCE(requested_lru < NR_LRU_LISTS && requested_lru != lru);
  Hum, so __lru_cache_add() calls this with 'requested_lru' set to whatever
LRU we currently want to add a page. How should this always be equal to the
LRU of all the pages we have cached in the pagevec?

And if I'm right, there doesn't seem to be a reason to pass requested_lru
to this function at all, does it?

>  	VM_BUG_ON(PageUnevictable(page));
>  	VM_BUG_ON(PageLRU(page));
>  
>  	SetPageLRU(page);
> -	if (active)
> -		SetPageActive(page);
>  	add_page_to_lru_list(page, lruvec, lru);
>  	update_page_reclaim_stat(lruvec, file, active);
>  }
> -- 
> 1.8.1.4
> 
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-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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