Re: [PATCH] mm: add free_hot_cold_page_list helper

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 29 Jul 2011 11:58:37 +0400
Konstantin Khlebnikov <khlebnikov@openvz.org> wrote:

> This patch adds helper free_hot_cold_page_list() to free list of 0-order pages.
> It frees pages directly from list without temporary page-vector.
> It also calls trace_mm_pagevec_free() to simulate pagevec_free() behaviour.
> 
> bloat-o-meter:
> 
> add/remove: 1/1 grow/shrink: 1/3 up/down: 267/-295 (-28)
> function                                     old     new   delta
> free_hot_cold_page_list                        -     264    +264
> get_page_from_freelist                      2129    2132      +3
> __pagevec_free                               243     239      -4
> split_free_page                              380     373      -7
> release_pages                                606     510     -96
> free_page_list                               188       -    -188
> 

It saves a total of 150 bytes for me.

> index cb40892..dd7b9cc 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -358,6 +358,7 @@ void *alloc_pages_exact_nid(int nid, size_t size, gfp_t gfp_mask);
>  extern void __free_pages(struct page *page, unsigned int order);
>  extern void free_pages(unsigned long addr, unsigned int order);
>  extern void free_hot_cold_page(struct page *page, int cold);
> +extern void free_hot_cold_page_list(struct list_head *list, int cold);
>  
>  #define __free_page(page) __free_pages((page), 0)
>  #define free_page(addr) free_pages((addr), 0)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1dbcf88..af486e4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1209,6 +1209,18 @@ out:
>  	local_irq_restore(flags);
>  }
>  
> +void free_hot_cold_page_list(struct list_head *list, int cold)
> +{
> +	struct page *page, *next;
> +
> +	list_for_each_entry_safe(page, next, list, lru) {
> +		trace_mm_pagevec_free(page, cold);
> +		free_hot_cold_page(page, cold);
> +	}
> +
> +	INIT_LIST_HEAD(list);
> +}
> +
>  /*
>   * split_page takes a non-compound higher-order page, and splits it into
>   * n (1<<order) sub-pages: page[0..n]
> diff --git a/mm/swap.c b/mm/swap.c
> index 3a442f1..b9138c7 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -562,11 +562,10 @@ int lru_add_drain_all(void)
>  void release_pages(struct page **pages, int nr, int cold)
>  {
>  	int i;
> -	struct pagevec pages_to_free;
> +	LIST_HEAD(pages_to_free);
>  	struct zone *zone = NULL;
>  	unsigned long uninitialized_var(flags);
>  
> -	pagevec_init(&pages_to_free, cold);
>  	for (i = 0; i < nr; i++) {
>  		struct page *page = pages[i];
>  
> @@ -597,19 +596,12 @@ void release_pages(struct page **pages, int nr, int cold)
>  			del_page_from_lru(zone, page);
>  		}
>  
> -		if (!pagevec_add(&pages_to_free, page)) {
> -			if (zone) {
> -				spin_unlock_irqrestore(&zone->lru_lock, flags);
> -				zone = NULL;
> -			}
> -			__pagevec_free(&pages_to_free);
> -			pagevec_reinit(&pages_to_free);
> -  		}
> +		list_add_tail(&page->lru, &pages_to_free);

There's a potential problem here with cache longevity.  If
release_pages() is called with a large number of pages then the current
code's approach of freeing pages 16-at-a-time will hopefully cause
those pageframes to still be in CPU cache when we get to actually
freeing them.

But after this change, we free all the pages in a single operation
right at the end, which adds risk that we'll have to reload all their
pageframes into CPU cache again.

That'll only be a problem if release_pages() _is_ called with a large
number of pages.  And manipulating large numbers of pages represents a
lot of work, so the additional work from one cachemiss per page will
presumably be tiny.


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