Re: [RFC PATCH 1/3 V1] mm, page_alloc: use __rmqueue_smallest whenborrow memory from MIGRATE_CMA

[Mel Gorman]

On Wed, Jul 04, 2012 at 03:26:16PM +0800, Lai Jiangshan wrote:
> The pages of MIGRATE_CMA can't not be changed to the other type,
> nor be moved to the other free list. 
> 
> ==>
> So when we use __rmqueue_fallback() to borrow memory from MIGRATE_CMA,
> one of the highest order page is borrowed and it is split.
> But the free pages resulted by splitting can NOT
> be moved to MIGRATE_MOVABLE.
> 
> ==>
> So in the next time of allocation, we NEED to borrow again,
> another one of the highest order page is borrowed from CMA and it is split.
> and results some other new split free pages.
> 

Then special case __rmqueue_fallback() to move pages stolen from
MIGRATE_CMA to the MIGRATE_MOVABLE lists but do not change the pageblock
type.

> ==>
> So when __rmqueue_fallback() borrows (highest order)memory from MIGRATE_CMA,
> it introduces fragments at the same time and may waste tlb(only one page is used in
> a pageblock).
> 
> Conclusion:
> We should borrows the smallest order memory from MIGRATE_CMA in such case
> 

That's excessive and unnecessary special casing. Just move the
MIGRATE_CMA pages to the MIGRATE_MOVABLE lists.

> Result(good):
> 1) use __rmqueue_smallest when borrow memory from MIGRATE_CMA
> 2) __rmqueue_fallback() don't handle CMA, it becomes much simpler
> Result(bad):
> __rmqueue_smallest() can't not be inlined to avoid function call overhead.
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> ---
>  include/linux/mmzone.h |    1 +
>  mm/page_alloc.c        |   63 ++++++++++++++++--------------------------------
>  2 files changed, 22 insertions(+), 42 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index bf3404e..979c333 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -40,6 +40,7 @@ enum {
>  	MIGRATE_RECLAIMABLE,
>  	MIGRATE_MOVABLE,
>  	MIGRATE_PCPTYPES,	/* the number of types on the pcp lists */
> +	MIGRATE_PRIME_TYPES = MIGRATE_PCPTYPES,

No explanation why this new name is necessary.

>  	MIGRATE_RESERVE = MIGRATE_PCPTYPES,
>  #ifdef CONFIG_CMA
>  	/*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 476ae3e..efc327f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -893,17 +893,10 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>   * This array describes the order lists are fallen back to when
>   * the free lists for the desirable migrate type are depleted
>   */
> -static int fallbacks[MIGRATE_TYPES][4] = {
> -	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,     MIGRATE_RESERVE },
> -	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,     MIGRATE_RESERVE },
> -#ifdef CONFIG_CMA
> -	[MIGRATE_MOVABLE]     = { MIGRATE_CMA,         MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
> -	[MIGRATE_CMA]         = { MIGRATE_RESERVE }, /* Never used */
> -#else
> -	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE,   MIGRATE_RESERVE },
> -#endif
> -	[MIGRATE_RESERVE]     = { MIGRATE_RESERVE }, /* Never used */
> -	[MIGRATE_ISOLATE]     = { MIGRATE_RESERVE }, /* Never used */
> +static int fallbacks[MIGRATE_PRIME_TYPES][2] = {
> +	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE   },
> +	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE   },
> +	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE },
>  };
>  

And it's completely unclear why it was necessary to rip out the existing
fallback lists. It reworks how fallback lists work for no clear benefit.

>  /*
> @@ -995,16 +988,15 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>  	struct page *page;
>  	int migratetype, i;
>  
> +	if (WARN_ON_ONCE(start_migratetype >= MIGRATE_PRIME_TYPES))
> +		start_migratetype = MIGRATE_UNMOVABLE;
> +

This should be completely unnecessary. If this warning is hit, the
callers are severely broken.

>  	/* Find the largest possible block of pages in the other list */
>  	for (current_order = MAX_ORDER-1; current_order >= order;
>  						--current_order) {
> -		for (i = 0;; i++) {
> +		for (i = 0; i < ARRAY_SIZE(fallbacks[0]); i++) {
>  			migratetype = fallbacks[start_migratetype][i];
>  
> -			/* MIGRATE_RESERVE handled later if necessary */
> -			if (migratetype == MIGRATE_RESERVE)
> -				break;
> -
>  			area = &(zone->free_area[current_order]);
>  			if (list_empty(&area->free_list[migratetype]))
>  				continue;
> @@ -1018,17 +1010,10 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>  			 * pages to the preferred allocation list. If falling
>  			 * back for a reclaimable kernel allocation, be more
>  			 * aggressive about taking ownership of free pages
> -			 *
> -			 * On the other hand, never change migration
> -			 * type of MIGRATE_CMA pageblocks nor move CMA
> -			 * pages on different free lists. We don't
> -			 * want unmovable pages to be allocated from
> -			 * MIGRATE_CMA areas.
>  			 */
> -			if (!is_migrate_cma(migratetype) &&
> -			    (unlikely(current_order >= pageblock_order / 2) ||
> -			     start_migratetype == MIGRATE_RECLAIMABLE ||
> -			     page_group_by_mobility_disabled)) {
> +			if (unlikely(current_order >= pageblock_order / 2) ||
> +			    start_migratetype == MIGRATE_RECLAIMABLE ||
> +			    page_group_by_mobility_disabled) {
>  				int pages;
>  				pages = move_freepages_block(zone, page,
>  								start_migratetype);
> @@ -1047,14 +1032,12 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>  			rmv_page_order(page);
>  
>  			/* Take ownership for orders >= pageblock_order */
> -			if (current_order >= pageblock_order &&
> -			    !is_migrate_cma(migratetype))
> +			if (current_order >= pageblock_order)
>  				change_pageblock_range(page, current_order,
>  							start_migratetype);
>  
>  			expand(zone, page, order, current_order, area,
> -			       is_migrate_cma(migratetype)
> -			     ? migratetype : start_migratetype);
> +			       start_migratetype);
>  
>  			trace_mm_page_alloc_extfrag(page, order, current_order,
>  				start_migratetype, migratetype);
> @@ -1075,22 +1058,18 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
>  {
>  	struct page *page;
>  
> -retry_reserve:
>  	page = __rmqueue_smallest(zone, order, migratetype);
>  
> -	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
> +#ifdef CONFIG_CMA
> +	if (unlikely(!page) && migratetype == MIGRATE_MOVABLE)
> +		page = __rmqueue_smallest(zone, order, MIGRATE_CMA);
> +#endif
> +
> +	if (unlikely(!page))
>  		page = __rmqueue_fallback(zone, order, migratetype);
>  
> -		/*
> -		 * Use MIGRATE_RESERVE rather than fail an allocation. goto
> -		 * is used because __rmqueue_smallest is an inline function
> -		 * and we want just one call site
> -		 */
> -		if (!page) {
> -			migratetype = MIGRATE_RESERVE;
> -			goto retry_reserve;
> -		}
> -	}
> +	if (unlikely(!page))
> +		page = __rmqueue_smallest(zone, order, MIGRATE_RESERVE);
>  
>  	trace_mm_page_alloc_zone_locked(page, order, migratetype);
>  	return page;

I didn't read this closely. It seems way more complex than necessary to
solve the problem described in the changelog. All you should need is to
special case that a __GFP_MOVABLE allocation using MIGRATE_CMA should
place the unused buddies on the MIGRATE_MOVABLE lists.

-- 
Mel Gorman
SUSE Labs
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