Re: [PATCH v2 4/4] mm/compaction: enhance compaction finish condition

[Vlastimil Babka]

On 01/30/2015 01:34 PM, Joonsoo Kim wrote:
> From: Joonsoo <iamjoonsoo.kim@xxxxxxx>
> 
> Compaction has anti fragmentation algorithm. It is that freepage
> should be more than pageblock order to finish the compaction if we don't
> find any freepage in requested migratetype buddy list. This is for
> mitigating fragmentation, but, there is a lack of migratetype
> consideration and it is too excessive compared to page allocator's anti
> fragmentation algorithm.
> 
> Not considering migratetype would cause premature finish of compaction.
> For example, if allocation request is for unmovable migratetype,
> freepage with CMA migratetype doesn't help that allocation and
> compaction should not be stopped. But, current logic regards this
> situation as compaction is no longer needed, so finish the compaction.
> 
> Secondly, condition is too excessive compared to page allocator's logic.
> We can steal freepage from other migratetype and change pageblock
> migratetype on more relaxed conditions in page allocator. This is designed
> to prevent fragmentation and we can use it here. Imposing hard constraint
> only to the compaction doesn't help much in this case since page allocator
> would cause fragmentation again.
> 
> To solve these problems, this patch borrows anti fragmentation logic from
> page allocator. It will reduce premature compaction finish in some cases
> and reduce excessive compaction work.
> 
> stress-highalloc test in mmtests with non movable order 7 allocation shows
> considerable increase of compaction success rate.
> 
> Compaction success rate (Compaction success * 100 / Compaction stalls, %)
> 31.82 : 42.20
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

I have some worries about longer-term fragmentation, some testing of
stress-highalloc several times without restarts could be helpful.

> ---
>  include/linux/mmzone.h |  3 +++
>  mm/compaction.c        | 30 ++++++++++++++++++++++++++++--
>  mm/internal.h          |  1 +
>  mm/page_alloc.c        |  5 ++---
>  4 files changed, 34 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index f279d9c..a2906bc 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -63,6 +63,9 @@ enum {
>  	MIGRATE_TYPES
>  };
>  
> +#define FALLBACK_MIGRATETYPES (4)
> +extern int fallbacks[MIGRATE_TYPES][FALLBACK_MIGRATETYPES];
> +
>  #ifdef CONFIG_CMA
>  #  define is_migrate_cma(migratetype) unlikely((migratetype) == MIGRATE_CMA)
>  #else
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 782772d..0460e4b 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1125,6 +1125,29 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
>  	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
>  }
>  
> +static bool can_steal_fallbacks(struct free_area *area,
> +			unsigned int order, int migratetype)

Could you move this to page_alloc.c and then you don't have to export the
fallbacks arrays?

> +{
> +	int i;
> +	int fallback_mt;
> +
> +	if (area->nr_free == 0)
> +		return false;
> +
> +	for (i = 0; i < FALLBACK_MIGRATETYPES; i++) {
> +		fallback_mt = fallbacks[migratetype][i];
> +		if (fallback_mt == MIGRATE_RESERVE)
> +			break;
> +
> +		if (list_empty(&area->free_list[fallback_mt]))
> +			continue;
> +
> +		if (can_steal_freepages(order, migratetype, fallback_mt))
> +			return true;
> +	}
> +	return false;
> +}
> +
>  static int __compact_finished(struct zone *zone, struct compact_control *cc,
>  			    const int migratetype)
>  {
> @@ -1175,8 +1198,11 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
>  		if (!list_empty(&area->free_list[migratetype]))
>  			return COMPACT_PARTIAL;
>  
> -		/* Job done if allocation would set block type */
> -		if (order >= pageblock_order && area->nr_free)
> +		/*
> +		 * Job done if allocation would steal freepages from
> +		 * other migratetype buddy lists.
> +		 */
> +		if (can_steal_fallbacks(area, order, migratetype))
>  			return COMPACT_PARTIAL;

Seems somewhat wasteful in scenario where we want to satisfy a movable
allocation and it's an async compaction. Then we don't compact in
unmovable/reclaimable pageblock, and yet we will keep checking them for
fallbacks. A price to pay for having generic code?

Maybe can_steal_fallbacks could know the sync/async mode and use
migrate_async_suitable appropriately. But then I wouldn't move it to
page_alloc.c. More efficient could be a separate fallbacks array for async
compaction with the unsuitable types removed. But maybe I'm just overengineering
now.

>  	}
>  
> diff --git a/mm/internal.h b/mm/internal.h
> index c4d6c9b..0a89a14 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -201,6 +201,7 @@ unsigned long
>  isolate_migratepages_range(struct compact_control *cc,
>  			   unsigned long low_pfn, unsigned long end_pfn);
>  
> +bool can_steal_freepages(unsigned int order, int start_mt, int fallback_mt);
>  #endif
>  
>  /*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ef74750..4c3538b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1026,7 +1026,7 @@ 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] = {
> +int fallbacks[MIGRATE_TYPES][FALLBACK_MIGRATETYPES] = {
>  	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,     MIGRATE_RESERVE },
>  	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,     MIGRATE_RESERVE },
>  #ifdef CONFIG_CMA
> @@ -1122,8 +1122,7 @@ static void change_pageblock_range(struct page *pageblock_page,
>  	}
>  }
>  
> -static bool can_steal_freepages(unsigned int order,
> -				int start_mt, int fallback_mt)
> +bool can_steal_freepages(unsigned int order, int start_mt, int fallback_mt)
>  {
>  	if (is_migrate_cma(fallback_mt))
>  		return false;
> 

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