Re: [PATCH 3/4] mm/compaction: enhance compaction finish condition

[Vlastimil Babka]

On 12/08/2014 08:16 AM, Joonsoo Kim wrote:
> 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, it is a lack of migratetype consideration
> and too excessive.
>
> At first, it doesn't consider migratetype so there would be false positive
> on compaction finish decision. For example, if allocation request is
> for unmovable migratetype, freepage in CMA migratetype doesn't help that
> allocation, so compaction should not be stopped. But, current logic
> considers it as compaction is no longer needed and stop the compaction.
>
> Secondly, it is too excessive. We can steal freepage from other migratetype
> and change pageblock migratetype on more relaxed conditions. In page
> allocator, there is another conditions that can succeed to steal without
> introducing fragmentation.
>
> 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
> in allocation success rate on phase 1 and compaction success rate.
>
> Allocation success rate on phase 1 (%)
> 57.00 : 63.67
>
> Compaction success rate (Compaction success * 100 / Compaction stalls, %)
> 28.94 : 35.13
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>   include/linux/mmzone.h |    3 +++
>   mm/compaction.c        |   31 +++++++++++++++++++++++++++++--
>   mm/internal.h          |    1 +
>   mm/page_alloc.c        |    5 ++---
>   4 files changed, 35 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 2f0856d..87f5bb5 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 1a5f465..2fd5f79 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1054,6 +1054,30 @@ 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)
> +{
> +	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)
>   {
> @@ -1104,8 +1128,11 @@ 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)

So, can_steal_fallbacks() -> can_steal_freepages() is quite involved way 
if in the end we just realize that order >= pageblock_order and we are 
stealing whole pageblock. Given that often compaction is done for THP, 
it would be better to check order >= pageblock_order and handle it 
upfront. This goes together with my comments on previous patch that 
order >= pageblock_order is better handled separately.

> +		/*
> +		 * Job done if allocation would steal freepages from
> +		 * other migratetype buddy lists.
> +		 */
> +		if (can_steal_fallbacks(area, order, migratetype))
>   			return COMPACT_PARTIAL;
>   	}
>
> diff --git a/mm/internal.h b/mm/internal.h
> index efad241..7028d83 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -179,6 +179,7 @@ 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 7b4c9aa..dcb8523 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1031,7 +1031,7 @@ 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
> @@ -1161,8 +1161,7 @@ static void try_to_steal_freepages(struct zone *zone, struct page *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)
>   {
>   	/*
>   	 * When borrowing from MIGRATE_CMA, we need to release the excess

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