Re: [PATCH v4 3/3] mm/compaction: enhance compaction finish condition

[Joonsoo Kim]

On Tue, Feb 17, 2015 at 10:46:04AM +0100, Vlastimil Babka wrote:
> On 02/12/2015 08:15 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, 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
> >
> >I tested it on non-reboot 5 runs stress-highalloc benchmark and found that
> >there is no more degradation on allocation success rate than before. That
> >roughly means that this patch doesn't result in more fragmentations.
> >
> >Vlastimil suggests additional idea that we only test for fallbacks
> >when migration scanner has scanned a whole pageblock. It looked good for
> >fragmentation because chance of stealing increase due to making more
> >free pages in certain pageblock. So, I tested it, but, it results in
> >decreased compaction success rate, roughly 38.00. I guess the reason that
> >if system is low memory condition, watermark check could be failed due to
> >not enough order 0 free page and so, sometimes, we can't reach a fallback
> >check although migrate_pfn is aligned to pageblock_nr_pages. I can insert
> >code to cope with this situation but it makes code more complicated so
> >I don't include his idea at this patch.
> 
> Hm that's weird. I'll try to investigate this later. Meanwhile it
> can stay as it is.
> 
> >Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Okay. Thanks.

> 
> But you'll need to fix:
> 
> >---
> >  mm/compaction.c |   14 ++++++++++++--
> >  mm/internal.h   |    2 ++
> >  mm/page_alloc.c |   19 ++++++++++++++-----
> >  3 files changed, 28 insertions(+), 7 deletions(-)
> >
> >diff --git a/mm/compaction.c b/mm/compaction.c
> >index 782772d..d40c426 100644
> >--- a/mm/compaction.c
> >+++ b/mm/compaction.c
> >@@ -1170,13 +1170,23 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
> >  	/* Direct compactor: Is a suitable page free? */
> >  	for (order = cc->order; order < MAX_ORDER; order++) {
> >  		struct free_area *area = &zone->free_area[order];
> >+		bool can_steal;
> >
> >  		/* Job done if page is free of the right migratetype */
> >  		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)
> >+		/* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
> >+		if (migratetype == MIGRATE_MOVABLE &&
> >+			!list_empty(&area->free_list[MIGRATE_CMA]))
> 
> This won't compile with !CONFIG_CMA, right? I recall pointing it on
> v3 already (or something similar elsewhere).

Will fix.

> 
> >+			return COMPACT_PARTIAL;
> >+
> >+		/*
> >+		 * Job done if allocation would steal freepages from
> >+		 * other migratetype buddy lists.
> >+		 */
> >+		if (find_suitable_fallback(area, order, migratetype,
> >+						true, &can_steal) != -1)
> >  			return COMPACT_PARTIAL;
> >  	}
> >
> >diff --git a/mm/internal.h b/mm/internal.h
> >index c4d6c9b..9640650 100644
> >--- a/mm/internal.h
> >+++ b/mm/internal.h
> >@@ -200,6 +200,8 @@ isolate_freepages_range(struct compact_control *cc,
> >  unsigned long
> >  isolate_migratepages_range(struct compact_control *cc,
> >  			   unsigned long low_pfn, unsigned long end_pfn);
> >+int find_suitable_fallback(struct free_area *area, unsigned int order,
> >+			int migratetype, bool only_stealable, bool *can_steal);
> >
> >  #endif
> >
> >diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >index 64a4974..95654f9 100644
> >--- a/mm/page_alloc.c
> >+++ b/mm/page_alloc.c
> >@@ -1191,9 +1191,14 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
> >  		set_pageblock_migratetype(page, start_type);
> >  }
> >
> >-/* Check whether there is a suitable fallback freepage with requested order. */
> >-static int find_suitable_fallback(struct free_area *area, unsigned int order,
> >-					int migratetype, bool *can_steal)
> >+/*
> >+ * Check whether there is a suitable fallback freepage with requested order.
> >+ * If only_stealable is true, this function returns fallback_mt only if
> >+ * we can steal other freepages all together. This would help to reduce
> >+ * fragmentation due to mixed migratetype pages in one pageblock.
> >+ */
> >+int find_suitable_fallback(struct free_area *area, unsigned int order,
> >+			int migratetype, bool only_stealable, bool *can_steal)
> >  {
> >  	int i;
> >  	int fallback_mt;
> >@@ -1213,7 +1218,11 @@ static int find_suitable_fallback(struct free_area *area, unsigned int order,
> >  		if (can_steal_fallback(order, migratetype))
> >  			*can_steal = true;
> >
> >-		return fallback_mt;
> >+		if (!only_stealable)
> >+			return fallback_mt;
> >+
> >+		if (*can_steal)
> >+			return fallback_mt;
> 
> Why not just single if (!only_stealable || *can_steal)

Will fix, too.

Thanks.

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