[PATCH 26/97] mm: get rid of unnecessary pageblock scanning in setup_zone_migrate_reserve

[Mel Gorman]

From: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>

commit 943dca1a1fcbccb58de944669b833fd38a6c809b upstream.

Yasuaki Ishimatsu reported memory hot-add spent more than 5 _hours_ on
9TB memory machine since onlining memory sections is too slow.  And we
found out setup_zone_migrate_reserve spent >90% of the time.

The problem is, setup_zone_migrate_reserve scans all pageblocks
unconditionally, but it is only necessary if the number of reserved
block was reduced (i.e.  memory hot remove).

Moreover, maximum MIGRATE_RESERVE per zone is currently 2.  It means
that the number of reserved pageblocks is almost always unchanged.

This patch adds zone->nr_migrate_reserve_block to maintain the number of
MIGRATE_RESERVE pageblocks and it reduces the overhead of
setup_zone_migrate_reserve dramatically.  The following table shows time
of onlining a memory section.

  Amount of memory     | 128GB | 192GB | 256GB|
  ---------------------------------------------
  linux-3.12           |  23.9 |  31.4 | 44.5 |
  This patch           |   8.3 |   8.3 |  8.6 |
  Mel's proposal patch |  10.9 |  19.2 | 31.3 |
  ---------------------------------------------
                                   (millisecond)

  128GB : 4 nodes and each node has 32GB of memory
  192GB : 6 nodes and each node has 32GB of memory
  256GB : 8 nodes and each node has 32GB of memory

  (*1) Mel proposed his idea by the following threads.
       https://lkml.org/lkml/2013/10/30/272

[akpm@xxxxxxxxxxxxxxxxxxxx: tweak comment]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Tested-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 include/linux/mmzone.h |  6 ++++++
 mm/page_alloc.c        | 13 +++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 5648290..4719985 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -494,6 +494,12 @@ struct zone {
 	unsigned long		managed_pages;
 
 	/*
+	 * Number of MIGRATE_RESEVE page block. To maintain for just
+	 * optimization. Protected by zone->lock.
+	 */
+	int			nr_migrate_reserve_block;
+
+	/*
 	 * rarely used fields:
 	 */
 	const char		*name;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3078eaf..c34b582 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3934,6 +3934,7 @@ static void setup_zone_migrate_reserve(struct zone *zone)
 	struct page *page;
 	unsigned long block_migratetype;
 	int reserve;
+	int old_reserve;
 
 	/*
 	 * Get the start pfn, end pfn and the number of blocks to reserve
@@ -3955,6 +3956,12 @@ static void setup_zone_migrate_reserve(struct zone *zone)
 	 * future allocation of hugepages at runtime.
 	 */
 	reserve = min(2, reserve);
+	old_reserve = zone->nr_migrate_reserve_block;
+
+	/* When memory hot-add, we almost always need to do nothing */
+	if (reserve == old_reserve)
+		return;
+	zone->nr_migrate_reserve_block = reserve;
 
 	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
 		if (!pfn_valid(pfn))
@@ -3992,6 +3999,12 @@ static void setup_zone_migrate_reserve(struct zone *zone)
 				reserve--;
 				continue;
 			}
+		} else if (!old_reserve) {
+			/*
+			 * At boot time we don't need to scan the whole zone
+			 * for turning off MIGRATE_RESERVE.
+			 */
+			break;
 		}
 
 		/*
-- 
1.8.4.5

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