mm: get rid of unnecessary pageblock scanning in setup_zone_migrate_reserve

I resend this patch.
I added performance result into description of the patch.

------------------------------------------------------------------
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 spnet >90% time.

The problem is, setup_zone_migrate_reserve scan all pageblock unconditionally, but it is only necessary number of reserved block was reduced (i.e. memory hot remove).
Moreover, maximum MIGRATE_RESERVE per zone are currently 2. It mean, number of reserved pageblock are almost always unchanged.

This patch adds zone->nr_migrate_reserve_block to maintain number of MIGRATE_RESERVE pageblock and it reduce an overhead of setup_zone_migrate_reserve dramatically. 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	

---
 include/linux/mmzone.h |  6 ++++++
 mm/page_alloc.c        | 13 +++++++++++++
 2 files changed, 19 insertions(+)

index bd791e4..67ab5fe 100644
@@ -490,6 +490,12 @@ 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;

index 5a98836..da93109 100644
@@ -3908,6 +3908,7 @@ 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

@@ -3929,6 +3930,12 @@ 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))

@@ -3966,6 +3973,12 @@ static void setup_zone_migrate_reserve(struct zone *zone)
 				reserve--;
 				continue;
 			}
+		} else if (!old_reserve) {
+			/*
+			 * When boot time, we don't need scan whole zone
+			 * for turning off MIGRATE_RESERVE.
+			 */
+			break;
 		}
 
 		/*

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