mm: get rid of unnecessary pageblock scanning in setup_zone_migrate_reserve

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@linux-foundation.org: tweak comment]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Tested-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index bd791e4..67ab5fe 100644 (file)
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -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;

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5248fe0..89d81f4 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3901,6 +3901,7 @@ 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
@@ -3922,6 +3923,12 @@ 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))
@@ -3959,6 +3966,12 @@ 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;
                }
 
                /*