mm: reduce the amount of work done when updating min_free_kbytes

When min_free_kbytes is updated, some pageblocks are marked
MIGRATE_RESERVE.  Ordinarily, this work is unnoticable as it happens early
in boot but on large machines with 1TB of memory, this has been reported
to delay boot times, probably due to the NUMA distances involved.

The bulk of the work is due to calling calling pageblock_is_reserved() an
unnecessary amount of times and accessing far more struct page metadata
than is necessary.  This patch significantly reduces the amount of work
done by setup_zone_migrate_reserve() improving boot times on 1TB machines.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8ac737fedafd6773cd6794dc427e93e8affe2a8d..af605986f33483e0e0c017eb1f7a51c0b21cdcb5 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3447,25 +3447,28 @@ static void setup_zone_migrate_reserve(struct zone *zone)
                if (page_to_nid(page) != zone_to_nid(zone))
                        continue;
 
-               /* Blocks with reserved pages will never free, skip them. */
-               block_end_pfn = min(pfn + pageblock_nr_pages, end_pfn);
-               if (pageblock_is_reserved(pfn, block_end_pfn))
-                       continue;
-
                block_migratetype = get_pageblock_migratetype(page);
 
-               /* If this block is reserved, account for it */
-               if (reserve > 0 && block_migratetype == MIGRATE_RESERVE) {
-                       reserve--;
-                       continue;
-               }
+               /* Only test what is necessary when the reserves are not met */
+               if (reserve > 0) {
+                       /* Blocks with reserved pages will never free, skip them. */
+                       block_end_pfn = min(pfn + pageblock_nr_pages, end_pfn);
+                       if (pageblock_is_reserved(pfn, block_end_pfn))
+                               continue;
 
-               /* Suitable for reserving if this block is movable */
-               if (reserve > 0 && block_migratetype == MIGRATE_MOVABLE) {
-                       set_pageblock_migratetype(page, MIGRATE_RESERVE);
-                       move_freepages_block(zone, page, MIGRATE_RESERVE);
-                       reserve--;
-                       continue;
+                       /* If this block is reserved, account for it */
+                       if (block_migratetype == MIGRATE_RESERVE) {
+                               reserve--;
+                               continue;
+                       }
+
+                       /* Suitable for reserving if this block is movable */
+                       if (block_migratetype == MIGRATE_MOVABLE) {
+                               set_pageblock_migratetype(page, MIGRATE_RESERVE);
+                               move_freepages_block(zone, page, MIGRATE_RESERVE);
+                               reserve--;
+                               continue;
+                       }
                }
 
                /*