[PATCH v2 2/2] mm/pageblock: remove false sharing in pageblock_flags

[Alex Shi]

Current pageblock_flags is only 4 bits, so it has to share a char size
in cmpxchg when get set, the false sharing cause perf drop.

If we incrase the bits up to 8, false sharing would gone in cmpxchg. and
the only cost is half char per pageblock, which is half char per 128MB
on x86, 4 chars in 1 GB.

Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
---
 include/linux/pageblock-flags.h | 2 +-
 mm/page_alloc.c                 | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
index d189441568eb..f785c9d6d68c 100644
--- a/include/linux/pageblock-flags.h
+++ b/include/linux/pageblock-flags.h
@@ -25,7 +25,7 @@ enum pageblock_bits {
 	 * Assume the bits will always align on a word. If this assumption
 	 * changes then get/set pageblock needs updating.
 	 */
-	NR_PAGEBLOCK_BITS
+	NR_PAGEBLOCK_BITS = BITS_PER_BYTE
 };
 
 #ifdef CONFIG_HUGETLB_PAGE
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f60071e8a4e1..65f692c762d5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -517,7 +517,7 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 	unsigned long bitidx, byte_bitidx;
 	unsigned char old_byte, byte;
 
-	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
+	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != BITS_PER_BYTE);
 	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
 
 	bitmap = get_pageblock_bitmap(page, pfn);
-- 
1.8.3.1