[PATCH 2/6] mm/page_alloc: Use only one PCP list for THP-sized allocations

[Mel Gorman <mgorman@techsingularity.net>]

The per_cpu_pages is cache-aligned on a standard x86-64 distribution
configuration but a later patch will add a new field which would push
the structure into the next cache line. Use only one list to store
THP-sized pages on the per-cpu list. This assumes that the vast majority
of THP-sized allocations are GFP_MOVABLE but even if it was another type,
it would not contribute to serious fragmentation that potentially causes
a later THP allocation failure. Align per_cpu_pages on the cacheline
boundary to ensure there is no false cache sharing.

After this patch, the structure sizing is;

struct per_cpu_pages {
        int                        count;                /*     0     4 */
        int                        high;                 /*     4     4 */
        int                        batch;                /*     8     4 */
        short int                  free_factor;          /*    12     2 */
        short int                  expire;               /*    14     2 */
        struct list_head           lists[13];            /*    16   208 */

        /* size: 256, cachelines: 4, members: 6 */
        /* padding: 32 */
} __attribute__((__aligned__(64)));

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 include/linux/mmzone.h | 11 +++++++----
 mm/page_alloc.c        |  4 ++--
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 962b14d403e8..abe530748de6 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -358,15 +358,18 @@ enum zone_watermarks {
 };
 
 /*
- * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER plus one additional
- * for pageblock size for THP if configured.
+ * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. One additional list
+ * for THP which will usually be GFP_MOVABLE. Even if it is another type,
+ * it should not contribute to serious fragmentation causing THP allocation
+ * failures.
  */
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 #define NR_PCP_THP 1
 #else
 #define NR_PCP_THP 0
 #endif
-#define NR_PCP_LISTS (MIGRATE_PCPTYPES * (PAGE_ALLOC_COSTLY_ORDER + 1 + NR_PCP_THP))
+#define NR_LOWORDER_PCP_LISTS (MIGRATE_PCPTYPES * (PAGE_ALLOC_COSTLY_ORDER + 1))
+#define NR_PCP_LISTS (NR_LOWORDER_PCP_LISTS + NR_PCP_THP)
 
 /*
  * Shift to encode migratetype and order in the same integer, with order
@@ -392,7 +395,7 @@ struct per_cpu_pages {
 
 	/* Lists of pages, one per migrate type stored on the pcp-lists */
 	struct list_head lists[NR_PCP_LISTS];
-};
+} ____cacheline_aligned_in_smp;
 
 struct per_cpu_zonestat {
 #ifdef CONFIG_SMP
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 63976ad4b7f1..ed2deb93a758 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -648,7 +648,7 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	if (order > PAGE_ALLOC_COSTLY_ORDER) {
 		VM_BUG_ON(order != pageblock_order);
-		base = PAGE_ALLOC_COSTLY_ORDER + 1;
+		return NR_LOWORDER_PCP_LISTS;
 	}
 #else
 	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
@@ -662,7 +662,7 @@ static inline int pindex_to_order(unsigned int pindex)
 	int order = pindex / MIGRATE_PCPTYPES;
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	if (order > PAGE_ALLOC_COSTLY_ORDER)
+	if (pindex == NR_LOWORDER_PCP_LISTS)
 		order = pageblock_order;
 #else
 	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
-- 
2.34.1