[PATCH v4 3/4] maple_tree: optimize mas_wr_append(), also improve duplicating VMAs

[Peng Zhang]

When the new range can be completely covered by the original last range
without touching the boundaries on both sides, two new entries can be
appended to the end as a fast path. We update the original last pivot at
the end, and the newly appended two entries will not be accessed before
this, so it is also safe in RCU mode.

This is useful for sequential insertion, which is what we do in
dup_mmap(). Enabling BENCH_FORK in test_maple_tree and just running
bench_forking() gives the following time-consuming numbers:

before:               after:
17,874.83 msec        15,738.38 msec

It shows about a 12% performance improvement for duplicating VMAs.

Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
---
 lib/maple_tree.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index bfffbb7cab26..56b9b5be28c8 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -4266,10 +4266,10 @@ static inline unsigned char mas_wr_new_end(struct ma_wr_state *wr_mas)
  *
  * Return: True if appended, false otherwise
  */
-static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
+static inline bool mas_wr_append(struct ma_wr_state *wr_mas,
+				 unsigned char new_end)
 {
 	unsigned char end = wr_mas->node_end;
-	unsigned char new_end = end + 1;
 	struct ma_state *mas = wr_mas->mas;
 	unsigned char node_pivots = mt_pivots[wr_mas->type];
 
@@ -4281,16 +4281,27 @@ static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
 		ma_set_meta(wr_mas->node, maple_leaf_64, 0, new_end);
 	}
 
-	if (mas->last == wr_mas->r_max) {
-		/* Append to end of range */
-		rcu_assign_pointer(wr_mas->slots[new_end], wr_mas->entry);
-		wr_mas->pivots[end] = mas->index - 1;
-		mas->offset = new_end;
+	if (new_end == wr_mas->node_end + 1) {
+		if (mas->last == wr_mas->r_max) {
+			/* Append to end of range */
+			rcu_assign_pointer(wr_mas->slots[new_end],
+					   wr_mas->entry);
+			wr_mas->pivots[end] = mas->index - 1;
+			mas->offset = new_end;
+		} else {
+			/* Append to start of range */
+			rcu_assign_pointer(wr_mas->slots[new_end],
+					   wr_mas->content);
+			wr_mas->pivots[end] = mas->last;
+			rcu_assign_pointer(wr_mas->slots[end], wr_mas->entry);
+		}
 	} else {
-		/* Append to start of range */
+		/* Append to the range without touching any boundaries. */
 		rcu_assign_pointer(wr_mas->slots[new_end], wr_mas->content);
-		wr_mas->pivots[end] = mas->last;
-		rcu_assign_pointer(wr_mas->slots[end], wr_mas->entry);
+		wr_mas->pivots[end + 1] = mas->last;
+		rcu_assign_pointer(wr_mas->slots[end + 1], wr_mas->entry);
+		wr_mas->pivots[end] = mas->index - 1;
+		mas->offset = end + 1;
 	}
 
 	if (!wr_mas->content || !wr_mas->entry)
@@ -4337,7 +4348,7 @@ static inline void mas_wr_modify(struct ma_wr_state *wr_mas)
 		goto slow_path;
 
 	/* Attempt to append */
-	if (new_end == wr_mas->node_end + 1 && mas_wr_append(wr_mas))
+	if (mas_wr_append(wr_mas, new_end))
 		return;
 
 	if (new_end == wr_mas->node_end && mas_wr_slot_store(wr_mas))
-- 
2.20.1