[PATCH] Btrfs: avoid unnecessary ordered extent cache resets

[Filipe David Borba Manana <fdmanana@gmail.com>]

After an ordered extent completes, don't blindly reset the
inode's ordered tree last accessed ordered extent pointer.

While running the xfstests I noticed that about 29% of the
time the ordered extent to which tree->last pointed was not
the same as our just completed ordered extent. After that I
ran the following sysbench test (after a prepare phase) and
noticed that about 68% of the time tree->last pointed to
a different ordered extent too.

sysbench --test=fileio --file-num=32 --file-total-size=4G \
    --file-test-mode=rndwr --num-threads=512 \
    --file-block-size=32768 --max-time=60 --max-requests=0 run

Therefore reset tree->last on ordered extent removal only if
it pointed to the ordered extent we're removing from the tree.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
---
 fs/btrfs/ordered-data.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index 69582d5..b8c2ded 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -520,7 +520,8 @@ void btrfs_remove_ordered_extent(struct inode *inode,
 	spin_lock_irq(&tree->lock);
 	node = &entry->rb_node;
 	rb_erase(node, &tree->tree);
-	tree->last = NULL;
+	if (tree->last == node)
+		tree->last = NULL;
 	set_bit(BTRFS_ORDERED_COMPLETE, &entry->flags);
 	spin_unlock_irq(&tree->lock);
 
-- 
1.7.9.5