Btrfs: introduce qgroup_ulist to avoid frequently allocating/freeing ulist

When doing qgroup accounting, we call ulist_alloc()/ulist_free() every time
when we want to walk qgroup tree.

By introducing 'qgroup_ulist', we only need to call ulist_alloc()/ulist_free()
once. This reduce some sys time to allocate memory, see the measurements below

fsstress -p 4 -n 10000 -d $dir

With this patch:

real    0m50.153s
user    0m0.081s
sys     0m6.294s

real    0m51.113s
user    0m0.092s
sys     0m6.220s

real    0m52.610s
user    0m0.096s
sys     0m6.125s	avg 6.213
-----------------------------------------------------
Without the patch:

real    0m54.825s
user    0m0.061s
sys     0m10.665s

real    1m6.401s
user    0m0.089s
sys     0m11.218s

real    1m13.768s
user    0m0.087s
sys     0m10.665s       avg 10.849

we can see the sys time reduce ~43%.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index d6dd49b51ba8..a365400e38da 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1594,6 +1594,12 @@ struct btrfs_fs_info {
 	struct rb_root qgroup_tree;
 	spinlock_t qgroup_lock;
 
+	/*
+	 * used to avoid frequently calling ulist_alloc()/ulist_free()
+	 * when doing qgroup accounting, it must be protected by qgroup_lock.
+	 */
+	struct ulist *qgroup_ulist;
+
 	/* protect user change for quota operations */
 	struct mutex qgroup_ioctl_lock;