btrfs: use smaller type for btrfs_path locks
The values of btrfs_path::locks are 0 to 4, fit into a u8. Let's see:
* overall size of btrfs_path drops down from 136 to 112 (-24 bytes),
* better packing in a slab page +6 objects
* the whole structure now fits to 2 cachelines
* slight decrease in code size:
text data bss dec hex filename
938731 43670 23144 1005545 f57e9 fs/btrfs/btrfs.ko.before
938203 43670 23144 1005017 f55d9 fs/btrfs/btrfs.ko.after
(and the generated assembly does not change much)
The main purpose is to decrease the size of the structure without
affecting performance. The byte access is usually well behaving accross
arches, the locks are not accessed frequently and sometimes just
compared to zero.
Note for further size reduction attempts: the slots could be made u16
but this might generate worse code on some arches (non-byte and non-int
access). Also the range of operations on slots is wider compared to
locks and the potential performance drop should be evaluated first.
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 09ee92d..5e0fe09 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -595,7 +595,7 @@
struct extent_buffer *nodes[BTRFS_MAX_LEVEL];
int slots[BTRFS_MAX_LEVEL];
/* if there is real range locking, this locks field will change */
- int locks[BTRFS_MAX_LEVEL];
+ u8 locks[BTRFS_MAX_LEVEL];
u8 reada;
/* keep some upper locks as we walk down */
u8 lowest_level;