Re: [PATCH 11/13] btrfs: keep track of the root owner for relocation reads

[David Sterba <dsterba@suse.cz>]

On Mon, Jan 11, 2021 at 11:23:15PM +0100, David Sterba wrote:
> On Wed, Dec 16, 2020 at 11:22:15AM -0500, Josef Bacik wrote:
> > --- a/fs/btrfs/relocation.c
> > +++ b/fs/btrfs/relocation.c
> > @@ -98,6 +98,7 @@ struct tree_block {
> >  		u64 bytenr;
> >  	}; /* Use rb_simple_node for search/insert */
> >  	struct btrfs_key key;
> > +	u64 owner;
> >  	unsigned int level:8;
> >  	unsigned int key_ready:1;
> 
> This would probably lead to bad packing, key is 17 bytes and placing
> u64 after that adds 7 bytes for proper alignment. The bitfield members
> following the key are aligned to a byte so it would work if owner is
> before key.

Easy fix and size 64 is also more cache friendly.

@@ -12256,22 +12256,18 @@ struct tree_block {
                struct rb_node     rb_node __attribute__((__aligned__(8))); /*     0    24 */
                u64                bytenr;               /*    24     8 */
        } __attribute__((__aligned__(8))) __attribute__((__aligned__(8)));               /*     0    32 */
-       struct btrfs_key           key;                  /*    32    17 */
+       u64                        owner;                /*    32     8 */
+       struct btrfs_key           key;                  /*    40    17 */

-       /* XXX 7 bytes hole, try to pack */
+       /* Bitfield combined with next fields */

-       u64                        owner;                /*    56     8 */
-       /* --- cacheline 1 boundary (64 bytes) --- */
-       unsigned int               level:8;              /*    64: 0  4 */
-       unsigned int               key_ready:1;          /*    64: 8  4 */
+       unsigned int               level:8;              /*    56: 8  4 */
+       unsigned int               key_ready:1;          /*    56:16  4 */

-       /* size: 72, cachelines: 2, members: 5 */
-       /* sum members: 57, holes: 1, sum holes: 7 */
-       /* sum bitfield members: 9 bits (1 bytes) */
+       /* size: 64, cachelines: 1, members: 5 */
        /* padding: 4 */
-       /* bit_padding: 23 bits */
+       /* bit_padding: 15 bits */
        /* forced alignments: 1 */
-       /* last cacheline: 8 bytes */
 } __attribute__((__aligned__(8)));