Re: [PATCH net-next] netlabel: Reorder fields in 'struct netlbl_domaddr6_map'

On Sun, Jun 18, 2023 at 6:16 AM Christophe JAILLET
[off-list ref] wrote:

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct netlbl_domaddr6_map'
from 72 to 64 bytes.

It saves a few bytes of memory and is more cache-line friendly.

Signed-off-by: Christophe JAILLET <redacted>
---
Using pahole

Before:
======
struct netlbl_dom_map {
        char *                     domain;               /*     0     8 */
        u16                        family;               /*     8     2 */

        /* XXX 6 bytes hole, try to pack */

        struct netlbl_dommap_def   def;                  /*    16    16 */
        u32                        valid;                /*    32     4 */

        /* XXX 4 bytes hole, try to pack */

        struct list_head           list;                 /*    40    16 */
        struct callback_head       rcu __attribute__((__aligned__(8))); /*    56    16 */

        /* size: 72, cachelines: 2, members: 6 */
        /* sum members: 62, holes: 2, sum holes: 10 */
        /* forced alignments: 1 */
        /* last cacheline: 8 bytes */
} __attribute__((__aligned__(8)));


After:
=====
struct netlbl_dom_map {
        char *                     domain;               /*     0     8 */
        struct netlbl_dommap_def   def;                  /*     8    16 */
        u16                        family;               /*    24     2 */

        /* XXX 2 bytes hole, try to pack */

        u32                        valid;                /*    28     4 */
        struct list_head           list;                 /*    32    16 */
        struct callback_head       rcu __attribute__((__aligned__(8))); /*    48    16 */

        /* size: 64, cachelines: 1, members: 6 */
        /* sum members: 62, holes: 1, sum holes: 2 */
        /* forced alignments: 1 */
} __attribute__((__aligned__(8)));
---
 net/netlabel/netlabel_domainhash.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Acked-by: Paul Moore <paul@paul-moore.com>

-- 
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