linux-kernel - [PATCH net-next] netlabel: Reorder fields in 'struct netlbl_domaddr6

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Date:   Sun, 18 Jun 2023 12:16:41 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Paul Moore <paul@...l-moore.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        netdev@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: [PATCH net-next] netlabel: Reorder fields in 'struct netlbl_domaddr6_map'

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 <christophe.jaillet@...adoo.fr>
---
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(-)

diff --git a/net/netlabel/netlabel_domainhash.h b/net/netlabel/netlabel_domainhash.h
index 9f80972ae39b..7eaa35fdd9bd 100644
--- a/net/netlabel/netlabel_domainhash.h
+++ b/net/netlabel/netlabel_domainhash.h
@@ -57,8 +57,8 @@ struct netlbl_domaddr6_map {
 
 struct netlbl_dom_map {
 	char *domain;
-	u16 family;
 	struct netlbl_dommap_def def;
+	u16 family;
 
 	u32 valid;
 	struct list_head list;
-- 
2.34.1