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

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Date:   Sun, 18 Jun 2023 11:33:55 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Jeremy Kerr <jk@...econstruct.com.au>,
        Matt Johnston <matt@...econstruct.com.au>,
        "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
Subject: [PATCH net-next] mctp: Reorder fields in 'struct mctp_route'

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct mctp_route'
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 mctp_route {
	mctp_eid_t                 min;                  /*     0     1 */
	mctp_eid_t                 max;                  /*     1     1 */

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

	struct mctp_dev *          dev;                  /*     8     8 */
	unsigned int               mtu;                  /*    16     4 */
	unsigned char              type;                 /*    20     1 */

	/* XXX 3 bytes hole, try to pack */

	int                        (*output)(struct mctp_route *, struct sk_buff *); /*    24     8 */
	struct list_head           list;                 /*    32    16 */
	refcount_t                 refs;                 /*    48     4 */

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

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

	/* size: 72, cachelines: 2, members: 9 */
	/* sum members: 59, holes: 3, sum holes: 13 */
	/* forced alignments: 1, forced holes: 1, sum forced holes: 4 */
	/* last cacheline: 8 bytes */
} __attribute__((__aligned__(8)));


After:
=====
struct mctp_route {
	mctp_eid_t                 min;                  /*     0     1 */
	mctp_eid_t                 max;                  /*     1     1 */
	unsigned char              type;                 /*     2     1 */

	/* XXX 1 byte hole, try to pack */

	unsigned int               mtu;                  /*     4     4 */
	struct mctp_dev *          dev;                  /*     8     8 */
	int                        (*output)(struct mctp_route *, struct sk_buff *); /*    16     8 */
	struct list_head           list;                 /*    24    16 */
	refcount_t                 refs;                 /*    40     4 */

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

	struct callback_head       rcu __attribute__((__aligned__(8))); /*    48    16 */

	/* size: 64, cachelines: 1, members: 9 */
	/* sum members: 59, holes: 2, sum holes: 5 */
	/* forced alignments: 1, forced holes: 1, sum forced holes: 4 */
} __attribute__((__aligned__(8)));
---
 include/net/mctp.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/net/mctp.h b/include/net/mctp.h
index 82800d521c3d..da86e106c91d 100644
--- a/include/net/mctp.h
+++ b/include/net/mctp.h
@@ -234,9 +234,9 @@ struct mctp_flow {
 struct mctp_route {
 	mctp_eid_t		min, max;
 
-	struct mctp_dev		*dev;
-	unsigned int		mtu;
 	unsigned char		type;
+	unsigned int		mtu;
+	struct mctp_dev		*dev;
 	int			(*output)(struct mctp_route *route,
 					  struct sk_buff *skb);
 
-- 
2.34.1