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

Christophe JAILLET
2023-06-18T09:34:09Z

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@wanadoo.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