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

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Date:   Sun, 18 Jun 2023 11:46:46 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Matthieu Baerts <matthieu.baerts@...sares.net>,
        Mat Martineau <martineau@...nel.org>,
        "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, mptcp@...ts.linux.dev
Subject: [PATCH net-next] mptcp: Reorder fields in 'struct mptcp_pm_add_entry'

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct mptcp_pm_add_entry'
from 136 to 128 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 mptcp_pm_add_entry {
	struct list_head           list;                 /*     0    16 */
	struct mptcp_addr_info     addr;                 /*    16    12 */

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

	struct timer_list          add_timer;            /*    32    88 */
	/* --- cacheline 1 boundary (64 bytes) was 56 bytes ago --- */
	struct mptcp_sock *        sock;                 /*   120     8 */
	/* --- cacheline 2 boundary (128 bytes) --- */
	u8                         retrans_times;        /*   128     1 */

	/* size: 136, cachelines: 3, members: 5 */
	/* sum members: 125, holes: 1, sum holes: 4 */
	/* padding: 7 */
	/* last cacheline: 8 bytes */
};


After:
=====
struct mptcp_pm_add_entry {
	struct list_head           list;                 /*     0    16 */
	struct mptcp_addr_info     addr;                 /*    16    12 */
	u8                         retrans_times;        /*    28     1 */

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

	struct timer_list          add_timer;            /*    32    88 */
	/* --- cacheline 1 boundary (64 bytes) was 56 bytes ago --- */
	struct mptcp_sock *        sock;                 /*   120     8 */

	/* size: 128, cachelines: 2, members: 5 */
	/* sum members: 125, holes: 1, sum holes: 3 */
};
---
 net/mptcp/pm_netlink.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index a12a87b780f6..a56718ffdd02 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -25,9 +25,9 @@ static int pm_nl_pernet_id;
 struct mptcp_pm_add_entry {
 	struct list_head	list;
 	struct mptcp_addr_info	addr;
+	u8			retrans_times;
 	struct timer_list	add_timer;
 	struct mptcp_sock	*sock;
-	u8			retrans_times;
 };
 
 struct pm_nl_pernet {
-- 
2.34.1