linux-kernel - [PATCH net-next] netpoll: optimize struct layout for cache efficiency

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Message-Id: <20250324-netpoll_structstruct-v1-1-ff78f8a88dbb@debian.org>
Date: Mon, 24 Mar 2025 05:29:13 -0700
From: Breno Leitao <leitao@...ian.org>
To: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 kernel-team@...a.com, Breno Leitao <leitao@...ian.org>
Subject: [PATCH net-next] netpoll: optimize struct layout for cache
 efficiency

The struct netpoll serves two distinct purposes: it contains
configuration data needed only during setup (in netpoll_setup()), and
runtime data that's accessed on every packet transmission (in
netpoll_send_udp()).

Currently, this structure spans three cache lines with suboptimal
organization, where frequently accessed fields are mixed with rarely
accessed ones.

This commit reorganizes the structure to place all runtime fields used
during packet transmission together in the first cache line, while
moving the setup-only configuration fields to subsequent cache lines.
This approach follows the principle of placing hot fields together for
better cache locality during the performance-critical path.

The restructuring also eliminates structural inefficiencies, reducing
the number of holes. This provides a more compact memory layout while
maintaining the same functionality, resulting in better cache
utilization and potentially improves performance during packet
transmission operations.

  -   /* sum members: 137, holes: 3, sum holes: 7 */
  +   /* sum members: 137, holes: 1, sum holes: 3 */

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
 include/linux/netpoll.h | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/include/linux/netpoll.h b/include/linux/netpoll.h
index 0477208ed9ffa..a8de41d84be52 100644
--- a/include/linux/netpoll.h
+++ b/include/linux/netpoll.h
@@ -24,7 +24,16 @@ union inet_addr {

 struct netpoll {
 	struct net_device *dev;
+	u16 local_port, remote_port;
 	netdevice_tracker dev_tracker;
+	union inet_addr local_ip, remote_ip;
+	bool ipv6;
+
+	/* Hot fields above */
+
+	const char *name;
+	struct sk_buff_head skb_pool;
+	struct work_struct refill_wq;
 	/*
 	 * Either dev_name or dev_mac can be used to specify the local
 	 * interface - dev_name is used if it is a nonempty string, else
@@ -32,14 +41,7 @@ struct netpoll {
 	 */
 	char dev_name[IFNAMSIZ];
 	u8 dev_mac[ETH_ALEN];
-	const char *name;
-
-	union inet_addr local_ip, remote_ip;
-	bool ipv6;
-	u16 local_port, remote_port;
 	u8 remote_mac[ETH_ALEN];
-	struct sk_buff_head skb_pool;
-	struct work_struct refill_wq;
 };

 struct netpoll_info {

---
base-commit: bfc17c1658353f22843c7c13e27c2d31950f1887
change-id: 20250324-netpoll_structstruct-6ff56d1cc4d8

Best regards,
-- 
Breno Leitao <leitao@...ian.org>