linux-kernel - [PATCH net-next 5/6] net: Add rfs

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Message-Id: <20250901-net-next-mptcp-misc-feat-6-18-v1-5-80ae80d2b903@kernel.org>
Date: Mon, 01 Sep 2025 11:39:14 +0200
From: "Matthieu Baerts (NGI0)" <matttbe@...nel.org>
To: mptcp@...ts.linux.dev, Mat Martineau <martineau@...nel.org>, 
 Geliang Tang <geliang@...nel.org>, "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>, 
 Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 "Matthieu Baerts (NGI0)" <matttbe@...nel.org>, 
 Christoph Paasch <cpaasch@...nai.com>
Subject: [PATCH net-next 5/6] net: Add rfs_needed() helper

From: Christoph Paasch <cpaasch@...nai.com>

Add a helper to check if RFS is needed or not. Allows to make the code a
bit cleaner and the next patch to have MPTCP use this helper to decide
whether or not to iterate over the subflows.

tun_flow_update() was calling sock_rps_record_flow_hash() regardless of
the state of rfs_needed. This was not really a bug as sock_flow_table
simply ends up being NULL and thus everything will be fine.
This commit here thus also implicitly makes tun_flow_update() respect
the state of rfs_needed.

Suggested-by: Matthieu Baerts <matttbe@...nel.org>
Signed-off-by: Christoph Paasch <cpaasch@...nai.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
---
 include/net/rps.h | 85 ++++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 56 insertions(+), 29 deletions(-)

diff --git a/include/net/rps.h b/include/net/rps.h
index 9917dce42ca457e9c25d9e84ee450235f771d09b..f1794cd2e7fb32a36bde9959fab651663ab190fd 100644
--- a/include/net/rps.h
+++ b/include/net/rps.h
@@ -85,11 +85,8 @@ static inline void rps_record_sock_flow(struct rps_sock_flow_table *table,
 		WRITE_ONCE(table->ents[index], val);
 }
 
-#endif /* CONFIG_RPS */
-
-static inline void sock_rps_record_flow_hash(__u32 hash)
+static inline void _sock_rps_record_flow_hash(__u32 hash)
 {
-#ifdef CONFIG_RPS
 	struct rps_sock_flow_table *sock_flow_table;
 
 	if (!hash)
@@ -99,42 +96,33 @@ static inline void sock_rps_record_flow_hash(__u32 hash)
 	if (sock_flow_table)
 		rps_record_sock_flow(sock_flow_table, hash);
 	rcu_read_unlock();
-#endif
 }
 
-static inline void sock_rps_record_flow(const struct sock *sk)
+static inline void _sock_rps_record_flow(const struct sock *sk)
 {
-#ifdef CONFIG_RPS
-	if (static_branch_unlikely(&rfs_needed)) {
-		/* Reading sk->sk_rxhash might incur an expensive cache line
-		 * miss.
-		 *
-		 * TCP_ESTABLISHED does cover almost all states where RFS
-		 * might be useful, and is cheaper [1] than testing :
-		 *	IPv4: inet_sk(sk)->inet_daddr
-		 * 	IPv6: ipv6_addr_any(&sk->sk_v6_daddr)
-		 * OR	an additional socket flag
-		 * [1] : sk_state and sk_prot are in the same cache line.
+	/* Reading sk->sk_rxhash might incur an expensive cache line
+	 * miss.
+	 *
+	 * TCP_ESTABLISHED does cover almost all states where RFS
+	 * might be useful, and is cheaper [1] than testing :
+	 *	IPv4: inet_sk(sk)->inet_daddr
+	 *	IPv6: ipv6_addr_any(&sk->sk_v6_daddr)
+	 * OR	an additional socket flag
+	 * [1] : sk_state and sk_prot are in the same cache line.
+	 */
+	if (sk->sk_state == TCP_ESTABLISHED) {
+		/* This READ_ONCE() is paired with the WRITE_ONCE()
+		 * from sock_rps_save_rxhash() and sock_rps_reset_rxhash().
 		 */
-		if (sk->sk_state == TCP_ESTABLISHED) {
-			/* This READ_ONCE() is paired with the WRITE_ONCE()
-			 * from sock_rps_save_rxhash() and sock_rps_reset_rxhash().
-			 */
-			sock_rps_record_flow_hash(READ_ONCE(sk->sk_rxhash));
-		}
+		_sock_rps_record_flow_hash(READ_ONCE(sk->sk_rxhash));
 	}
-#endif
 }
 
-static inline void sock_rps_delete_flow(const struct sock *sk)
+static inline void _sock_rps_delete_flow(const struct sock *sk)
 {
-#ifdef CONFIG_RPS
 	struct rps_sock_flow_table *table;
 	u32 hash, index;
 
-	if (!static_branch_unlikely(&rfs_needed))
-		return;
-
 	hash = READ_ONCE(sk->sk_rxhash);
 	if (!hash)
 		return;
@@ -147,6 +135,45 @@ static inline void sock_rps_delete_flow(const struct sock *sk)
 			WRITE_ONCE(table->ents[index], RPS_NO_CPU);
 	}
 	rcu_read_unlock();
+}
+#endif /* CONFIG_RPS */
+
+static inline bool rfs_is_needed(void)
+{
+#ifdef CONFIG_RPS
+	return static_branch_unlikely(&rfs_needed);
+#else
+	return false;
+#endif
+}
+
+static inline void sock_rps_record_flow_hash(__u32 hash)
+{
+#ifdef CONFIG_RPS
+	if (!rfs_is_needed())
+		return;
+
+	_sock_rps_record_flow_hash(hash);
+#endif
+}
+
+static inline void sock_rps_record_flow(const struct sock *sk)
+{
+#ifdef CONFIG_RPS
+	if (!rfs_is_needed())
+		return;
+
+	_sock_rps_record_flow(sk);
+#endif
+}
+
+static inline void sock_rps_delete_flow(const struct sock *sk)
+{
+#ifdef CONFIG_RPS
+	if (!rfs_is_needed())
+		return;
+
+	_sock_rps_delete_flow(sk);
 #endif
 }
 

-- 
2.50.1