net: Add rfs_needed() helper

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 <[e-mail]>
Signed-off-by: Christoph Paasch <[e-mail]>
Acked-by: Matthieu Baerts (NGI0) <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/rps.h

···
 		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)
···
 	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)
···
 			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
 }