udp: add local "peek offset enabled" flag

We want to re-organize the struct sock layout. The sk_peek_off
field location is problematic, as most protocols want it in the
RX read area, while UDP wants it on a cacheline different from
sk_receive_queue.

Create a local (inside udp_sock) copy of the 'peek offset is enabled'
flag and place it inside the same cacheline of reader_queue.

Check such flag before reading sk_peek_off. This will save potential
false sharing and cache misses in the fast-path.

Tested under UDP flood with small packets. The struct sock layout
update causes a 4% performance drop, and this patch restores completely
the original tput.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/67ab679c15fbf49fa05b3ffe05d91c47ab84f147.1708426665.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

include/linux/udp.h

@@ -92,6 +92,9 @@ struct udp_sock {
 
 	/* This fields follows rcvbuf value, and is touched by udp_recvmsg */
 	int		forward_threshold;
+
+	/* Cache friendly copy of sk->sk_peek_off >= 0 */
+	bool		peeking_with_offset;
 };
 
 #define udp_test_bit(nr, sk)			\
@@ -109,6 +112,13 @@ struct udp_sock {
 
 #define udp_sk(ptr) container_of_const(ptr, struct udp_sock, inet.sk)
 
+static inline int udp_set_peek_off(struct sock *sk, int val)
+{
+	sk_set_peek_off(sk, val);
+	WRITE_ONCE(udp_sk(sk)->peeking_with_offset, val >= 0);
+	return 0;
+}
+
 static inline void udp_set_no_check6_tx(struct sock *sk, bool val)
 {
 	udp_assign_bit(NO_CHECK6_TX, sk, val);

net/ipv4/af_inet.c

@@ -1103,7 +1103,7 @@ const struct proto_ops inet_dgram_ops = {
 	.recvmsg	   = inet_recvmsg,
 	.mmap		   = sock_no_mmap,
 	.splice_eof	   = inet_splice_eof,
-	.set_peek_off	   = sk_set_peek_off,
+	.set_peek_off	   = udp_set_peek_off,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	   = inet_compat_ioctl,
 #endif

net/ipv4/udp.c

@@ -1589,7 +1589,7 @@ int udp_init_sock(struct sock *sk)
 
 void skb_consume_udp(struct sock *sk, struct sk_buff *skb, int len)
 {
-	if (unlikely(READ_ONCE(sk->sk_peek_off) >= 0)) {
+	if (unlikely(READ_ONCE(udp_sk(sk)->peeking_with_offset))) {
 		bool slow = lock_sock_fast(sk);
 
 		sk_peek_offset_bwd(sk, len);

net/ipv6/af_inet6.c

@@ -736,7 +736,7 @@ const struct proto_ops inet6_dgram_ops = {
 	.recvmsg	   = inet6_recvmsg,		/* retpoline's sake */
 	.read_skb	   = udp_read_skb,
 	.mmap		   = sock_no_mmap,
-	.set_peek_off	   = sk_set_peek_off,
+	.set_peek_off	   = udp_set_peek_off,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	   = inet6_compat_ioctl,
 #endif