tcp: leverage support of SO_PEEK_OFF socket option when available - dev

1 Feb 2024

The kernel may support recvmsg(MSG_PEEK), starting reading data from a
given offset set by the SO_PEEK_OFF socket option. This makes it
possible to avoid repeated reading of already read initial bytes of a
received message, hence saving read cycles when forwarding TCP messages
in the host->name space direction.

In this commit, we add functionality to leverage this feature when available,
while we fall back to the previous behavior when not.

Measurements with iperf3 shows that throughput increases with 15-20 percent
in the host->namespace direction when this feature is used.

Signed-off-by: Jon Maloy &lt;jmaloy(a)redhat.com&gt;
---
 tcp.c | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/tcp.c b/tcp.c
index 905d26f..58674eb 100644
--- a/tcp.c
+++ b/tcp.c
@@ -505,6 +505,7 @@ static struct tcp_buf_seq_update
tcp6_l2_buf_seq_update[TCP_FRAMES_MEM];
 static unsigned int tcp6_l2_buf_used;
 
 /* recvmsg()/sendmsg() data for tap */
+static bool peek_offset_cap = false;
 static char 		tcp_buf_discard		[MAX_WINDOW];
 static struct iovec	iov_sock		[TCP_FRAMES_MEM + 1];
 
@@ -582,6 +583,14 @@ static_assert(ARRAY_SIZE(tc_hash) >= FLOW_MAX,
 int init_sock_pool4		[TCP_SOCK_POOL_SIZE];
 int init_sock_pool6		[TCP_SOCK_POOL_SIZE];
 
+static void set_peek_offset(int s, int offset)
+{
+	if (!peek_offset_cap)
+		return;
+	if (setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &offset, sizeof(offset)))
+		perror("Failed to set SO_PEEK_OFF\n");
+}
+
 /**
  * tcp_conn_epoll_events() - epoll events mask for given connection state
  * @events:	Current connection events
@@ -1951,7 +1960,7 @@ static void tcp_conn_from_tap(struct ctx *c,
 		if (bind(s, (struct sockaddr *)&addr6_ll, sizeof(addr6_ll)))
 			goto cancel;
 	}
-
+	set_peek_offset(s, 0);
 	conn = &flow->tcp;
 	conn->f.type = FLOW_TCP;
 	conn->sock = s;
@@ -2174,6 +2183,12 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn
*conn)
 	if (iov_rem)
 		iov_sock[fill_bufs].iov_len = iov_rem;
 
+	/* Don't use discard buffer if SO_PEEK_OFF is supported. */
+	if (peek_offset_cap) {
+		mh_sock.msg_iov = &iov_sock[1];
+		mh_sock.msg_iovlen -= 1;
+	}
+
 	/* Receive into buffers, don't dequeue until acknowledged by guest. */
 	do
 		len = recvmsg(s, &mh_sock, MSG_PEEK);
@@ -2195,7 +2210,9 @@ static int tcp_data_from_sock(struct ctx *c, struct tcp_tap_conn
*conn)
 		return 0;
 	}
 
-	sendlen = len - already_sent;
+	sendlen = len;
+	if (!peek_offset_cap)
+		sendlen -= already_sent;
 	if (sendlen <= 0) {
 		conn_flag(c, conn, STALLED);
 		return 0;
@@ -2367,6 +2384,7 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn
*conn,
 			   max_ack_seq, conn->seq_to_tap);
 		conn->seq_ack_from_tap = max_ack_seq;
 		conn->seq_to_tap = max_ack_seq;
+		set_peek_offset(conn->sock, 0);
 		tcp_data_from_sock(c, conn);
 	}
 
@@ -2718,6 +2736,7 @@ void tcp_listen_handler(struct ctx *c, union epoll_ref ref,
 	    tcp_splice_conn_from_sock(c, ref.tcp_listen, &flow->tcp_splice,
 				      s, (struct sockaddr *)&sa))
 		return;
+	set_peek_offset(s, 0);
 
 	tcp_tap_conn_from_sock(c, ref.tcp_listen, &flow->tcp, s,
 			       (struct sockaddr *)&sa, now);
@@ -3042,6 +3061,7 @@ static void tcp_sock_refill_init(const struct ctx *c)
 int tcp_init(struct ctx *c)
 {
 	unsigned b;
+	int s;
 
 	for (b = 0; b < TCP_HASH_TABLE_SIZE; b++)
 		tc_hash[b] = FLOW_SIDX_NONE;
@@ -3065,6 +3085,17 @@ int tcp_init(struct ctx *c)
 		NS_CALL(tcp_ns_socks_init, c);
 	}
 
+	/* Probe for SO_PEEK_OFF support */
+	s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
+	if (s < 0) {
+		perror("Temporary tcp socket creation failed\n");
+	} else {
+		if (!setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &(int){0}, sizeof(int))) {
+			peek_offset_cap = true;
+		}
+		close(s);
+	}
+	printf("SO_PEEK_OFF%ssupported\n", peek_offset_cap ? " " : "
not ");
 	return 0;
 }
 
-- 
2.42.0


    