tcp: add SNMP counter for zero-window drops

It will be helpful if we could display the drops due to zero window or no
enough window space.
So a new SNMP MIB entry is added to track this behavior.
This entry is named LINUX_MIB_TCPZEROWINDOWDROP and published in
/proc/net/netstat in TcpExt line as TCPZeroWindowDrop.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/snmp.h

@@ -279,6 +279,7 @@ enum
 	LINUX_MIB_TCPDELIVERED,			/* TCPDelivered */
 	LINUX_MIB_TCPDELIVEREDCE,		/* TCPDeliveredCE */
 	LINUX_MIB_TCPACKCOMPRESSED,		/* TCPAckCompressed */
+	LINUX_MIB_TCPZEROWINDOWDROP,		/* TCPZeroWindowDrop */
 	__LINUX_MIB_MAX
 };
 

net/ipv4/proc.c

@@ -287,6 +287,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPDelivered", LINUX_MIB_TCPDELIVERED),
 	SNMP_MIB_ITEM("TCPDeliveredCE", LINUX_MIB_TCPDELIVEREDCE),
 	SNMP_MIB_ITEM("TCPAckCompressed", LINUX_MIB_TCPACKCOMPRESSED),
+	SNMP_MIB_ITEM("TCPZeroWindowDrop", LINUX_MIB_TCPZEROWINDOWDROP),
 	SNMP_MIB_SENTINEL
 };
 

net/ipv4/tcp_input.c

@@ -4668,8 +4668,10 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 	 *  Out of sequence packets to the out_of_order_queue.
 	 */
 	if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
-		if (tcp_receive_window(tp) == 0)
+		if (tcp_receive_window(tp) == 0) {
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
 			goto out_of_window;
+		}
 
 		/* Ok. In sequence. In window. */
 queue_and_out:
@@ -4735,8 +4737,10 @@ drop:
 		/* If window is closed, drop tail of packet. But after
 		 * remembering D-SACK for its head made in previous line.
 		 */
-		if (!tcp_receive_window(tp))
+		if (!tcp_receive_window(tp)) {
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
 			goto out_of_window;
+		}
 		goto queue_and_out;
 	}