[RFC PATCH net-next] tcp: introduce a new MIB for CLOSE-WAIT sockets

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

CLOSE-WAIT is a relatively special state which "represents waiting for
a connection termination request from the local user" (RFC 793). Some
issues may happen because of unexpected/too many CLOSE-WAIT sockets,
like user application mistakenly handling close() syscall.

We want to trace this total number of CLOSE-WAIT sockets fastly and
frequently instead of resorting to displaying them altogether by using:

  netstat -anlp | grep CLOSE_WAIT

or something like this, which does harm to the performance especially in
heavy load. That's the reason why I chose to introduce this new MIB counter
like CurrEstab does. It do help us diagnose/find issues in production.

Besides, in the group of TCP_MIB_* defined by RFC 1213, TCP_MIB_CURRESTAB
should include both ESTABLISHED and CLOSE-WAIT sockets in theory:

  "tcpCurrEstab OBJECT-TYPE
   ...
   The number of TCP connections for which the current state
   is either ESTABLISHED or CLOSE- WAIT."

Apparently, at least since 2005, we don't count CLOSE-WAIT sockets. I think
there is a need to count it separately to avoid polluting the existing
TCP_MIB_CURRESTAB counter.

After this patch, we can see the counter by running 'cat /proc/net/netstat'
or 'nstat -s | grep CloseWait'

Suggested-by: Yongming Liu <yomiliu@tencent.com>
Suggested-by: Wangzi Yong <curuwang@tencent.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 include/uapi/linux/snmp.h | 1 +
 net/ipv4/proc.c           | 1 +
 net/ipv4/tcp.c            | 2 ++
 3 files changed, 4 insertions(+)

diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h
index adf5fd78dd50..c0feefb4d88b 100644
--- a/include/uapi/linux/snmp.h
+++ b/include/uapi/linux/snmp.h
@@ -302,6 +302,7 @@ enum
 	LINUX_MIB_TCPAOKEYNOTFOUND,		/* TCPAOKeyNotFound */
 	LINUX_MIB_TCPAOGOOD,			/* TCPAOGood */
 	LINUX_MIB_TCPAODROPPEDICMPS,		/* TCPAODroppedIcmps */
+	LINUX_MIB_TCPCLOSEWAIT,			/* TCPCloseWait */
 	__LINUX_MIB_MAX
 };
 
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 6c4664c681ca..964897dc6eb8 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -305,6 +305,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPAOKeyNotFound", LINUX_MIB_TCPAOKEYNOTFOUND),
 	SNMP_MIB_ITEM("TCPAOGood", LINUX_MIB_TCPAOGOOD),
 	SNMP_MIB_ITEM("TCPAODroppedIcmps", LINUX_MIB_TCPAODROPPEDICMPS),
+	SNMP_MIB_ITEM("TCPCloseWait", LINUX_MIB_TCPCLOSEWAIT),
 	SNMP_MIB_SENTINEL
 };
 
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 681b54e1f3a6..7abaa2660cc8 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2659,6 +2659,8 @@ void tcp_set_state(struct sock *sk, int state)
 	default:
 		if (oldstate == TCP_ESTABLISHED)
 			TCP_DEC_STATS(sock_net(sk), TCP_MIB_CURRESTAB);
+		if (state == TCP_CLOSE_WAIT)
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPCLOSEWAIT);
 	}
 
 	/* Change state AFTER socket is unhashed to avoid closed
-- 
2.37.3