tcp: add SNMP counter for no. of duplicate segments reported by DSACK

There are two existing SNMP counters, TCPDSACKRecv and TCPDSACKOfoRecv,
which are incremented depending on whether the DSACKed range is below
the cumulative ACK sequence number or not. Unfortunately, these both
implicitly assume each DSACK covers only one segment. This makes these
counters unusable for estimating spurious retransmit rates,
or real/non-spurious loss rate.

This patch introduces a new SNMP counter, TCPDSACKRecvSegs, which tracks
the estimated number of duplicate segments based on:
(DSACKed sequence range) / MSS. This counter is usable for estimating
spurious retransmit rates, or real/non-spurious loss rate.

Signed-off-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/snmp.h

@@ -287,6 +287,7 @@ enum
 	LINUX_MIB_TCPFASTOPENPASSIVEALTKEY,	/* TCPFastOpenPassiveAltKey */
 	LINUX_MIB_TCPTIMEOUTREHASH,		/* TCPTimeoutRehash */
 	LINUX_MIB_TCPDUPLICATEDATAREHASH,	/* TCPDuplicateDataRehash */
+	LINUX_MIB_TCPDSACKRECVSEGS,		/* TCPDSACKRecvSegs */
 	__LINUX_MIB_MAX
 };
 

net/ipv4/proc.c

@@ -292,6 +292,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPFastOpenPassiveAltKey", LINUX_MIB_TCPFASTOPENPASSIVEALTKEY),
 	SNMP_MIB_ITEM("TcpTimeoutRehash", LINUX_MIB_TCPTIMEOUTREHASH),
 	SNMP_MIB_ITEM("TcpDuplicateDataRehash", LINUX_MIB_TCPDUPLICATEDATAREHASH),
+	SNMP_MIB_ITEM("TCPDSACKRecvSegs", LINUX_MIB_TCPDSACKRECVSEGS),
 	SNMP_MIB_SENTINEL
 };
 

net/ipv4/tcp_input.c

@@ -1153,6 +1153,7 @@ static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
 	}
 
 	dup_segs = tcp_dsack_seen(tp, start_seq_0, end_seq_0, state);
+	NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPDSACKRECVSEGS, dup_segs);
 
 	/* D-SACK for already forgotten data... Do dumb counting. */
 	if (tp->undo_marker && tp->undo_retrans > 0 &&