tcp: introduce TCPSpuriousRtxHostQueues SNMP counter

Host queues (Qdisc + NIC) can hold packets so long that TCP can
eventually retransmit a packet before the first transmit even left
the host.

Its not clear right now if we could avoid this in the first place :

- We could arm RTO timer not at the time we enqueue packets, but
  at the time we TX complete them (tcp_wfree())

- Cancel the sending of the new copy of the packet if prior one
  is still in queue.

This patch adds instrumentation so that we can at least see how
often this problem happens.

TCPSpuriousRtxHostQueues SNMP counter is incremented every time
we detect the fast clone is not yet freed in tcp_transmit_skb()
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/snmp.h

@@ -247,6 +247,7 @@ enum
 	LINUX_MIB_TCPFASTOPENPASSIVEFAIL,	/* TCPFastOpenPassiveFail */
 	LINUX_MIB_TCPFASTOPENLISTENOVERFLOW,	/* TCPFastOpenListenOverflow */
 	LINUX_MIB_TCPFASTOPENCOOKIEREQD,	/* TCPFastOpenCookieReqd */
+	LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES, /* TCPSpuriousRtxHostQueues */
 	__LINUX_MIB_MAX
 };
 

net/ipv4/proc.c

@@ -269,6 +269,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPFastOpenPassiveFail", LINUX_MIB_TCPFASTOPENPASSIVEFAIL),
 	SNMP_MIB_ITEM("TCPFastOpenListenOverflow", LINUX_MIB_TCPFASTOPENLISTENOVERFLOW),
 	SNMP_MIB_ITEM("TCPFastOpenCookieReqd", LINUX_MIB_TCPFASTOPENCOOKIEREQD),
+	SNMP_MIB_ITEM("TCPSpuriousRtxHostQueues", LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES),
 	SNMP_MIB_SENTINEL
 };
 

net/ipv4/tcp_output.c

@@ -846,6 +846,13 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 		__net_timestamp(skb);
 
 	if (likely(clone_it)) {
+		const struct sk_buff *fclone = skb + 1;
+
+		if (unlikely(skb->fclone == SKB_FCLONE_ORIG &&
+			     fclone->fclone == SKB_FCLONE_CLONE))
+			NET_INC_STATS_BH(sock_net(sk),
+					 LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
+
 		if (unlikely(skb_cloned(skb)))
 			skb = pskb_copy(skb, gfp_mask);
 		else