tcp: include locally failed retries in retransmission stats

Currently the retransmission stats are not incremented if the
retransmit fails locally. But we always increment the other packet
counters that track total packet/bytes sent.  Awkwardly while we
don't count these failed retransmits in RETRANSSEGS, we do count
them in FAILEDRETRANS.

If the qdisc is dropping many packets this could under-estimate
TCP retransmission rate substantially from both SNMP or per-socket
TCP_INFO stats. This patch changes this by always incrementing
retransmission stats on retransmission attempts and failures.

Another motivation is to properly track retransmists in
SCM_TIMESTAMPING_OPT_STATS. Since SCM_TSTAMP_SCHED collection is
triggered in tcp_transmit_skb(), If tp->total_retrans is incremented
after the function, we'll always mis-count by the amount of the
latest retransmission.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -2771,6 +2771,13 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
 		tcp_ecn_clear_syn(sk, skb);
 
+	/* Update global and local TCP statistics. */
+	segs = tcp_skb_pcount(skb);
+	TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
+	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
+		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
+	tp->total_retrans += segs;
+
 	/* make sure skb->data is aligned on arches that require it
 	 * and check if ack-trimming & collapsing extended the headroom
 	 * beyond what csum_start can cover.
@@ -2788,14 +2795,9 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	}
 
 	if (likely(!err)) {
-		segs = tcp_skb_pcount(skb);
-
 		TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
-		/* Update global TCP statistics. */
-		TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
-		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
-			__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
-		tp->total_retrans += segs;
+	} else if (err != -EBUSY) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
 	}
 	return err;
 }
@@ -2818,8 +2820,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		if (!tp->retrans_stamp)
 			tp->retrans_stamp = tcp_skb_timestamp(skb);
 
-	} else if (err != -EBUSY) {
-		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
 	}
 
 	if (tp->undo_retrans < 0)