tcp: retry more conservatively on local congestion

Previously when the sender fails to retransmit a data packet on
timeout due to congestion in the local host (e.g. throttling in
qdisc), it'll retry within an RTO up to 500ms.

In low-RTT networks such as data-centers, RTO is often far
below the default minimum 200ms (and the cap 500ms). Then local
host congestion could trigger a retry storm pouring gas to the
fire. Worse yet, the retry counter (icsk_retransmits) is not
properly updated so the aggressive retry may exceed the system
limit (15 rounds) until the packet finally slips through.

On such rare events, it's wise to retry more conservatively (500ms)
and update the stats properly to reflect these incidents and follow
the system limit. Note that this is consistent with the behavior
when a keep-alive probe is dropped due to local congestion.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_timer.c

@@ -500,14 +500,13 @@ void tcp_retransmit_timer(struct sock *sk)
 
 	tcp_enter_loss(sk);
 
+	icsk->icsk_retransmits++;
 	if (tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1) > 0) {
 		/* Retransmission failed because of local congestion,
-		 * do not backoff.
+		 * Let senders fight for local resources conservatively.
 		 */
-		if (!icsk->icsk_retransmits)
-			icsk->icsk_retransmits = 1;
 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
-					  min(icsk->icsk_rto, TCP_RESOURCE_PROBE_INTERVAL),
+					  TCP_RESOURCE_PROBE_INTERVAL,
 					  TCP_RTO_MAX);
 		goto out;
 	}
@@ -528,7 +527,6 @@ void tcp_retransmit_timer(struct sock *sk)
 	 * the 120 second clamps though!
 	 */
 	icsk->icsk_backoff++;
-	icsk->icsk_retransmits++;
 
 out_reset_timer:
 	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is