tcp: adjust tail loss probe timeout

This patch adjusts the timeout formula to schedule the TCP loss probe
(TLP). The previous formula uses 2*SRTT or 1.5*RTT + DelayACKMax if
only one packet is in flight. It keeps a lower bound of 10 msec which
is too large for short RTT connections (e.g. within a data-center).
The new formula = 2*RTT + (inflight == 1 ? 200ms : 2ticks) which
performs better for short and fast connections.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -139,6 +139,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 #endif
 #define TCP_RTO_MAX	((unsigned)(120*HZ))
 #define TCP_RTO_MIN	((unsigned)(HZ/5))
+#define TCP_TIMEOUT_MIN	(2U) /* Min timeout for TCP timers in jiffies */
 #define TCP_TIMEOUT_INIT ((unsigned)(1*HZ))	/* RFC6298 2.1 initial RTO value	*/
 #define TCP_TIMEOUT_FALLBACK ((unsigned)(3*HZ))	/* RFC 1122 initial RTO value, now
 						 * used as a fallback RTO for the
@@ -150,8 +151,6 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCP_RESOURCE_PROBE_INTERVAL ((unsigned)(HZ/2U)) /* Maximal interval between probes
 					                 * for local resources.
 					                 */
-#define TCP_REO_TIMEOUT_MIN	(2000) /* Min RACK reordering timeout in usec */
-
 #define TCP_KEEPALIVE_TIME	(120*60*HZ)	/* two hours */
 #define TCP_KEEPALIVE_PROBES	9		/* Max of 9 keepalive probes	*/
 #define TCP_KEEPALIVE_INTVL	(75*HZ)

net/ipv4/tcp_output.c

@@ -2377,7 +2377,6 @@ bool tcp_schedule_loss_probe(struct sock *sk)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 timeout, tlp_time_stamp, rto_time_stamp;
-	u32 rtt = usecs_to_jiffies(tp->srtt_us >> 3);
 
 	/* No consecutive loss probes. */
 	if (WARN_ON(icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)) {
@@ -2406,15 +2405,19 @@ bool tcp_schedule_loss_probe(struct sock *sk)
 	     tcp_send_head(sk))
 		return false;
 
-	/* Probe timeout is at least 1.5*rtt + TCP_DELACK_MAX to account
+	/* Probe timeout is 2*rtt. Add minimum RTO to account
 	 * for delayed ack when there's one outstanding packet. If no RTT
 	 * sample is available then probe after TCP_TIMEOUT_INIT.
 	 */
-	timeout = rtt << 1 ? : TCP_TIMEOUT_INIT;
-	if (tp->packets_out == 1)
-		timeout = max_t(u32, timeout,
-				(rtt + (rtt >> 1) + TCP_DELACK_MAX));
-	timeout = max_t(u32, timeout, msecs_to_jiffies(10));
+	if (tp->srtt_us) {
+		timeout = usecs_to_jiffies(tp->srtt_us >> 2);
+		if (tp->packets_out == 1)
+			timeout += TCP_RTO_MIN;
+		else
+			timeout += TCP_TIMEOUT_MIN;
+	} else {
+		timeout = TCP_TIMEOUT_INIT;
+	}
 
 	/* If RTO is shorter, just schedule TLP in its place. */
 	tlp_time_stamp = tcp_jiffies32 + timeout;

net/ipv4/tcp_recovery.c

@@ -113,7 +113,7 @@ void tcp_rack_mark_lost(struct sock *sk)
 	tp->rack.advanced = 0;
 	tcp_rack_detect_loss(sk, &timeout);
 	if (timeout) {
-		timeout = usecs_to_jiffies(timeout + TCP_REO_TIMEOUT_MIN);
+		timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
 					  timeout, inet_csk(sk)->icsk_rto);
 	}