Commit bb4d991a authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
Browse files

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: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent c4b2bf6b

include/net/tcp.h

+1 −2
Original line number Diff line number Diff line
@@ -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

+10 −7
Original line number Diff line number Diff line
@@ -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->srtt_us) {

		timeout = usecs_to_jiffies(tp->srtt_us >> 2);

		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));

			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

+1 −1
Original line number Diff line number Diff line
@@ -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);

	}

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