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>

1 files changed, 10 insertions, 7 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 4e985dea1dd2..886d874775df 100644
--- a/net/ipv4/tcp_output.c
+++ b/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;