tcp: consolidate SYNACK RTT sampling

The first patch consolidates SYNACK and other RTT measurement to use a
central function tcp_ack_update_rtt(). A (small) bonus is now SYNACK
RTT measurement happens after PAWS check, potentially reducing the
impact of RTO seeding on bad TCP timestamps values.

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

1 files changed, 12 insertions, 2 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 28af45abe062..b531710596ec 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2853,6 +2853,17 @@ static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,
                 tcp_ack_no_tstamp(sk, seq_rtt, flag);
 }
 
+/* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
+static void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
+{
+        struct tcp_sock *tp = tcp_sk(sk);
+        s32 seq_rtt = -1;
+
+        if (tp->lsndtime && !tp->total_retrans)
+                seq_rtt = tcp_time_stamp - tp->lsndtime;
+        tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt);
+}
+
 static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 {
         const struct inet_connection_sock *icsk = inet_csk(sk);
@@ -5624,9 +5635,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                  * so release it.
                  */
                 if (req) {
-                        tcp_synack_rtt_meas(sk, req);
                         tp->total_retrans = req->num_retrans;
-
                         reqsk_fastopen_remove(sk, req, false);
                 } else {
                         /* Make sure socket is routed, for correct metrics. */
@@ -5651,6 +5660,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                 tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
                 tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
                 tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
+                tcp_synack_rtt_meas(sk, req);
 
                 if (tp->rx_opt.tstamp_ok)
                         tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;