tcp: fix bogus RTT for CC when retransmissions are acked

Since retransmitted segments are not used for RTT estimation, previously
SACKed segments present in the rtx queue are used. This estimation can be
several times larger than the actual RTT. When a cumulative ack covers both
previously SACKed and retransmitted segments, CC may thus get a bogus RTT.

Such segments previously had an RTT estimation in tcp_sacktag_one(), so it
seems reasonable to not reuse them in tcp_clean_rtx_queue() at all.

Afaik, this has had no effect on SRTT/RTO because of Karn's check.

Signed-off-by: Kenneth Klette Jonassen <kennetkl@ifi.uio.no>
Acked-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 031cf72cd05c8094de8a9a76cd4cffa13e45c0d5..a7ef679dd3ea434e5b3bda48d391e1a872ed2478 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3099,17 +3099,15 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
                        if (sacked & TCPCB_SACKED_RETRANS)
                                tp->retrans_out -= acked_pcount;
                        flag |= FLAG_RETRANS_DATA_ACKED;
-               } else {
+               } else if (!(sacked & TCPCB_SACKED_ACKED)) {
                        last_ackt = skb->skb_mstamp;
                        WARN_ON_ONCE(last_ackt.v64 == 0);
                        if (!first_ackt.v64)
                                first_ackt = last_ackt;
 
-                       if (!(sacked & TCPCB_SACKED_ACKED)) {
-                               reord = min(pkts_acked, reord);
-                               if (!after(scb->end_seq, tp->high_seq))
-                                       flag |= FLAG_ORIG_SACK_ACKED;
-                       }
+                       reord = min(pkts_acked, reord);
+                       if (!after(scb->end_seq, tp->high_seq))
+                               flag |= FLAG_ORIG_SACK_ACKED;
                }
 
                if (sacked & TCPCB_SACKED_ACKED)