[PATCH net] tcp: fix potential underestimation on rcv_rtt

[Wei Wang]

From: Wei Wang <wei...@google.com>

When ms timestamp is used, current logic uses 1us in
tcp_rcv_rtt_update() when the real rcv_rtt is within 1 - 999us.
This could cause rcv_rtt underestimation.
Fix it by always using a min value of 1ms if ms timestamp is used.

Fixes: 645f4c6f2ebd ("tcp: switch rcv_rtt_est and rcvq_space to high
resolution timestamps")

Signed-off-by: Wei Wang <wei...@google.com>
Signed-off-by: Eric Dumazet <eduma...@google.com>
---
 net/ipv4/tcp_input.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9550cc42de2d..45f750e85714 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -508,9 +508,6 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 
sample, int win_dep)
        u32 new_sample = tp->rcv_rtt_est.rtt_us;
        long m = sample;
 
-       if (m == 0)
-               m = 1;
-
        if (new_sample != 0) {
                /* If we sample in larger samples in the non-timestamp
                 * case, we could grossly overestimate the RTT especially
@@ -547,6 +544,8 @@ static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
        if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
                return;
        delta_us = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcv_rtt_est.time);
+       if (!delta_us)
+               delta_us = 1;
        tcp_rcv_rtt_update(tp, delta_us, 1);
 
 new_measure:
@@ -563,8 +562,11 @@ static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
            (TCP_SKB_CB(skb)->end_seq -
             TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss)) {
                u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
-               u32 delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
+               u32 delta_us;
 
+               if (!delta)
+                       delta = 1;
+               delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
                tcp_rcv_rtt_update(tp, delta_us, 0);
        }
 }
-- 
2.15.1.424.g9478a66081-goog