[PATCH 4.14 101/146] tcp: fix potential underestimation on rcv_rtt

Greg Kroah-Hartman
2018-01-01T14:44:58Z

From: Wei Wang <weiwan@google.com>

[ Upstream commit 9ee11bd03cb1a5c3ca33c2bb70e7ed325f68890f ]

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 <weiwan@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ipv4/tcp_input.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -521,9 +521,6 @@ static void tcp_rcv_rtt_update(struct tc
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
@@ -560,6 +557,8 @@ static inline void tcp_rcv_rtt_measure(s
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:
@@ -576,8 +575,11 @@ static inline void tcp_rcv_rtt_measure_t
(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);
}
}