Commit 730e700e authored by Jianfeng Wang's avatar Jianfeng Wang Committed by David S. Miller

tcp: apply a floor of 1 for RTT samples from TCP timestamps

For retransmitted packets, TCP needs to resort to using TCP timestamps
for computing RTT samples. In the common case where the data and ACK
fall in the same 1-millisecond interval, TCP senders with millisecond-
granularity TCP timestamps compute a ca_rtt_us of 0. This ca_rtt_us
of 0 propagates to rs->rtt_us.

This value of 0 can cause performance problems for congestion control
modules. For example, in BBR, the zero min_rtt sample can bring the
min_rtt and BDP estimate down to 0, reduce snd_cwnd and result in a
low throughput. It would be hard to mitigate this with filtering in
the congestion control module, because the proper floor to apply would
depend on the method of RTT sampling (using timestamp options or
internally-saved transmission timestamps).

This fix applies a floor of 1 for the RTT sample delta from TCP
timestamps, so that seq_rtt_us, ca_rtt_us, and rs->rtt_us will be at
least 1 * (USEC_PER_SEC / TCP_TS_HZ).

Note that the receiver RTT computation in tcp_rcv_rtt_measure() and
min_rtt computation in tcp_update_rtt_min() both already apply a floor
of 1 timestamp tick, so this commit makes the code more consistent in
avoiding this edge case of a value of 0.
Signed-off-by: default avatarJianfeng Wang <jfwang@google.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Acked-by: default avatarKevin Yang <yyd@google.com>
Acked-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent fbc97de8
...@@ -2950,6 +2950,8 @@ static bool tcp_ack_update_rtt(struct sock *sk, const int flag, ...@@ -2950,6 +2950,8 @@ static bool tcp_ack_update_rtt(struct sock *sk, const int flag,
u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr; u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) { if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
if (!delta)
delta = 1;
seq_rtt_us = delta * (USEC_PER_SEC / TCP_TS_HZ); seq_rtt_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
ca_rtt_us = seq_rtt_us; ca_rtt_us = seq_rtt_us;
} }
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment