This is a note to let you know that I've just added the patch titled tcp: apply a floor of 1 for RTT samples from TCP timestamps to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: tcp-apply-a-floor-of-1-for-rtt-samples-from-tcp-timestamps.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From foo@baz Mon 10 Aug 2020 02:07:44 PM CEST From: Jianfeng Wang <jfwang@xxxxxxxxxx> Date: Thu, 30 Jul 2020 23:49:16 +0000 Subject: tcp: apply a floor of 1 for RTT samples from TCP timestamps From: Jianfeng Wang <jfwang@xxxxxxxxxx> [ Upstream commit 730e700e2c19d87e578ff0e7d8cb1d4a02b036d2 ] 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: Jianfeng Wang <jfwang@xxxxxxxxxx> Signed-off-by: Neal Cardwell <ncardwell@xxxxxxxxxx> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx> Acked-by: Kevin Yang <yyd@xxxxxxxxxx> Acked-by: Yuchung Cheng <ycheng@xxxxxxxxxx> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- net/ipv4/tcp_input.c | 2 ++ 1 file changed, 2 insertions(+) --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2944,6 +2944,8 @@ static bool tcp_ack_update_rtt(struct so u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr; 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); ca_rtt_us = seq_rtt_us; } Patches currently in stable-queue which might be from jfwang@xxxxxxxxxx are queue-5.4/tcp-apply-a-floor-of-1-for-rtt-samples-from-tcp-timestamps.patch