From: Jianfeng Wang <jfw...@google.com> Date: Thu, 30 Jul 2020 23:49:16 +0000
> 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 <jfw...@google.com> > Signed-off-by: Neal Cardwell <ncardw...@google.com> > Signed-off-by: Eric Dumazet <eduma...@google.com> > Acked-by: Kevin Yang <y...@google.com> > Acked-by: Yuchung Cheng <ych...@google.com> Applied and queued up for -stable, thanks.
