From: Yuchung Cheng <ych...@google.com> Date: Fri, 18 Sep 2015 11:36:14 -0700
> Currently SYN/ACK RTT is measured in jiffies. For LAN the SYN/ACK > RTT is often measured as 0ms or sometimes 1ms, which would affect > RTT estimation and min RTT samping used by some congestion control. > > This patch improves SYN/ACK RTT to be usec resolution if platform > supports it. While the timestamping of SYN/ACK is done in request > sock, the RTT measurement is carefully arranged to avoid storing > another u64 timestamp in tcp_sock. > > For regular handshake w/o SYNACK retransmission, the RTT is sampled > right after the child socket is created and right before the request > sock is released (tcp_check_req() in tcp_minisocks.c) > > For Fast Open the child socket is already created when SYN/ACK was > sent, the RTT is sampled in tcp_rcv_state_process() after processing > the final ACK an right before the request socket is released. > > If the SYN/ACK was retransmistted or SYN-cookie was used, we rely > on TCP timestamps to measure the RTT. The sample is taken at the > same place in tcp_rcv_state_process() after the timestamp values > are validated in tcp_validate_incoming(). Note that we do not store > TS echo value in request_sock for SYN-cookies, because the value > is already stored in tp->rx_opt used by tcp_ack_update_rtt(). > > One side benefit is that the RTT measurement now happens before > initializing congestion control (of the passive side). Therefore > the congestion control can use the SYN/ACK RTT. > > Signed-off-by: Yuchung Cheng <ych...@google.com> > Signed-off-by: Neal Cardwell <ncardw...@google.com> > Signed-off-by: Eric Dumazet <eduma...@google.com> Applied. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
