tcp: switch rtt estimations to usec resolution

Upcoming congestion controls for TCP require usec resolution for RTT
estimations. Millisecond resolution is simply not enough these days.

FQ/pacing in DC environments also require this change for finer control
and removal of bimodal behavior due to the current hack in
tcp_update_pacing_rate() for 'small rtt'

TCP_CONG_RTT_STAMP is no longer needed.

As Julian Anastasov pointed out, we need to keep user compatibility :
tcp_metrics used to export RTT and RTTVAR in msec resolution,
so we added RTT_US and RTTVAR_US. An iproute2 patch is needed
to use the new attributes if provided by the kernel.

In this example ss command displays a srtt of 32 usecs (10Gbit link)

lpk51:~# ./ss -i dst lpk52
Netid  State      Recv-Q Send-Q   Local Address:Port       Peer
Address:Port
tcp    ESTAB      0      1         10.246.11.51:42959
10.246.11.52:64614
         cubic wscale:6,6 rto:201 rtt:0.032/0.001 ato:40 mss:1448
cwnd:10 send
3620.0Mbps pacing_rate 7240.0Mbps unacked:1 rcv_rtt:993 rcv_space:29559

Updated iproute2 ip command displays :

lpk51:~# ./ip tcp_metrics | grep 10.246.11.52
10.246.11.52 age 561.914sec cwnd 10 rtt 274us rttvar 213us source
10.246.11.51

Old binary displays :

lpk51:~# ip tcp_metrics | grep 10.246.11.52
10.246.11.52 age 561.914sec cwnd 10 rtt 250us rttvar 125us source
10.246.11.51

With help from Julian Anastasov, Stephen Hemminger and Yuchung Cheng

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Larry Brakmo <brakmo@google.com>
Cc: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 4 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index bed379c7abcd..7374905b3701 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -387,7 +387,7 @@ void tcp_init_sock(struct sock *sk)
         INIT_LIST_HEAD(&tp->tsq_node);
 
         icsk->icsk_rto = TCP_TIMEOUT_INIT;
-        tp->mdev = TCP_TIMEOUT_INIT;
+        tp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT);
 
         /* So many TCP implementations out there (incorrectly) count the
          * initial SYN frame in their delayed-ACK and congestion control
@@ -2339,7 +2339,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 
         sk->sk_shutdown = 0;
         sock_reset_flag(sk, SOCK_DONE);
-        tp->srtt = 0;
+        tp->srtt_us = 0;
         if ((tp->write_seq += tp->max_window + 2) == 0)
                 tp->write_seq = 1;
         icsk->icsk_backoff = 0;
@@ -2783,8 +2783,8 @@ void tcp_get_info(const struct sock *sk, struct tcp_info *info)
 
         info->tcpi_pmtu = icsk->icsk_pmtu_cookie;
         info->tcpi_rcv_ssthresh = tp->rcv_ssthresh;
-        info->tcpi_rtt = jiffies_to_usecs(tp->srtt)>>3;
-        info->tcpi_rttvar = jiffies_to_usecs(tp->mdev)>>2;
+        info->tcpi_rtt = tp->srtt_us >> 3;
+        info->tcpi_rttvar = tp->mdev_us >> 2;
         info->tcpi_snd_ssthresh = tp->snd_ssthresh;
         info->tcpi_snd_cwnd = tp->snd_cwnd;
         info->tcpi_advmss = tp->advmss;