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>
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c5eadec..bf38b1f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -866,11 +866,7 @@
if (clone_it) {
const struct sk_buff *fclone = skb + 1;
- /* If congestion control is doing timestamping, we must
- * take such a timestamp before we potentially clone/copy.
- */
- if (icsk->icsk_ca_ops->flags & TCP_CONG_RTT_STAMP)
- __net_timestamp(skb);
+ skb_mstamp_get(&skb->skb_mstamp);
if (unlikely(skb->fclone == SKB_FCLONE_ORIG &&
fclone->fclone == SKB_FCLONE_CLONE))
@@ -1974,7 +1970,7 @@
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
u32 timeout, tlp_time_stamp, rto_time_stamp;
- u32 rtt = tp->srtt >> 3;
+ u32 rtt = usecs_to_jiffies(tp->srtt_us >> 3);
if (WARN_ON(icsk->icsk_pending == ICSK_TIME_EARLY_RETRANS))
return false;
@@ -1996,7 +1992,7 @@
/* Schedule a loss probe in 2*RTT for SACK capable connections
* in Open state, that are either limited by cwnd or application.
*/
- if (sysctl_tcp_early_retrans < 3 || !tp->srtt || !tp->packets_out ||
+ if (sysctl_tcp_early_retrans < 3 || !tp->srtt_us || !tp->packets_out ||
!tcp_is_sack(tp) || inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
return false;
@@ -3050,8 +3046,9 @@
* Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
* directly.
*/
- if (tp->srtt) {
- int rtt = max(tp->srtt >> 3, TCP_DELACK_MIN);
+ if (tp->srtt_us) {
+ int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 3),
+ TCP_DELACK_MIN);
if (rtt < max_ato)
max_ato = rtt;