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, 7 insertions, 5 deletions

diff --git a/net/ipv4/tcp_hybla.c b/net/ipv4/tcp_hybla.c
index 2a1a9e2a4e51..a15a799bf768 100644
--- a/net/ipv4/tcp_hybla.c
+++ b/net/ipv4/tcp_hybla.c
@@ -21,7 +21,7 @@ struct hybla {
 	u32   rho2;	      /* Rho * Rho, integer part */
 	u32   rho_3ls;	      /* Rho parameter, <<3 */
 	u32   rho2_7ls;	      /* Rho^2, <<7	*/
-	u32   minrtt;	      /* Minimum smoothed round trip time value seen */
+	u32   minrtt_us;      /* Minimum smoothed round trip time value seen */
 };
 
 /* Hybla reference round trip time (default= 1/40 sec = 25 ms), in ms */
@@ -35,7 +35,9 @@ static inline void hybla_recalc_param (struct sock *sk)
 {
 	struct hybla *ca = inet_csk_ca(sk);
 
-	ca->rho_3ls = max_t(u32, tcp_sk(sk)->srtt / msecs_to_jiffies(rtt0), 8);
+	ca->rho_3ls = max_t(u32,
+			    tcp_sk(sk)->srtt_us / (rtt0 * USEC_PER_MSEC),
+			    8U);
 	ca->rho = ca->rho_3ls >> 3;
 	ca->rho2_7ls = (ca->rho_3ls * ca->rho_3ls) << 1;
 	ca->rho2 = ca->rho2_7ls >> 7;
@@ -59,7 +61,7 @@ static void hybla_init(struct sock *sk)
 	hybla_recalc_param(sk);
 
 	/* set minimum rtt as this is the 1st ever seen */
-	ca->minrtt = tp->srtt;
+	ca->minrtt_us = tp->srtt_us;
 	tp->snd_cwnd = ca->rho;
 }
 
@@ -94,9 +96,9 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked,
 	int is_slowstart = 0;
 
 	/*  Recalculate rho only if this srtt is the lowest */
-	if (tp->srtt < ca->minrtt){
+	if (tp->srtt_us < ca->minrtt_us) {
 		hybla_recalc_param(sk);
-		ca->minrtt = tp->srtt;
+		ca->minrtt_us = tp->srtt_us;
 	}
 
 	if (!tcp_is_cwnd_limited(sk, in_flight))