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>

3 files changed, 16 insertions, 9 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 4ad0706d40eb..239946868142 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -201,10 +201,10 @@ struct tcp_sock {
         u32     tlp_high_seq;   /* snd_nxt at the time of TLP retransmit. */
 
 /* RTT measurement */
-        u32     srtt;           /* smoothed round trip time << 3        */
-        u32     mdev;           /* medium deviation                     */
-        u32     mdev_max;       /* maximal mdev for the last rtt period */
-        u32     rttvar;         /* smoothed mdev_max                    */
+        u32     srtt_us;        /* smoothed round trip time << 3 in usecs */
+        u32     mdev_us;        /* medium deviation                     */
+        u32     mdev_max_us;    /* maximal mdev for the last rtt period */
+        u32     rttvar_us;      /* smoothed mdev_max                    */
         u32     rtt_seq;        /* sequence number to update rttvar     */
 
         u32     packets_out;    /* Packets which are "in flight"        */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 1f820537741a..93eab0b9da60 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -31,6 +31,7 @@
 #include <linux/crypto.h>
 #include <linux/cryptohash.h>
 #include <linux/kref.h>
+#include <linux/ktime.h>
 
 #include <net/inet_connection_sock.h>
 #include <net/inet_timewait_sock.h>
@@ -478,7 +479,6 @@ int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
                              struct ip_options *opt);
 #ifdef CONFIG_SYN_COOKIES
-#include <linux/ktime.h>
 
 /* Syncookies use a monotonic timer which increments every 64 seconds.
  * This counter is used both as a hash input and partially encoded into
@@ -619,7 +619,7 @@ static inline void tcp_bound_rto(const struct sock *sk)
 
 static inline u32 __tcp_set_rto(const struct tcp_sock *tp)
 {
-        return (tp->srtt >> 3) + tp->rttvar;
+        return usecs_to_jiffies((tp->srtt_us >> 3) + tp->rttvar_us);
 }
 
 static inline void __tcp_fast_path_on(struct tcp_sock *tp, u32 snd_wnd)
@@ -656,6 +656,11 @@ static inline u32 tcp_rto_min(struct sock *sk)
         return rto_min;
 }
 
+static inline u32 tcp_rto_min_us(struct sock *sk)
+{
+        return jiffies_to_usecs(tcp_rto_min(sk));
+}
+
 /* Compute the actual receive window we are currently advertising.
  * Rcv_nxt can be after the window if our peer push more data
  * than the offered window.
@@ -778,7 +783,6 @@ enum tcp_ca_event {
 #define TCP_CA_BUF_MAX  (TCP_CA_NAME_MAX*TCP_CA_MAX)
 
 #define TCP_CONG_NON_RESTRICTED 0x1
-#define TCP_CONG_RTT_STAMP      0x2
 
 struct tcp_congestion_ops {
         struct list_head        list;
diff --git a/include/uapi/linux/tcp_metrics.h b/include/uapi/linux/tcp_metrics.h
index 54a37b13f2c4..93533926035c 100644
--- a/include/uapi/linux/tcp_metrics.h
+++ b/include/uapi/linux/tcp_metrics.h
@@ -11,12 +11,15 @@
 #define TCP_METRICS_GENL_VERSION        0x1
 
 enum tcp_metric_index {
-        TCP_METRIC_RTT,
-        TCP_METRIC_RTTVAR,
+        TCP_METRIC_RTT,         /* in ms units */
+        TCP_METRIC_RTTVAR,      /* in ms units */
         TCP_METRIC_SSTHRESH,
         TCP_METRIC_CWND,
         TCP_METRIC_REORDERING,
 
+        TCP_METRIC_RTT_US,      /* in usec units */
+        TCP_METRIC_RTTVAR_US,   /* in usec units */
+
         /* Always last.  */
         __TCP_METRIC_MAX,
 };