tcp: switch rcv_rtt_est and rcvq_space to high resolution timestamps

Some devices or distributions use HZ=100 or HZ=250

TCP receive buffer autotuning has poor behavior caused by this choice.
Since autotuning happens after 4 ms or 10 ms, short distance flows
get their receive buffer tuned to a very high value, but after an initial
period where it was frozen to (too small) initial value.

With tp->tcp_mstamp introduction, we can switch to high resolution
timestamps almost for free (at the expense of 8 additional bytes per
TCP structure)

Note that some TCP stacks use usec TCP timestamps where this
patch makes even more sense : Many TCP flows have < 500 usec RTT.
Hopefully this finer TS option can be standardized soon.

Tested:
 HZ=100 kernel
 ./netperf -H lpaa24 -t TCP_RR -l 1000 -- -r 10000,10000 &

 Peer without patch :
 lpaa24:~# ss -tmi dst lpaa23
 ...
 skmem:(r0,rb8388608,...)
 rcv_rtt:10 rcv_space:3210000 minrtt:0.017

 Peer with the patch :
 lpaa23:~# ss -tmi dst lpaa24
 ...
 skmem:(r0,rb428800,...)
 rcv_rtt:0.069 rcv_space:30000 minrtt:0.017

We can see saner RCVBUF, and more precise rcv_rtt information.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 17 insertions, 11 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index f475f0b53bfe..9739962bfb3f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -442,7 +442,8 @@ void tcp_init_buffer_space(struct sock *sk)
                 tcp_sndbuf_expand(sk);
 
         tp->rcvq_space.space = tp->rcv_wnd;
-        tp->rcvq_space.time = tcp_time_stamp;
+        skb_mstamp_get(&tp->tcp_mstamp);
+        tp->rcvq_space.time = tp->tcp_mstamp;
         tp->rcvq_space.seq = tp->copied_seq;
 
         maxwin = tcp_full_space(sk);
@@ -518,7 +519,7 @@ EXPORT_SYMBOL(tcp_initialize_rcv_mss);
  */
 static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
 {
-        u32 new_sample = tp->rcv_rtt_est.rtt;
+        u32 new_sample = tp->rcv_rtt_est.rtt_us;
         long m = sample;
 
         if (m == 0)
@@ -548,21 +549,23 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
                 new_sample = m << 3;
         }
 
-        if (tp->rcv_rtt_est.rtt != new_sample)
-                tp->rcv_rtt_est.rtt = new_sample;
+        tp->rcv_rtt_est.rtt_us = new_sample;
 }
 
 static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
 {
-        if (tp->rcv_rtt_est.time == 0)
+        u32 delta_us;
+
+        if (tp->rcv_rtt_est.time.v64 == 0)
                 goto new_measure;
         if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
                 return;
-        tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rcv_rtt_est.time, 1);
+        delta_us = skb_mstamp_us_delta(&tp->tcp_mstamp, &tp->rcv_rtt_est.time);
+        tcp_rcv_rtt_update(tp, delta_us, 1);
 
 new_measure:
         tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
-        tp->rcv_rtt_est.time = tcp_time_stamp;
+        tp->rcv_rtt_est.time = tp->tcp_mstamp;
 }
 
 static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
@@ -572,7 +575,10 @@ static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
         if (tp->rx_opt.rcv_tsecr &&
             (TCP_SKB_CB(skb)->end_seq -
              TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss))
-                tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rx_opt.rcv_tsecr, 0);
+                tcp_rcv_rtt_update(tp,
+                                   jiffies_to_usecs(tcp_time_stamp -
+                                                    tp->rx_opt.rcv_tsecr),
+                                   0);
 }
 
 /*
@@ -585,8 +591,8 @@ void tcp_rcv_space_adjust(struct sock *sk)
         int time;
         int copied;
 
-        time = tcp_time_stamp - tp->rcvq_space.time;
-        if (time < (tp->rcv_rtt_est.rtt >> 3) || tp->rcv_rtt_est.rtt == 0)
+        time = skb_mstamp_us_delta(&tp->tcp_mstamp, &tp->rcvq_space.time);
+        if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)
                 return;
 
         /* Number of bytes copied to user in last RTT */
@@ -642,7 +648,7 @@ void tcp_rcv_space_adjust(struct sock *sk)
 
 new_measure:
         tp->rcvq_space.seq = tp->copied_seq;
-        tp->rcvq_space.time = tcp_time_stamp;
+        tp->rcvq_space.time = tp->tcp_mstamp;
 }
 
 /* There is something which you must keep in mind when you analyze the