tcp: remove 1ms offset in srtt computation

[~andy/linux] / net / ipv4 / tcp_input.c

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 65cf90e063d5adcc98f15b044e35b20e1668352a..227cba79fa6b1f490f27a3aaf013070fbf9c8015 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -671,6 +671,7 @@ static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt)
 {
        struct tcp_sock *tp = tcp_sk(sk);
        long m = mrtt; /* RTT */
+       u32 srtt = tp->srtt;
 
        /*      The following amusing code comes from Jacobson's
         *      article in SIGCOMM '88.  Note that rtt and mdev
@@ -688,11 +689,9 @@ static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt)
         * does not matter how to _calculate_ it. Seems, it was trap
         * that VJ failed to avoid. 8)
         */
-       if (m == 0)
-               m = 1;
-       if (tp->srtt != 0) {
-               m -= (tp->srtt >> 3);   /* m is now error in rtt est */
-               tp->srtt += m;          /* rtt = 7/8 rtt + 1/8 new */
+       if (srtt != 0) {
+               m -= (srtt >> 3);       /* m is now error in rtt est */
+               srtt += m;              /* rtt = 7/8 rtt + 1/8 new */
                if (m < 0) {
                        m = -m;         /* m is now abs(error) */
                        m -= (tp->mdev >> 2);   /* similar update on mdev */
@@ -723,11 +722,12 @@ static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt)
                }
        } else {
                /* no previous measure. */
-               tp->srtt = m << 3;      /* take the measured time to be rtt */
+               srtt = m << 3;          /* take the measured time to be rtt */
                tp->mdev = m << 1;      /* make sure rto = 3*rtt */
                tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
                tp->rtt_seq = tp->snd_nxt;
        }
+       tp->srtt = max(1U, srtt);
 }
 
 /* Set the sk_pacing_rate to allow proper sizing of TSO packets.
@@ -746,8 +746,10 @@ static void tcp_update_pacing_rate(struct sock *sk)
 
        rate *= max(tp->snd_cwnd, tp->packets_out);
 
-       /* Correction for small srtt : minimum srtt being 8 (1 jiffy << 3),
-        * be conservative and assume srtt = 1 (125 us instead of 1.25 ms)
+       /* Correction for small srtt and scheduling constraints.
+        * For small rtt, consider noise is too high, and use
+        * the minimal value (srtt = 1 -> 125 us for HZ=1000)
+        *
         * We probably need usec resolution in the future.
         * Note: This also takes care of possible srtt=0 case,
         * when tcp_rtt_estimator() was not yet called.