tcp: fix cubic scaling error

Doug Leith observed a discrepancy between the version of CUBIC described
in the papers and the version in 2.6.18. A math error related to scaling
causes Cubic to grow too slowly.

Patch is from "Sangtae Ha" <sha2@ncsu.edu>. I validated that
it does fix the problems.

See the following to show behavior over 500ms 100 Mbit link.

Sender (2.6.19-rc3) ---  Bridge (2.6.18-rt7) ------- Receiver (2.6.19-rc3)
                    1G      [netem]           100M

        http://developer.osdl.org/shemminger/tcp/2.6.19-rc3/cubic-orig.png
        http://developer.osdl.org/shemminger/tcp/2.6.19-rc3/cubic-fix.png

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 31a4986dfbf77baf7d439f4a1c775891fa29fc81..4dae9a866806e67547da7d0a67b1529f8eb6a91f 100644 (file)
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -191,7 +191,7 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
          */
 
        /* change the unit from HZ to bictcp_HZ */
-        t = ((tcp_time_stamp + ca->delay_min - ca->epoch_start)
+        t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
             << BICTCP_HZ) / HZ;
 
         if (t < ca->bic_K)             /* t - K */
@@ -260,7 +260,7 @@ static inline void measure_delay(struct sock *sk)
            (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
                return;
 
-       delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
+       delay = (tcp_time_stamp - tp->rx_opt.rcv_tsecr)<<3;
        if (delay == 0)
                delay = 1;
 
@@ -373,7 +373,7 @@ static int __init cubictcp_register(void)
 
        beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
 
-       cube_rtt_scale = (bic_scale << 3) / 10; /* 1024*c/rtt */
+       cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */
 
        /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
         *  so K = cubic_root( (wmax-cwnd)*rtt/c )