tcp_cubic: enable high resolution ack time if needed

This is a refined version of an earlier patch by Lucas Nussbaum.
Cubic needs RTT values in milliseconds. If HZ < 1000 then
the values will be too coarse.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Reported-by: Lucas Nussbaum <lucas.nussbaum@loria.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 0 deletions

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index f4fb2d4b90ad..5e0491d742ca 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -459,6 +459,10 @@ static int __init cubictcp_register(void)
         /* divide by bic_scale and by constant Srtt (100ms) */
         do_div(cube_factor, bic_scale * 10);
 
+        /* hystart needs ms clock resolution */
+        if (hystart && HZ < 1000)
+                cubictcp.flags |= TCP_CONG_RTT_STAMP;
+
         return tcp_register_congestion_control(&cubictcp);
 }