target/linux/generic/patches-3.3/049-codel-refine-one-condition-to-avoid-a-nul-rec_inv_sqrt.patch

   1 From b8fc328668a74e1314a19266755a54abd875e5a6 Mon Sep 17 00:00:00 2001
   2 From: Eric Dumazet <edumazet@google.com>
   3 Date: Sun, 29 Jul 2012 20:52:21 +0000
   4 Subject: [PATCH] codel: refine one condition to avoid a nul rec_inv_sqrt
   5
   6 commit 2359a47671fc4fb0fe5e9945f76c2cb10792c0f8 upstream.
   7
   8 One condition before codel_Newton_step() was not good if
   9 we never left the dropping state for a flow. As a result
  10 rec_inv_sqrt was 0, instead of the ~0 initial value.
  11
  12 codel control law was then set to a very aggressive mode, dropping
  13 many packets before reaching 'target' and recovering from this problem.
  14
  15 To keep codel_vars_init() as efficient as possible, refine
  16 the condition to make sure rec_inv_sqrt initial value is correct
  17
  18 Many thanks to Anton Mich for discovering the issue and suggesting
  19 a fix.
  20
  21 Reported-by: Anton Mich <lp2s1h@gmail.com>
  22 Signed-off-by: Eric Dumazet <edumazet@google.com>
  23 Signed-off-by: David S. Miller <davem@davemloft.net>
  24 ---
  25  include/net/codel.h |    8 ++++++--
  26  1 file changed, 6 insertions(+), 2 deletions(-)
  27
  28 --- a/include/net/codel.h
  29 +++ b/include/net/codel.h
  30 @@ -305,6 +305,8 @@ static struct sk_buff *codel_dequeue(str
  31                         }
  32                 }
  33         } else if (drop) {
  34 +               u32 delta;
  35 +
  36                 if (params->ecn && INET_ECN_set_ce(skb)) {
  37                         stats->ecn_mark++;
  38                 } else {
  39 @@ -320,9 +322,11 @@ static struct sk_buff *codel_dequeue(str
  40                  * assume that the drop rate that controlled the queue on the
  41                  * last cycle is a good starting point to control it now.
  42                  */
  43 -               if (codel_time_before(now - vars->drop_next,
  44 +               delta = vars->count - vars->lastcount;
  45 +               if (delta > 1 &&
  46 +                   codel_time_before(now - vars->drop_next,
  47                                       16 * params->interval)) {
  48 -                       vars->count = (vars->count - vars->lastcount) | 1;
  49 +                       vars->count = delta;
  50                         /* we dont care if rec_inv_sqrt approximation
  51                          * is not very precise :
  52                          * Next Newton steps will correct it quadratically.