codel: refine one condition to avoid a nul rec_inv_sqrt

One condition before codel_Newton_step() was not good if
we never left the dropping state for a flow. As a result
rec_inv_sqrt was 0, instead of the ~0 initial value.

codel control law was then set to a very aggressive mode, dropping
many packets before reaching 'target' and recovering from this problem.

To keep codel_vars_init() as efficient as possible, refine
the condition to make sure rec_inv_sqrt initial value is correct

Many thanks to Anton Mich for discovering the issue and suggesting
a fix.

Reported-by: Anton Mich <lp2s1h@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 2 deletions

diff --git a/include/net/codel.h b/include/net/codel.h
index 550debfc2403..389cf621161d 100644
--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -305,6 +305,8 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
                         }
                 }
         } else if (drop) {
+                u32 delta;
+
                 if (params->ecn && INET_ECN_set_ce(skb)) {
                         stats->ecn_mark++;
                 } else {
@@ -320,9 +322,11 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
                  * assume that the drop rate that controlled the queue on the
                  * last cycle is a good starting point to control it now.
                  */
-                if (codel_time_before(now - vars->drop_next,
+                delta = vars->count - vars->lastcount;
+                if (delta > 1 &&
+                    codel_time_before(now - vars->drop_next,
                                       16 * params->interval)) {
-                        vars->count = (vars->count - vars->lastcount) | 1;
+                        vars->count = delta;
                         /* we dont care if rec_inv_sqrt approximation
                          * is not very precise :
                          * Next Newton steps will correct it quadratically.