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>

diff --git a/include/net/codel.h b/include/net/codel.h
index 550debfc240384f529562e3cc10cafcd342725f4..389cf621161d6903510c023e04e57a4ccd03072c 100644 (file)
--- 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.