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: NAnton Mich <lp2s1h@gmail.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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.