pie: remove pie_vars->accu_prob_overflows

The variable pie_vars->accu_prob is used as an accumulator for
probability values. Since probabilty values are scaled using the
MAX_PROB macro denoting (2^64 - 1), pie_vars->accu_prob is
likely to overflow as it is of type u64.

The variable pie_vars->accu_prob_overflows counts the number of
times the variable pie_vars->accu_prob overflows.

The MAX_PROB macro needs to be equal to at least (2^39 - 1) in
order to do precise calculations without any underflow. Thus
MAX_PROB can be reduced to (2^56 - 1) without affecting the
precision in calculations drastically. Doing so will eliminate
the need for the variable pie_vars->accu_prob_overflows as the
variable pie_vars->accu_prob will never overflow.

Removing the variable pie_vars->accu_prob_overflows also reduces
the size of the structure pie_vars to exactly 64 bytes.

Signed-off-by: Mohit P. Tahiliani <tahiliani@nitk.edu.in>
Signed-off-by: Gautam Ramakrishnan <gautamramk@gmail.com>
Signed-off-by: Leslie Monis <lesliemonis@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fq_pie.c

@@ -189,7 +189,6 @@ static int fq_pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 out:
 	q->stats.dropped++;
 	sel_flow->vars.accu_prob = 0;
-	sel_flow->vars.accu_prob_overflows = 0;
 	__qdisc_drop(skb, to_free);
 	qdisc_qstats_drop(sch);
 	return NET_XMIT_CN;