[PATCH net-next v2 3/4] pie: remove pie_vars->accu_prob_overflows

[Leslie Monis <lesliemonis@gmail.com>]

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>
---
 include/net/pie.h      |  5 +----
 net/sched/sch_fq_pie.c |  1 -
 net/sched/sch_pie.c    | 21 ++++++---------------
 3 files changed, 7 insertions(+), 20 deletions(-)

diff --git a/include/net/pie.h b/include/net/pie.h
index 24f68c1e9919..1c645b76a2ed 100644
--- a/include/net/pie.h
+++ b/include/net/pie.h
@@ -8,7 +8,7 @@
 #include <net/inet_ecn.h>
 #include <net/pkt_sched.h>
 
-#define MAX_PROB	U64_MAX
+#define MAX_PROB	(U64_MAX >> BITS_PER_BYTE)
 #define DTIME_INVALID	U64_MAX
 #define QUEUE_THRESHOLD	16384
 #define DQCOUNT_INVALID	-1
@@ -47,7 +47,6 @@ struct pie_params {
  * @dq_count:			number of bytes dequeued in a measurement cycle
  * @avg_dq_rate:		calculated average dq rate
  * @backlog_old:		queue backlog during previous qdelay calculation
- * @accu_prob_overflows:	number of times accu_prob overflows
  */
 struct pie_vars {
 	psched_time_t qdelay;
@@ -59,7 +58,6 @@ struct pie_vars {
 	u64 dq_count;
 	u32 avg_dq_rate;
 	u32 backlog_old;
-	u8 accu_prob_overflows;
 };
 
 /**
@@ -107,7 +105,6 @@ static inline void pie_vars_init(struct pie_vars *vars)
 	vars->accu_prob = 0;
 	vars->dq_count = DQCOUNT_INVALID;
 	vars->avg_dq_rate = 0;
-	vars->accu_prob_overflows = 0;
 }
 
 static inline struct pie_skb_cb *get_pie_cb(const struct sk_buff *skb)
diff --git a/net/sched/sch_fq_pie.c b/net/sched/sch_fq_pie.c
index 214657eb3dfd..a9da8776bf5b 100644
--- a/net/sched/sch_fq_pie.c
+++ b/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;
diff --git a/net/sched/sch_pie.c b/net/sched/sch_pie.c
index 198cfa34a00a..f52442d39bf5 100644
--- a/net/sched/sch_pie.c
+++ b/net/sched/sch_pie.c
@@ -62,27 +62,19 @@ bool pie_drop_early(struct Qdisc *sch, struct pie_params *params,
 	else
 		local_prob = vars->prob;
 
-	if (local_prob == 0) {
+	if (local_prob == 0)
 		vars->accu_prob = 0;
-		vars->accu_prob_overflows = 0;
-	}
-
-	if (local_prob > MAX_PROB - vars->accu_prob)
-		vars->accu_prob_overflows++;
-
-	vars->accu_prob += local_prob;
+	else
+		vars->accu_prob += local_prob;
 
-	if (vars->accu_prob_overflows == 0 &&
-	    vars->accu_prob < (MAX_PROB / 100) * 85)
+	if (vars->accu_prob < (MAX_PROB / 100) * 85)
 		return false;
-	if (vars->accu_prob_overflows == 8 &&
-	    vars->accu_prob >= MAX_PROB / 2)
+	if (vars->accu_prob >= (MAX_PROB / 2) * 17)
 		return true;
 
 	prandom_bytes(&rnd, 8);
-	if (rnd < local_prob) {
+	if ((rnd >> BITS_PER_BYTE) < local_prob) {
 		vars->accu_prob = 0;
-		vars->accu_prob_overflows = 0;
 		return true;
 	}
 
@@ -129,7 +121,6 @@ static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 out:
 	q->stats.dropped++;
 	q->vars.accu_prob = 0;
-	q->vars.accu_prob_overflows = 0;
 	return qdisc_drop(skb, sch, to_free);
 }
 
-- 
2.17.1