netdev - [PATCH net-next] net: sched: pie: avoid slow division in drop probability decay

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Message-Id: <20190228123654.26330-1-lesliemonis@gmail.com>
Date:   Thu, 28 Feb 2019 18:06:54 +0530
From:   Leslie Monis <lesliemonis@...il.com>
To:     davem@...emloft.net
Cc:     netdev@...r.kernel.org, tahiliani@...k.edu.in, dave.taht@...il.com,
        Leslie Monis <lesliemonis@...il.com>
Subject: [PATCH net-next] net: sched: pie: avoid slow division in drop probability decay

As per RFC 8033, it is sufficient for the drop probability
decay factor to have a value of (1 - 1/64) instead of 98%.
This avoids the need to do slow division.

Suggested-by: David Laight <David.Laight@...lab.com>
Signed-off-by: Leslie Monis <lesliemonis@...il.com>
---
 net/sched/sch_pie.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_pie.c b/net/sched/sch_pie.c
index f93cfe034c72..1cc0c7b74aa3 100644
--- a/net/sched/sch_pie.c
+++ b/net/sched/sch_pie.c
@@ -429,7 +429,8 @@ static void calculate_probability(struct Qdisc *sch)
 	 */
 
 	if (qdelay == 0 && qdelay_old == 0 && update_prob)
-		q->vars.prob = 98 * div_u64(q->vars.prob, 100);
+		/* Reduce drop probability to 98.4% */
+		q->vars.prob -= q->vars.prob / 64u;
 
 	q->vars.qdelay = qdelay;
 	q->vars.qlen_old = qlen;
-- 
2.17.1