netdev - [PATCH] tcp_nv: use do_div() instead of expensive div64

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Date:   Thu, 02 Nov 2017 17:07:05 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Lawrence Brakmo <brakmo@...com>, linux-kernel@...r.kernel.org,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH] tcp_nv: use do_div() instead of expensive div64_u64()

Average RTT is 32-bit thus full 64-bit division is redundant.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Suggested-by: Stephen Hemminger <stephen@...workplumber.org>
Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
---
 net/ipv4/tcp_nv.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_nv.c b/net/ipv4/tcp_nv.c
index 125fc1450b01..17e290ffcd80 100644
--- a/net/ipv4/tcp_nv.c
+++ b/net/ipv4/tcp_nv.c
@@ -209,7 +209,7 @@ static void tcpnv_acked(struct sock *sk, const struct ack_sample *sample)
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct tcpnv *ca = inet_csk_ca(sk);
 	unsigned long now = jiffies;
-	s64 rate64 = 0;
+	u64 rate64;
 	u32 rate, max_win, cwnd_by_slope;
 	u32 avg_rtt;
 	u32 bytes_acked = 0;
@@ -251,8 +251,9 @@ static void tcpnv_acked(struct sock *sk, const struct ack_sample *sample)
 	}
 
 	/* rate in 100's bits per second */
-	rate64 = ((u64)sample->in_flight) * 8000000;
-	rate = (u32)div64_u64(rate64, (u64)(avg_rtt ?: 1) * 100);
+	rate64 = ((u64)sample->in_flight) * 80000;
+	do_div(rate64, avg_rtt ?: 1);
+	rate = (u32)rate64;
 
 	/* Remember the maximum rate seen during this RTT
 	 * Note: It may be more than one RTT. This function should be