linux-kernel - [PATCH] net/tun: use reciprocal

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Date: Thu, 25 Jan 2024 16:25:11 -0800
From: Stephen Hemminger <stephen@...workplumber.org>
To: netdev@...r.kernel.org
Cc: Stephen Hemminger <stephen@...workplumber.org>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] net/tun: use reciprocal_scale

Use the inline function reciprocal_scale rather than open coding
the scale optimization.  Also, remove unnecessary initializations.
Resulting compiled code is unchanged (according to godbolt).

Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>
---
 drivers/net/tun.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 4a4f8c8e79fa..e335ece47dec 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -54,6 +54,7 @@
 #include <linux/if_tun.h>
 #include <linux/if_vlan.h>
 #include <linux/crc32.h>
+#include <linux/math.h>
 #include <linux/nsproxy.h>
 #include <linux/virtio_net.h>
 #include <linux/rcupdate.h>
@@ -523,8 +524,7 @@ static inline void tun_flow_save_rps_rxhash(struct tun_flow_entry *e, u32 hash)
 static u16 tun_automq_select_queue(struct tun_struct *tun, struct sk_buff *skb)
 {
 	struct tun_flow_entry *e;
-	u32 txq = 0;
-	u32 numqueues = 0;
+	u32 txq, numqueues;
 
 	numqueues = READ_ONCE(tun->numqueues);
 
@@ -534,8 +534,7 @@ static u16 tun_automq_select_queue(struct tun_struct *tun, struct sk_buff *skb)
 		tun_flow_save_rps_rxhash(e, txq);
 		txq = e->queue_index;
 	} else {
-		/* use multiply and shift instead of expensive divide */
-		txq = ((u64)txq * numqueues) >> 32;
+		txq = reciprocal_scale(txq, numqueues);
 	}
 
 	return txq;
-- 
2.43.0