linux-kernel - [PATCH] hv: use substraction to update ring buffer index

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Message-Id: <1483589302-27177-1-git-send-email-longli@exchange.microsoft.com>
Date:   Wed,  4 Jan 2017 20:08:22 -0800
From:   Long Li <longli@...hange.microsoft.com>
To:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>
Cc:     devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
        Long Li <longli@...rosoft.com>
Subject: [PATCH] hv: use substraction to update ring buffer index

From: Long Li <longli@...rosoft.com>

The ring buffer code uses %= to calculate index. For x86/64, %= compiles to
div, more than 10 times slower than sub.

Replace div with sub for this data heavy code path.

Signed-off-by: Long Li <longli@...rosoft.com>
---
 drivers/hv/ring_buffer.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index cd49cb1..f8eee6e 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -135,7 +135,8 @@ hv_get_next_readlocation_withoffset(struct hv_ring_buffer_info *ring_info,
 	u32 next = ring_info->ring_buffer->read_index;
 
 	next += offset;
-	next %= ring_info->ring_datasize;
+	if (next >= ring_info->ring_datasize)
+		next -= ring_info->ring_datasize;
 
 	return next;
 }
@@ -179,7 +180,8 @@ static u32 hv_copyfrom_ringbuffer(
 	memcpy(dest, ring_buffer + start_read_offset, destlen);
 
 	start_read_offset += destlen;
-	start_read_offset %= ring_buffer_size;
+	if (start_read_offset >= ring_buffer_size)
+		start_read_offset -= ring_buffer_size;
 
 	return start_read_offset;
 }
@@ -201,7 +203,8 @@ static u32 hv_copyto_ringbuffer(
 	memcpy(ring_buffer + start_write_offset, src, srclen);
 
 	start_write_offset += srclen;
-	start_write_offset %= ring_buffer_size;
+	if (start_write_offset >= ring_buffer_size)
+		start_write_offset -= ring_buffer_size;
 
 	return start_write_offset;
 }
-- 
2.7.4