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

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Date:   Mon, 16 Jan 2017 03:12:03 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Long Li <longli@...rosoft.com>, KY Srinivasan <kys@...rosoft.com>,
        "Haiyang Zhang" <haiyangz@...rosoft.com>
CC:     "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] hv: use substraction to update ring buffer index

> From: devel [mailto:driverdev-devel-bounces@...uxdriverproject.org] On
> Behalf Of Long Li
> Sent: Thursday, January 5, 2017 12:08
> To: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> <haiyangz@...rosoft.com>
> Cc: devel@...uxdriverproject.org; linux-kernel@...r.kernel.org
> 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;
>  }

Hi Long,
I guess you want to fix put_pkt_raw() too. :-)

Thanks,
-- Dexuan