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[PATCH] virtio: Reorder fields in 'struct virtqueue'

Christophe JAILLET posted 1 patch 2 years, 6 months ago 
include/linux/virtio.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
[PATCH] virtio: Reorder fields in 'struct virtqueue'
Posted by Christophe JAILLET 2 years, 6 months ago 
Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct virtqueue'
from 72 to 68 bytes.

It saves a few bytes of memory.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
Using pahole

Before:
======
struct virtqueue {
	struct list_head           list;                 /*     0    16 */
	void                       (*callback)(struct virtqueue *); /*    16     8 */
	const char  *              name;                 /*    24     8 */
	struct virtio_device *     vdev;                 /*    32     8 */
	unsigned int               index;                /*    40     4 */
	unsigned int               num_free;             /*    44     4 */
	unsigned int               num_max;              /*    48     4 */

	/* XXX 4 bytes hole, try to pack */

	void *                     priv;                 /*    56     8 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	bool                       reset;                /*    64     1 */

	/* size: 72, cachelines: 2, members: 9 */
	/* sum members: 61, holes: 1, sum holes: 4 */
	/* padding: 7 */
	/* last cacheline: 8 bytes */
};

After:
=====
struct virtqueue {
	struct list_head           list;                 /*     0    16 */
	void                       (*callback)(struct virtqueue *); /*    16     8 */
	const char  *              name;                 /*    24     8 */
	struct virtio_device *     vdev;                 /*    32     8 */
	unsigned int               index;                /*    40     4 */
	unsigned int               num_free;             /*    44     4 */
	unsigned int               num_max;              /*    48     4 */
	bool                       reset;                /*    52     1 */

	/* XXX 3 bytes hole, try to pack */

	void *                     priv;                 /*    56     8 */

	/* size: 64, cachelines: 1, members: 9 */
	/* sum members: 61, holes: 1, sum holes: 3 */
};
---
 include/linux/virtio.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 6ac2655500dc..9439ae898310 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -35,8 +35,8 @@ struct virtqueue {
 	unsigned int index;
 	unsigned int num_free;
 	unsigned int num_max;
-	void *priv;
 	bool reset;
+	void *priv;
 };
 
 int virtqueue_add_outbuf(struct virtqueue *vq,
-- 
2.34.1
Re: [PATCH] virtio: Reorder fields in 'struct virtqueue'
Posted by Stefano Garzarella 2 years, 6 months ago 
On Sat, Feb 18, 2023 at 09:10:31AM +0100, Christophe JAILLET wrote:
>Group some variables based on their sizes to reduce hole and avoid padding.
>On x86_64, this shrinks the size of 'struct virtqueue'
>from 72 to 68 bytes.
>
>It saves a few bytes of memory.
>
>Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
>---

LGTM!

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

>Using pahole
>
>Before:
>======
>struct virtqueue {
>	struct list_head           list;                 /*     0    16 */
>	void                       (*callback)(struct virtqueue *); /*    16     8 */
>	const char  *              name;                 /*    24     8 */
>	struct virtio_device *     vdev;                 /*    32     8 */
>	unsigned int               index;                /*    40     4 */
>	unsigned int               num_free;             /*    44     4 */
>	unsigned int               num_max;              /*    48     4 */
>
>	/* XXX 4 bytes hole, try to pack */
>
>	void *                     priv;                 /*    56     8 */
>	/* --- cacheline 1 boundary (64 bytes) --- */
>	bool                       reset;                /*    64     1 */
>
>	/* size: 72, cachelines: 2, members: 9 */
>	/* sum members: 61, holes: 1, sum holes: 4 */
>	/* padding: 7 */
>	/* last cacheline: 8 bytes */
>};
>
>After:
>=====
>struct virtqueue {
>	struct list_head           list;                 /*     0    16 */
>	void                       (*callback)(struct virtqueue *); /*    16     8 */
>	const char  *              name;                 /*    24     8 */
>	struct virtio_device *     vdev;                 /*    32     8 */
>	unsigned int               index;                /*    40     4 */
>	unsigned int               num_free;             /*    44     4 */
>	unsigned int               num_max;              /*    48     4 */
>	bool                       reset;                /*    52     1 */
>
>	/* XXX 3 bytes hole, try to pack */
>
>	void *                     priv;                 /*    56     8 */
>
>	/* size: 64, cachelines: 1, members: 9 */
>	/* sum members: 61, holes: 1, sum holes: 3 */
>};
>---
> include/linux/virtio.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>index 6ac2655500dc..9439ae898310 100644
>--- a/include/linux/virtio.h
>+++ b/include/linux/virtio.h
>@@ -35,8 +35,8 @@ struct virtqueue {
> 	unsigned int index;
> 	unsigned int num_free;
> 	unsigned int num_max;
>-	void *priv;
> 	bool reset;
>+	void *priv;
> };
>
> int virtqueue_add_outbuf(struct virtqueue *vq,
>-- 
>2.34.1
>
Re: [PATCH] virtio: Reorder fields in 'struct virtqueue'
Posted by Peter Lafreniere 2 years, 6 months ago 
On Saturday, February 18th, 2023 at 03:10, Christophe JAILLET <christophe.jaillet@wanadoo.fr> wrote:
> 
> 
> Group some variables based on their sizes to reduce hole and avoid padding.
> On x86_64, this shrinks the size of 'struct virtqueue'
> from 72 to 68 bytes.
> 
> It saves a few bytes of memory.
> 
> Signed-off-by: Christophe JAILLET christophe.jaillet@wanadoo.fr
> 
> ---
> Using pahole
> 
> Before:
> ======
> struct virtqueue {
> struct list_head list; /* 0 16 */
> void (*callback)(struct virtqueue ); / 16 8 /
> const char * name; / 24 8 /
> struct virtio_device * vdev; / 32 8 /
> unsigned int index; / 40 4 /
> unsigned int num_free; / 44 4 /
> unsigned int num_max; / 48 4 /
> 
> / XXX 4 bytes hole, try to pack /
> 
> void * priv; / 56 8 /
> / --- cacheline 1 boundary (64 bytes) --- /
> bool reset; / 64 1 /
> 
> / size: 72, cachelines: 2, members: 9 /
> / sum members: 61, holes: 1, sum holes: 4 /
> / padding: 7 /
> / last cacheline: 8 bytes /
> };
> 
> After:
> =====
> struct virtqueue {
> struct list_head list; / 0 16 */
> void (*callback)(struct virtqueue ); / 16 8 /
> const char * name; / 24 8 /
> struct virtio_device * vdev; / 32 8 /
> unsigned int index; / 40 4 /
> unsigned int num_free; / 44 4 /
> unsigned int num_max; / 48 4 /
> bool reset; / 52 1 /
> 
> / XXX 3 bytes hole, try to pack /
> 
> void * priv; / 56 8 /
> 
> / size: 64, cachelines: 1, members: 9 /
> / sum members: 61, holes: 1, sum holes: 3 */
> };
> ---
> include/linux/virtio.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 6ac2655500dc..9439ae898310 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -35,8 +35,8 @@ struct virtqueue {
> unsigned int index;
> unsigned int num_free;
> unsigned int num_max;
> - void *priv;
> bool reset;
> + void *priv;
> };
> 
> int virtqueue_add_outbuf(struct virtqueue *vq,
> --
> 2.34.1

This will shrink the struct size on 32 bit archs too.

Acked-by: Peter Lafreniere <peter@n8pjl.ca>

- Peter

Patchew 2.1-devel-b67e4c2

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