linux-kernel - [PATCH] virtio: Reorder fields in 'struct virtqueue'

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Message-Id: <8f3d2e49270a2158717e15008e7ed7228196ba02.1676707807.git.christophe.jaillet@wanadoo.fr>
Date:   Sat, 18 Feb 2023 09:10:31 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH] virtio: Reorder fields in 'struct virtqueue'

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@...adoo.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