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

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Date:   Sat, 18 Feb 2023 17:51:40 +0000
From:   Peter Lafreniere <peter@...jl.ca>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio: Reorder fields in 'struct virtqueue'

On Saturday, February 18th, 2023 at 03:10, Christophe JAILLET <christophe.jaillet@...adoo.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@...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

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

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

- Peter