Re: [PATCH] virtio_balloon: don't call virtio_has_feature() twice on init_vqs()

[Leandro Dorileo <l@xxxxxxxxxxx>]

Hi Rusty,

On Mon, Jan 27, 2014 at 10:18 PM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:

Leandro Dorileo <l@xxxxxxxxxxx> writes:
> Cchange init_vqs() to avoid calling twice the virtio_has_feature()
> - attempting to find out if VIRTIO_BALLOON_F_STATS_VQ feature was negotiated -
> consequently we prevent unnecessarily running the drivers' feature_table more
> than needed.
>
> Signed-off-by: Leandro Dorileo <l@xxxxxxxxxxx>

Hi Leandro,

        This seems like a premature optimization.  The current code is
fairly clear, and there's no performance issue with init_vqs.

Indeed, rethinking the patch and reviewing the code path it seems to be no gain,

It's just called on probe and restore operations. As you said a premature optimization.

Thanks...

 

Am I missing something?
Rusty.

> ---
>  drivers/virtio/virtio_balloon.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 34bdaba..41771c1 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -320,19 +320,21 @@ static int init_vqs(struct virtio_balloon *vb)
>       vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
>       const char *names[] = { "inflate", "deflate", "stats" };
>       int err, nvqs;
> +     bool stats;
>
>       /*
>        * We expect two virtqueues: inflate and deflate, and
>        * optionally stat.
>        */
> -     nvqs = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
> +     stats = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ);
> +     nvqs = stats ? 3 : 2;
>       err = vb->vdev->config->find_vqs(vb->vdev, nvqs, vqs, callbacks, names);
>       if (err)
>               return err;
>
>       vb->inflate_vq = vqs[0];
>       vb->deflate_vq = vqs[1];
> -     if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> +     if (stats) {
>               struct scatterlist sg;
>               vb->stats_vq = vqs[2];
>
> --
> 1.8.5.3

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