linux-kernel - Re: [PATCH] virtio_balloon: don't call virtio_has_feature() twice on init

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Message-ID: <87ppndyn54.fsf@rustcorp.com.au>
Date:	Tue, 28 Jan 2014 10:48:31 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Leandro Dorileo <l@...ileo.org>, linux-kernel@...r.kernel.org
Cc:	virtio-dev@...ts.oasis-open.org,
	virtualization@...ts.linux-foundation.org, mst@...hat.com,
	Leandro Dorileo <l@...ileo.org>
Subject: Re: [PATCH] virtio_balloon: don't call virtio_has_feature() twice on init_vqs()

Leandro Dorileo <l@...ileo.org> 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@...ileo.org>

Hi Leandro,

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

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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