2014-01-25 20:06:56

by Leandro Dorileo

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

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 <[email protected]>
---
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

2014-01-28 00:38:26

by Rusty Russell

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

Leandro Dorileo <[email protected]> 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 <[email protected]>

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

2014-01-28 02:06:51

by Leandro Dorileo

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

Hi Rusty,

On Tue, Jan 28, 2014 at 10:48:31AM +1030, Rusty Russell wrote:
> Leandro Dorileo <[email protected]> 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 <[email protected]>
>
> 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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--
Leandro Dorileo