Re: [PATCH] virtio_ring: remove unnecessary to_vvq call in vring hot path

[Xianting Tian]

在 2022/3/28 下午5:05, Stefano Garzarella 写道:

> On Thu, Mar 24, 2022 at 03:33:40PM +0800, Xianting Tian wrote:
> > It passes '_vq' to virtqueue_use_indirect(), which still calls
> > to_vvq to get 'vq', let's directly pass 'vq'. It can avoid
> > unnecessary call of to_vvq in hot path.
>
> It seems reasonable to me.
>
> > Other tiny optimization:
> > Add unlikely to "if (vq->vq.num_free < descs_used).
>
> Better to do this change in another patch.
Thanks for the reply, I will send the patchset, which contains the two 
patches, soon.
> Thanks,
> Stefano
>
> > Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
> > ---
> > drivers/virtio/virtio_ring.c | 10 ++++------
> > 1 file changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 962f1477b1fa..ab6d5f0cb579 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -205,11 +205,9 @@ struct vring_virtqueue {
> >
> > #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
> >
> > -static inline bool virtqueue_use_indirect(struct virtqueue *_vq,
> > +static inline bool virtqueue_use_indirect(struct vring_virtqueue *vq,
> >                       unsigned int total_sg)
> > {
> > -    struct vring_virtqueue *vq = to_vvq(_vq);
> > -
> >     /*
> >      * If the host supports indirect descriptor tables, and we have 
> > multiple
> >      * buffers, then go indirect. FIXME: tune this threshold
> > @@ -507,7 +505,7 @@ static inline int virtqueue_add_split(struct 
> > virtqueue *_vq,
> >
> >     head = vq->free_head;
> >
> > -    if (virtqueue_use_indirect(_vq, total_sg))
> > +    if (virtqueue_use_indirect(vq, total_sg))
> >         desc = alloc_indirect_split(_vq, total_sg, gfp);
> >     else {
> >         desc = NULL;
> > @@ -527,7 +525,7 @@ static inline int virtqueue_add_split(struct 
> > virtqueue *_vq,
> >         descs_used = total_sg;
> >     }
> >
> > -    if (vq->vq.num_free < descs_used) {
> > +    if (unlikely(vq->vq.num_free < descs_used)) {
> >         pr_debug("Can't add buf len %i - avail = %i\n",
> >              descs_used, vq->vq.num_free);
> >         /* FIXME: for historical reasons, we force a notify here if
> > @@ -1194,7 +1192,7 @@ static inline int virtqueue_add_packed(struct 
> > virtqueue *_vq,
> >
> >     BUG_ON(total_sg == 0);
> >
> > -    if (virtqueue_use_indirect(_vq, total_sg)) {
> > +    if (virtqueue_use_indirect(vq, total_sg)) {
> >         err = virtqueue_add_indirect_packed(vq, sgs, total_sg, out_sgs,
> >                             in_sgs, data, gfp);
> >         if (err != -ENOMEM) {
> > --
> > 2.17.1