On Fri, Jan 06, 2017 at 10:13:15AM +0800, Jason Wang wrote: > This patch tries to do several tweaks on vhost_vq_avail_empty() for a > better performance: > > - check cached avail index first which could avoid userspace memory access. > - using unlikely() for the failure of userspace access > - check vq->last_avail_idx instead of cached avail index as the last > step. > > This patch is need for batching supports which needs to peek whether > or not there's still available buffers in the ring. > > Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> > Signed-off-by: Jason Wang <jasow...@redhat.com> > --- > drivers/vhost/vhost.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index d643260..9f11838 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, > struct vhost_virtqueue *vq) > __virtio16 avail_idx; > int r; > > + if (vq->avail_idx != vq->last_avail_idx) > + return false; > + > r = vhost_get_user(vq, avail_idx, &vq->avail->idx); > - if (r) > + if (unlikely(r)) > return false; > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > - return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx; > + return vq->avail_idx == vq->last_avail_idx; > } > EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
So again, this did not address the issue I pointed out in v1: if we have 1 buffer in RX queue and that is not enough to store the whole packet, vhost_vq_avail_empty returns false, then we re-read the descriptors again and again. You have saved a single index access but not the more expensive descriptor access. I think that a way to address this could be to have this return current index for the caller. Then as long as that index isn't changed, you don't poke at descriptor ring. > -- > 2.7.4
