Subject: Re: [PATCH V4 net-next 1/3] vhost: better detection of available buffers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 10 Jan 2017 01:10:41 +0200

On Mon, Jan 09, 2017 at 10:59:16AM +0800, Jason Wang wrote:
> 
> 
> On 2017年01月07日 03:55, Michael S. Tsirkin wrote:
> > 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 <[email-masked]>
> > > Signed-off-by: Jason Wang <[email-masked]>
> > > ---
> > >   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.
> 
> Looks not, if I understand the code correctly, in this case, get_rx_bufs()
> will return zero, and we will try to enable rx kick and exit the loop.
> 
> Thanks

I mean this:

                while (vhost_can_busy_poll(vq->dev, endtime) &&
                       vhost_vq_avail_empty(vq->dev, vq))
                        cpu_relax();
                preempt_enable();
                r = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
                                      out_num, in_num, NULL, NULL);

vhost_vq_avail_empty returns false so we break out of the loop
and call vhost_get_vq_desc.

-- 
MST
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