Re: [PATCH v2] vhost: batch used descs chains write-back with packed ring

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 20, 2018 at 12:44:46PM +0800, Tiwei Bie wrote:
> On Wed, Dec 19, 2018 at 10:29:52AM +0100, Maxime Coquelin wrote:
> > Instead of writing back descriptors chains in order, let's
> > write the first chain flags last in order to improve batching.
> > 
> > With Kernel's pktgen benchmark, ~3% performance gain is measured.
> > 
> > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > ---
> > 
> > V2:
> > Revert back to initial implementation to have a write
> > barrier before every descs flags store, but still
> > store first desc flags last. (Missing barrier reported
> > by Ilya)
> > 
> > 
> >  lib/librte_vhost/virtio_net.c | 19 ++++++++++++++++---
> >  1 file changed, 16 insertions(+), 3 deletions(-)
> > 
> > diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> > index 8c657a101..de436af79 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -97,6 +97,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> >  {
> >  	int i;
> >  	uint16_t used_idx = vq->last_used_idx;
> > +	uint16_t head_idx = vq->last_used_idx;
> > +	uint16_t head_flags = 0;
> >  
> >  	/* Split loop in two to save memory barriers */
> >  	for (i = 0; i < vq->shadow_used_idx; i++) {
> > @@ -126,12 +128,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> >  			flags &= ~VRING_DESC_F_AVAIL;
> >  		}
> >  
> > -		vq->desc_packed[vq->last_used_idx].flags = flags;
> > +		if (i > 0) {
> > +			vq->desc_packed[vq->last_used_idx].flags = flags;
> >  
> > -		vhost_log_cache_used_vring(dev, vq,
> > +			vhost_log_cache_used_vring(dev, vq,
> >  					vq->last_used_idx *
> >  					sizeof(struct vring_packed_desc),
> >  					sizeof(struct vring_packed_desc));
> > +		} else {
> > +			head_idx = vq->last_used_idx;
> > +			head_flags = flags;
> > +		}
> >  
> >  		vq->last_used_idx += vq->shadow_used_packed[i].count;
> >  		if (vq->last_used_idx >= vq->size) {
> > @@ -140,7 +147,13 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> >  		}
> >  	}
> >  
> > -	rte_smp_wmb();
> > +	vq->desc_packed[head_idx].flags = head_flags;
> > +
> > +	vhost_log_cache_used_vring(dev, vq,
> > +				vq->last_used_idx *
> 
> Should be head_idx.
> 
> > +				sizeof(struct vring_packed_desc),
> > +				sizeof(struct vring_packed_desc));
> > +
> >  	vq->shadow_used_idx = 0;
> 
> A wmb() is needed before log_cache_sync?
> 
> >  	vhost_log_cache_sync(dev, vq);


Probably smarter to have the wmb in there.
This way we can skip it if not logging.

> >  }
> > -- 
> > 2.17.2
> >