Re: vhost: batch used descriptors chains write-back with packed ring

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

On Wed, Dec 05, 2018 at 07:01:23PM +0300, Ilya Maximets wrote:
> On 28.11.2018 12:47, Maxime Coquelin wrote:
> > Instead of writing back descriptors chains in order, let's
> > write the first chain flags last in order to improve batching.
> 
> I'm not sure if this fully compliant with virtio spec.
> It says that 'each side (driver and device) are only required to poll
> (or test) a single location in memory', but it does not forbid to
> test other descriptors. So, if the driver will try to check not only
> 'the next device descriptor after the one they processed previously,
> in circular order' but a few descriptors ahead, it could read an
> inconsistent memory because there are no more write barriers between
> updates for flags and id/len for them.
> 
> What do you think ?

Write barriers for SMP effects are quite cheap on most architectures.
So adding them before each flag write is probably not a big deal.


> > 
> > With Kernel's pktgen benchmark, ~3% performance gain is measured.
> > 
> > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > Tested-by: Jens Freimann <jfreimann@redhat.com>
> > Reviewed-by: Jens Freimann <jfreimann@redhat.com>
> > ---
> >  lib/librte_vhost/virtio_net.c | 37 ++++++++++++++++++++++-------------
> >  1 file changed, 23 insertions(+), 14 deletions(-)
> > 
> > diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> > index 5e1a1a727..f54642c2d 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -135,19 +135,10 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> >  			struct vhost_virtqueue *vq)
> >  {
> >  	int i;
> > -	uint16_t used_idx = vq->last_used_idx;
> > +	uint16_t head_flags, head_idx = vq->last_used_idx;
> >  
> > -	/* Split loop in two to save memory barriers */
> > -	for (i = 0; i < vq->shadow_used_idx; i++) {
> > -		vq->desc_packed[used_idx].id = vq->shadow_used_packed[i].id;
> > -		vq->desc_packed[used_idx].len = vq->shadow_used_packed[i].len;
> > -
> > -		used_idx += vq->shadow_used_packed[i].count;
> > -		if (used_idx >= vq->size)
> > -			used_idx -= vq->size;
> > -	}
> > -
> > -	rte_smp_wmb();
> > +	if (unlikely(vq->shadow_used_idx == 0))
> > +		return;
> >  
> >  	for (i = 0; i < vq->shadow_used_idx; i++) {
> >  		uint16_t flags;
> > @@ -165,12 +156,22 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> >  			flags &= ~VRING_DESC_F_AVAIL;
> >  		}
> >  
> > -		vq->desc_packed[vq->last_used_idx].flags = flags;
> > +		vq->desc_packed[vq->last_used_idx].id =
> > +			vq->shadow_used_packed[i].id;
> > +		vq->desc_packed[vq->last_used_idx].len =
> > +			vq->shadow_used_packed[i].len;
> > +
> > +		if (i > 0) {

Specifically here?

> > +			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) {
> > @@ -180,7 +181,15 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> >  	}
> >  
> >  	rte_smp_wmb();
> > +
> > +	vq->desc_packed[head_idx].flags = head_flags;
> >  	vq->shadow_used_idx = 0;
> > +
> > +	vhost_log_cache_used_vring(dev, vq,
> > +				head_idx *
> > +				sizeof(struct vring_packed_desc),
> > +				sizeof(struct vring_packed_desc));
> > +
> >  	vhost_log_cache_sync(dev, vq);
> >  }
> >  
> >