Re: [dpdk-dev] [PATCH] vhost: avoid memory barriers when no descriptors dequeued

[Maxime Coquelin]

On 10/22/2018 09:15 AM, Tiwei Bie wrote:

On Fri, Oct 19, 2018 at 04:00:58PM +0200, Maxime Coquelin wrote:

In both split and packed dequeue paths, flush_shadow_used_ring
and vhost_ring_call variants gets called even if not packets
have been dequeued, and so no descriptors updates happened.

It has an impact on CPU pipeline, as memory barriers are used
in these functions.

This patch don't call these functions if no descriptors have
been dequeued. The performance gain with split ring when
dequeue zero-copy is disabled should be null, but should be
noticeable with packed ring or dequeue zero-copy enabled.

Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring")
Fixes: 915cf9404225 ("vhost: use shadow used ring in dequeue path")
Cc: sta...@dpdk.org

Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
---
  lib/librte_vhost/virtio_net.c | 16 ++++++++++------
  1 file changed, 10 insertions(+), 6 deletions(-)

[...]

- if (likely(dev->dequeue_zero_copy == 0)) {

+       if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {
                do_data_copy_dequeue(vq);
                if (unlikely(i < count))
                        vq->shadow_used_idx = i;

When i is 0, we may need to update vq->shadow_used_idx to 0,
e.g. when error happens after update_shadow_used_ring_split()
in the first iteration of the loop.

I totally agree, it is broken when error happens.
I will fix that in next revision.

Thanks,
Maxime

@@ -1475,8 +1477,10 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
                        }
                }

- flush_shadow_used_ring_packed(dev, vq);

-               vhost_vring_call_packed(dev, vq);
+               if (likely(vq->shadow_used_idx)) {
+                       flush_shadow_used_ring_packed(dev, vq);
+                       vhost_vring_call_packed(dev, vq);
+               }
        }

VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);

@@ -1550,7 +1554,7 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct 
vhost_virtqueue *vq,
                }
        }

- if (likely(dev->dequeue_zero_copy == 0)) {

+       if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {

Ditto

                do_data_copy_dequeue(vq);
                if (unlikely(i < count))
                        vq->shadow_used_idx = i;
--
2.17.1