[dpdk-stable] [PATCH v2] vhost: fix shadow update

[dpdk-stable] [PATCH v2] vhost: fix shadow update

[Marvin Liu]

Defer shadow ring update introduces functional issue which has been
described in Eugenio's fix patch.

The current implementation of vhost_net in packed vring tries to fill
the shadow vector before send any actual changes to the guest. While
this can be beneficial for the throughput, it conflicts with some
bufferfloats methods like the linux kernel napi, that stops
transmitting packets if there are too much bytes/buffers in the
driver.

It also introduces performance issue when frontend run much faster than
backend. Frontend may not be able to collect available descs when shadow
update is deferred. That will harm RFC2544 throughput.

Appropriate choice is to remove deferred shadowed update method.
Now shadowed used descs are flushed at the end of dequeue function.

Fixes: 31d6c6a5b820 ("vhost: optimize packed ring dequeue")
Cc: stable@dpdk.org

Signed-off-by: Marvin Liu <yong.liu@intel.com>
Tested-by: Wang, Yinan <yinan.wang@intel.com>

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 210415904..4a7531943 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -382,25 +382,6 @@ vhost_shadow_enqueue_single_packed(struct virtio_net *dev,
 	}
 }
 
-static __rte_always_inline void
-vhost_flush_dequeue_packed(struct virtio_net *dev,
-			   struct vhost_virtqueue *vq)
-{
-	int shadow_count;
-	if (!vq->shadow_used_idx)
-		return;
-
-	shadow_count = vq->last_used_idx - vq->shadow_last_used_idx;
-	if (shadow_count <= 0)
-		shadow_count += vq->size;
-
-	if ((uint32_t)shadow_count >= (vq->size - MAX_PKT_BURST)) {
-		do_data_copy_dequeue(vq);
-		vhost_flush_dequeue_shadow_packed(dev, vq);
-		vhost_vring_call_packed(dev, vq);
-	}
-}
-
 /* avoid write operation when necessary, to lessen cache issues */
 #define ASSIGN_UNLESS_EQUAL(var, val) do {	\
 	if ((var) != (val))			\
@@ -2133,20 +2114,6 @@ virtio_dev_tx_packed_zmbuf(struct virtio_net *dev,
 	return pkt_idx;
 }
 
-static __rte_always_inline bool
-next_desc_is_avail(const struct vhost_virtqueue *vq)
-{
-	bool wrap_counter = vq->avail_wrap_counter;
-	uint16_t next_used_idx = vq->last_used_idx + 1;
-
-	if (next_used_idx >= vq->size) {
-		next_used_idx -= vq->size;
-		wrap_counter ^= 1;
-	}
-
-	return desc_is_avail(&vq->desc_packed[next_used_idx], wrap_counter);
-}
-
 static __rte_noinline uint16_t
 virtio_dev_tx_packed(struct virtio_net *dev,
 		     struct vhost_virtqueue *vq,
@@ -2163,7 +2130,6 @@ virtio_dev_tx_packed(struct virtio_net *dev,
 		if (remained >= PACKED_BATCH_SIZE) {
 			if (!virtio_dev_tx_batch_packed(dev, vq, mbuf_pool,
 							&pkts[pkt_idx])) {
-				vhost_flush_dequeue_packed(dev, vq);
 				pkt_idx += PACKED_BATCH_SIZE;
 				remained -= PACKED_BATCH_SIZE;
 				continue;
@@ -2173,7 +2139,6 @@ virtio_dev_tx_packed(struct virtio_net *dev,
 		if (virtio_dev_tx_single_packed(dev, vq, mbuf_pool,
 						&pkts[pkt_idx]))
 			break;
-		vhost_flush_dequeue_packed(dev, vq);
 		pkt_idx++;
 		remained--;
 
@@ -2182,15 +2147,8 @@ virtio_dev_tx_packed(struct virtio_net *dev,
 	if (vq->shadow_used_idx) {
 		do_data_copy_dequeue(vq);
 
-		if (remained && !next_desc_is_avail(vq)) {
-			/*
-			 * The guest may be waiting to TX some buffers to
-			 * enqueue more to avoid bufferfloat, so we try to
-			 * reduce latency here.
-			 */
-			vhost_flush_dequeue_shadow_packed(dev, vq);
-			vhost_vring_call_packed(dev, vq);
-		}
+		vhost_flush_dequeue_shadow_packed(dev, vq);
+		vhost_vring_call_packed(dev, vq);
 	}
 
 	return pkt_idx;
-- 
2.17.1

Re: [dpdk-stable] [PATCH v2] vhost: fix shadow update

[Maxime Coquelin]

On 4/17/20 4:39 AM, Marvin Liu wrote:
> Defer shadow ring update introduces functional issue which has been
> described in Eugenio's fix patch.
> 
> The current implementation of vhost_net in packed vring tries to fill
> the shadow vector before send any actual changes to the guest. While
> this can be beneficial for the throughput, it conflicts with some
> bufferfloats methods like the linux kernel napi, that stops
> transmitting packets if there are too much bytes/buffers in the
> driver.
> 
> It also introduces performance issue when frontend run much faster than
> backend. Frontend may not be able to collect available descs when shadow
> update is deferred. That will harm RFC2544 throughput.
> 
> Appropriate choice is to remove deferred shadowed update method.
> Now shadowed used descs are flushed at the end of dequeue function.
> 
> Fixes: 31d6c6a5b820 ("vhost: optimize packed ring dequeue")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Marvin Liu <yong.liu@intel.com>
> Tested-by: Wang, Yinan <yinan.wang@intel.com>
> 

Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Thanks,
Maxime

Re: [dpdk-stable] [PATCH v2] vhost: fix shadow update

[Maxime Coquelin]

On 4/17/20 4:39 AM, Marvin Liu wrote:
> Defer shadow ring update introduces functional issue which has been
> described in Eugenio's fix patch.
> 
> The current implementation of vhost_net in packed vring tries to fill
> the shadow vector before send any actual changes to the guest. While
> this can be beneficial for the throughput, it conflicts with some
> bufferfloats methods like the linux kernel napi, that stops
> transmitting packets if there are too much bytes/buffers in the
> driver.
> 
> It also introduces performance issue when frontend run much faster than
> backend. Frontend may not be able to collect available descs when shadow
> update is deferred. That will harm RFC2544 throughput.
> 
> Appropriate choice is to remove deferred shadowed update method.
> Now shadowed used descs are flushed at the end of dequeue function.
> 
> Fixes: 31d6c6a5b820 ("vhost: optimize packed ring dequeue")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Marvin Liu <yong.liu@intel.com>
> Tested-by: Wang, Yinan <yinan.wang@intel.com>

Applied to dpdk-next-virtio/master

Thanks,
Maxime