[dpdk-dev] [PATCH v2] virtio: fix rx ring descriptor starvation

[Kyle Larose]

Virtio has an mbuf descriptor ring containing mbufs to be used for receiving
traffic. When the host queues traffic to be sent to the guest, it consumes
these descriptors. If none exist, it discards the packet.

The virtio pmd allocates mbufs to the descriptor ring every time it
succesfully receives a packet. However, it never does it if it does not
receive a valid packet. If the descriptor ring is exhausted, and the mbuf
mempool does not have any mbufs free (which can happen for various reasons,
such as queueing along the processing pipeline), then the receive call will
not allocate any mbufs to the descriptor ring, and when it finishes, the
descriptor ring will be empty. The ring being empty means that we will never
receive a packet again, which means we will never allocate mbufs to the ring:
we are stuck.

Ultimately, the problem arises because there is a dependency between receiving
packets and making the descriptor ring not be empty, and a dependency between
the descriptor ring not being empty, and receiving packets.

To fix the problem, this pakes makes virtio always try to allocate mbufs to the
descriptor ring, if necessary, when polling for packets. Do this by removing
the early exit if no packets were received. Since the packet loop later will do
nothing if there are no packets, this is fine.

I reproduced the problem by pushing packets through a pipelined systems (such
as the client_server sample application) after artificially decreasing the
size of the mbuf pool and introducing a delay in a secondary stage.

Without the fix, the process stops receiving packets fairly quicky. With the
fix, it continues to receive packets.

Signed-off-by: Kyle Larose <klarose at sandvine.com>
---
v2:
 * Added missing sign-off.
 * Cleaned up the commit message a bit.
---
 drivers/net/virtio/virtio_rxtx.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
index 41a1366..9d2f7d6 100644
--- a/drivers/net/virtio/virtio_rxtx.c
+++ b/drivers/net/virtio/virtio_rxtx.c
@@ -571,9 +571,6 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
 	if (likely(num > DESC_PER_CACHELINE))
 		num = num - ((rxvq->vq_used_cons_idx + num) % DESC_PER_CACHELINE);
 
-	if (num == 0)
-		return 0;
-
 	num = virtqueue_dequeue_burst_rx(rxvq, rcv_pkts, len, num);
 	PMD_RX_LOG(DEBUG, "used:%d dequeue:%d", nb_used, num);
 
@@ -671,9 +668,6 @@ virtio_recv_mergeable_pkts(void *rx_queue,
 
 	virtio_rmb();
 
-	if (nb_used == 0)
-		return 0;
-
 	PMD_RX_LOG(DEBUG, "used:%d\n", nb_used);
 
 	hw = rxvq->hw;
-- 
1.8.3.1