From 3eabd79c5036ca82cd04d35388f9d013ed25020d Mon Sep 17 00:00:00 2001 From: Kyle Larose Date: Thu, 10 Mar 2016 09:44:11 -0500 Subject: [PATCH] virtio: fix Rx ring descriptor starvation 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 successfully 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. Fixes: c1f86306a026 ("virtio: add new driver") Signed-off-by: Kyle Larose Acked-by: Huawei Xie --- 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 b4017bee9e..2b88efddd9 100644 --- a/drivers/net/virtio/virtio_rxtx.c +++ b/drivers/net/virtio/virtio_rxtx.c @@ -613,9 +613,6 @@ 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); @@ -713,9 +710,6 @@ 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; -- 2.20.1