On Sun, Apr 02, 2017 at 04:10:11PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@xxxxxxxxxx>
>
> Convert virtio-net to a standard napi tx completion path. This enables
> better TCP pacing using TCP small queues and increases single stream
> throughput.
>
> The virtio-net driver currently cleans tx descriptors on transmission
> of new packets in ndo_start_xmit. Latency depends on new traffic, so
> is unbounded. To avoid deadlock when a socket reaches its snd limit,
> packets are orphaned on tranmission. This breaks socket backpressure,
> including TSQ.
>
> Napi increases the number of interrupts generated compared to the
> current model, which keeps interrupts disabled as long as the ring
> has enough free descriptors. Keep tx napi optional for now. Follow-on
> patches will reduce the interrupt cost.
>
> Signed-off-by: Willem de Bruijn <willemb@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> drivers/net/virtio_net.c | 63 ++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 53 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6aac0ad0d9b2..95d938e82080 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -33,9 +33,10 @@
> static int napi_weight = NAPI_POLL_WEIGHT;
> module_param(napi_weight, int, 0444);
>
> -static bool csum = true, gso = true;
> +static bool csum = true, gso = true, napi_tx = true;
> module_param(csum, bool, 0444);
> module_param(gso, bool, 0444);
> +module_param(napi_tx, bool, 0644);
>
> /* FIXME: MTU in config. */
> #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
Off by default seems safer until we can find better ways
to reduce the overhead, esp for UDP.
> @@ -86,6 +87,8 @@ struct send_queue {
>
> /* Name of the send queue: output.$index */
> char name[40];
> +
> + struct napi_struct napi;
> };
>
> /* Internal representation of a receive virtqueue */
> @@ -262,12 +265,16 @@ static void virtqueue_napi_complete(struct napi_struct *napi,
> static void skb_xmit_done(struct virtqueue *vq)
> {
> struct virtnet_info *vi = vq->vdev->priv;
> + struct napi_struct *napi = &vi->sq[vq2txq(vq)].napi;
>
> /* Suppress further interrupts. */
> virtqueue_disable_cb(vq);
>
> - /* We were probably waiting for more output buffers. */
> - netif_wake_subqueue(vi->dev, vq2txq(vq));
> + if (napi->weight)
> + virtqueue_napi_schedule(napi, vq);
> + else
> + /* We were probably waiting for more output buffers. */
> + netif_wake_subqueue(vi->dev, vq2txq(vq));
> }
>
> static unsigned int mergeable_ctx_to_buf_truesize(unsigned long mrg_ctx)
> @@ -961,6 +968,9 @@ static void skb_recv_done(struct virtqueue *rvq)
>
> static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
> {
> + if (!napi->weight)
> + return;
> +
> napi_enable(napi);
>
> /* If all buffers were filled by other side before we napi_enabled, we
> @@ -1046,6 +1056,7 @@ static int virtnet_open(struct net_device *dev)
> if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> schedule_delayed_work(&vi->refill, 0);
> virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
> + virtnet_napi_enable(vi->sq[i].vq, &vi->sq[i].napi);
> }
>
> return 0;
> @@ -1081,6 +1092,24 @@ static void free_old_xmit_skbs(struct send_queue *sq)
> u64_stats_update_end(&stats->tx_syncp);
> }
>
> +static int virtnet_poll_tx(struct napi_struct *napi, int budget)
> +{
> + struct send_queue *sq = container_of(napi, struct send_queue, napi);
> + struct virtnet_info *vi = sq->vq->vdev->priv;
> + struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, vq2txq(sq->vq));
> +
> + __netif_tx_lock(txq, smp_processor_id());
> + free_old_xmit_skbs(sq);
> + __netif_tx_unlock(txq);
> +
> + virtqueue_napi_complete(napi, sq->vq, 0);
> +
> + if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> + netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
> +
> + return 0;
> +}
> +
> static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
> {
> struct virtio_net_hdr_mrg_rxbuf *hdr;
> @@ -1130,9 +1159,11 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> int err;
> struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
> bool kick = !skb->xmit_more;
> + bool use_napi = sq->napi.weight;
>
> /* Free up any pending old buffers before queueing new ones. */
> - free_old_xmit_skbs(sq);
> + if (!use_napi)
> + free_old_xmit_skbs(sq);
>
> /* timestamp packet in software */
> skb_tx_timestamp(skb);
> @@ -1152,8 +1183,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> }
>
> /* Don't wait up for transmitted skbs to be freed. */
> - skb_orphan(skb);
> - nf_reset(skb);
> + if (!use_napi) {
> + skb_orphan(skb);
> + nf_reset(skb);
> + }
>
> /* If running out of space, stop queue to avoid getting packets that we
> * are then unable to transmit.
> @@ -1167,7 +1200,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> */
> if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> netif_stop_subqueue(dev, qnum);
> - if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> + if (!use_napi &&
> + unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> /* More just got used, free them then recheck. */
> free_old_xmit_skbs(sq);
> if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> @@ -1371,8 +1405,10 @@ static int virtnet_close(struct net_device *dev)
> /* Make sure refill_work doesn't re-enable napi! */
> cancel_delayed_work_sync(&vi->refill);
>
> - for (i = 0; i < vi->max_queue_pairs; i++)
> + for (i = 0; i < vi->max_queue_pairs; i++) {
> napi_disable(&vi->rq[i].napi);
> + napi_disable(&vi->sq[i].napi);
> + }
>
> return 0;
> }
> @@ -1727,8 +1763,10 @@ static void virtnet_freeze_down(struct virtio_device *vdev)
> cancel_delayed_work_sync(&vi->refill);
>
> if (netif_running(vi->dev)) {
> - for (i = 0; i < vi->max_queue_pairs; i++)
> + for (i = 0; i < vi->max_queue_pairs; i++) {
> napi_disable(&vi->rq[i].napi);
> + napi_disable(&vi->sq[i].napi);
> + }
> }
> }
>
> @@ -1751,8 +1789,10 @@ static int virtnet_restore_up(struct virtio_device *vdev)
> if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> schedule_delayed_work(&vi->refill, 0);
>
> - for (i = 0; i < vi->max_queue_pairs; i++)
> + for (i = 0; i < vi->max_queue_pairs; i++) {
> virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
> + virtnet_napi_enable(vi->sq[i].vq, &vi->sq[i].napi);
> + }
> }
>
> netif_device_attach(vi->dev);
> @@ -1957,6 +1997,7 @@ static void virtnet_free_queues(struct virtnet_info *vi)
> for (i = 0; i < vi->max_queue_pairs; i++) {
> napi_hash_del(&vi->rq[i].napi);
> netif_napi_del(&vi->rq[i].napi);
> + netif_napi_del(&vi->sq[i].napi);
> }
>
> /* We called napi_hash_del() before netif_napi_del(),
> @@ -2142,6 +2183,8 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
> vi->rq[i].pages = NULL;
> netif_napi_add(vi->dev, &vi->rq[i].napi, virtnet_poll,
> napi_weight);
> + netif_napi_add(vi->dev, &vi->sq[i].napi, virtnet_poll_tx,
> + napi_tx ? napi_weight : 0);
>
> sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
> ewma_pkt_len_init(&vi->rq[i].mrg_avg_pkt_len);
> --
> 2.12.2.564.g063fe858b8-goog
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