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 and disabled 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 | 77 +++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 67 insertions(+), 10 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b9c1df29892c..c173e85dc7b8 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;
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)
@@ -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)
@@ -972,6 +979,21 @@ static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
local_bh_enable();
}
+static void virtnet_napi_tx_enable(struct virtnet_info *vi,
+ struct virtqueue *vq,
+ struct napi_struct *napi)
+{
+ if (!napi->weight)
+ return;
+
+ if (!vi->affinity_hint_set) {
+ napi->weight = 0;
+ return;
+ }
+
+ return virtnet_napi_enable(vq, napi);
+}
+
static void refill_work(struct work_struct *work)
{
struct virtnet_info *vi =
@@ -1046,6 +1068,7 @@ 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_tx_enable(vi, vi->sq[i].vq, &vi->sq[i].napi);
}
return 0;
@@ -1081,6 +1104,25 @@ 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));
+
+ if (__netif_tx_trylock(txq)) {
+ 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_tx_wake_queue(txq);
+
+ return 0;
+}
+
static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
{
struct virtio_net_hdr_mrg_rxbuf *hdr;
@@ -1130,9 +1172,11 @@ 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);
