[PATCH RFC v5 net-next 3/6] virtio_net: enable tx interrupt

Jason Wang
2015-02-09T08:39:59Z

On newer hosts that support delayed tx interrupts,
we probably don't have much to gain from orphaning
packets early.

Note: this might degrade performance for
hosts without event idx support.
Should be addressed by the next patch.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
Changes from RFCv4:
- change:
virtqueue_disable_cb(sq->vq);
napi_schedule(&sq->napi);

in skb_xmit_done() to:

if (__napi_schedule_prep(&sq->napi)) {
virtqueue_diable_cb(sq->vq);
__napi_schedule(&sq->napi);
}

to solve the race on architectures that atomic operations were not
serialized. And do solve a similar issue in virtnet_poll_tx().
- use netif_wake_subqueue() instead of netif_start_subqueue() in
free_old_xmit_skbs(), since it may be called in tx napi.
- in start_xmit(), try to enable the callback only when current skb is
the last in the list or tx has already been stopped. This avoid the
callbacks enabling in heavy load.
---
drivers/net/virtio_net.c | 136 +++++++++++++++++++++++++++++++----------------
1 file changed, 90 insertions(+), 46 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 11e2e81..cc5f5de 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -72,6 +72,8 @@ struct send_queue {

/* Name of the send queue: output.$index */
char name[40];
+
+ struct napi_struct napi;
};

/* Internal representation of a receive virtqueue */
@@ -140,6 +142,9 @@ struct virtnet_info {

/* CPU hot plug notifier */
struct notifier_block nb;
+
+ /* Budget for polling tx completion */
+ u32 tx_work_limit;
};

struct padded_vnet_hdr {
@@ -207,15 +212,43 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
return p;
}

+static unsigned int free_old_xmit_skbs(struct netdev_queue *txq,
+ struct send_queue *sq, int budget)
+{
+ struct sk_buff *skb;
+ unsigned int len;
+ struct virtnet_info *vi = sq->vq->vdev->priv;
+ struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
+ unsigned int packets = 0;
+
+ while (packets < budget &&
+ (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
+ pr_debug("Sent skb %p\n", skb);
+
+ u64_stats_update_begin(&stats->tx_syncp);
+ stats->tx_bytes += skb->len;
+ stats->tx_packets++;
+ u64_stats_update_end(&stats->tx_syncp);
+
+ dev_kfree_skb_any(skb);
+ packets++;
+ }
+
+ if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+ netif_wake_subqueue(vi->dev, vq2txq(sq->vq));
+
+ return packets;
+}
+
static void skb_xmit_done(struct virtqueue *vq)
{
struct virtnet_info *vi = vq->vdev->priv;
+ struct send_queue *sq = &vi->sq[vq2txq(vq)];

- /* Suppress further interrupts. */
- virtqueue_disable_cb(vq);
-
- /* We were probably waiting for more output buffers. */
- netif_wake_subqueue(vi->dev, vq2txq(vq));
+ if (napi_schedule_prep(&sq->napi)) {
+ virtqueue_disable_cb(sq->vq);
+ __napi_schedule(&sq->napi);
+ }
}

static unsigned int mergeable_ctx_to_buf_truesize(unsigned long mrg_ctx)
@@ -776,6 +809,30 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
return received;
}

+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));
+ u32 limit = vi->tx_work_limit;
+ unsigned int r, sent;
+
+ __netif_tx_lock(txq, smp_processor_id());
+ sent = free_old_xmit_skbs(txq, sq, limit);
+ if (sent < limit) {
+ r = virtqueue_enable_cb_prepare(sq->vq);
+ napi_complete(napi);
+ if (unlikely(virtqueue_poll(sq->vq, r)) &&
+ napi_schedule_prep(napi)) {
+ virtqueue_disable_cb(sq->vq);
+ __napi_schedule(napi);
+ }
+ }
+ __netif_tx_unlock(txq);
+ return sent < limit ? 0 : budget;
+}
+
#ifdef CONFIG_NET_RX_BUSY_POLL
/* must be called with local_bh_disable()d */
static int virtnet_busy_poll(struct napi_struct *napi)
@@ -824,30 +881,12 @@ 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]);
+ napi_enable(&vi->sq[i].napi);
}

return 0;
}

-static void free_old_xmit_skbs(struct send_queue *sq)
-{
- struct sk_buff *skb;
- unsigned int len;
- struct virtnet_info *vi = sq->vq->vdev->priv;
- struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
-
- while ((skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
- pr_debug("Sent skb %p\n", skb);
-
- u64_stats_update_begin(&stats->tx_syncp);
- stats->tx_bytes += skb->len;
- stats->tx_packets++;
- u64_stats_update_end(&stats->tx_syncp);
-
- dev_kfree_skb_any(skb);
- }
-}
-
static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
{
struct virtio_net_hdr_mrg_rxbuf *hdr;
@@ -910,7 +949,9 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
sg_set_buf(sq->sg, hdr, hdr_len);
num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
}
- return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
+
+ return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb,
+ GFP_ATOMIC);
}

static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
@@ -922,8 +963,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
bool kick = !skb->xmit_more;

- /* Free up any pending old buffers before queueing new ones. */
- free_old_xmit_skbs(sq);
+ virtqueue_disable_cb(sq->vq);

/* Try to transmit */
err = xmit_skb(sq, skb);
@@ -939,27 +979,19 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
return NETDEV_TX_OK;
}

- /* Don't wait up for transmitted skbs to be freed. */
- skb_orphan(skb);
- nf_reset(skb);
-
/* Apparently nice girls don't return TX_BUSY; stop the queue
* before it gets out of hand. Naturally, this wastes entries. */
- if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
+ if (sq->vq->num_free < 2+MAX_SKB_FRAGS)
netif_stop_subqueue(dev, qnum);
- if (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) {
- netif_start_subqueue(dev, qnum);
- virtqueue_disable_cb(sq->vq);
- }
- }
- }

- if (kick || netif_xmit_stopped(txq))
+ if (kick || netif_xmit_stopped(txq)) {
virtqueue_kick(sq->vq);
-
+ if (!virtqueue_enable_cb_delayed(sq->vq) &&
+ napi_schedule_prep(&sq->napi)) {
+ virtqueue_disable_cb(sq->vq);
+ __napi_schedule(&sq->napi);
+ }
+ }
return NETDEV_TX_OK;
}

@@ -1137,8 +1169,10 @@ 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;
}
@@ -1451,8 +1485,10 @@ static void virtnet_free_queues(struct virtnet_info *vi)
{
int i;

- for (i = 0; i < vi->max_queue_pairs; i++)
+ for (i = 0; i < vi->max_queue_pairs; i++) {
netif_napi_del(&vi->rq[i].napi);
+ netif_napi_del(&vi->sq[i].napi);
+ }

kfree(vi->rq);
kfree(vi->sq);
@@ -1606,6 +1642,8 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
netif_napi_add(vi->dev, &vi->rq[i].napi, virtnet_poll,
napi_weight);
napi_hash_add(&vi->rq[i].napi);
+ netif_napi_add(vi->dev, &vi->sq[i].napi, virtnet_poll_tx,
+ napi_weight);

sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
ewma_init(&vi->rq[i].mrg_avg_pkt_len, 1, RECEIVE_AVG_WEIGHT);
@@ -1830,6 +1868,8 @@ static int virtnet_probe(struct virtio_device *vdev)
if (err)
goto free_stats;

+ vi->tx_work_limit = napi_weight;
+
#ifdef CONFIG_SYSFS
if (vi->mergeable_rx_bufs)
dev->sysfs_rx_queue_group = &virtio_net_mrg_rx_group;
@@ -1944,8 +1984,10 @@ static int virtnet_freeze(struct virtio_device *vdev)
if (netif_running(vi->dev)) {
for (i = 0; i < vi->max_queue_pairs; i++) {
napi_disable(&vi->rq[i].napi);
+ napi_disable(&vi->sq[i].napi);
napi_hash_del(&vi->rq[i].napi);
netif_napi_del(&vi->rq[i].napi);
+ netif_napi_del(&vi->sq[i].napi);
}
}

@@ -1970,8 +2012,10 @@ static int virtnet_restore(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]);
+ napi_enable(&vi->sq[i].napi);
+ }
}

netif_device_attach(vi->dev);
--
1.8.3.1