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Subject[PATCH RFC 3/3] virtio_net: limit xmit polling
Current code might introduce a lot of latency variation
if there are many pending bufs at the time we
attempt to transmit a new one. This is bad for
real-time applications and can't be good for TCP either.

Free up just enough to both clean up all buffers
eventually and to be able to xmit the next packet.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/net/virtio_net.c | 62 ++++++++++++++++++++++++++--------------------
1 files changed, 35 insertions(+), 27 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index a0ee78d..6045510 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -509,17 +509,27 @@ again:
return received;
}

-static void free_old_xmit_skbs(struct virtnet_info *vi)
+/* Check capacity and try to free enough pending old buffers to enable queueing
+ * new ones. If min_skbs > 0, try to free at least the specified number of skbs
+ * even if the ring already has sufficient capacity. Return true if we can
+ * guarantee that a following virtqueue_add_buf will succeed. */
+static bool free_old_xmit_skbs(struct virtnet_info *vi, int min_skbs)
{
struct sk_buff *skb;
unsigned int len;
+ bool r;

- while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
+ while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
+ min_skbs-- > 0) {
+ skb = virtqueue_get_buf(vi->svq, &len);
+ if (unlikely(!skb))
+ break;
pr_debug("Sent skb %p\n", skb);
vi->dev->stats.tx_bytes += skb->len;
vi->dev->stats.tx_packets++;
dev_kfree_skb_any(skb);
}
+ return r;
}

static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
@@ -572,27 +582,24 @@ static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
{
struct virtnet_info *vi = netdev_priv(dev);
- int capacity;
+ int ret, n;

- /* Free up any pending old buffers before queueing new ones. */
- free_old_xmit_skbs(vi);
+ /* Free up space in the ring in case this is the first time we get
+ * woken up after ring full condition. Note: this might try to free
+ * more than strictly necessary if the skb has a small
+ * number of fragments, but keep it simple. */
+ free_old_xmit_skbs(vi, 0);

/* Try to transmit */
- capacity = xmit_skb(vi, skb);
-
- /* This can happen with OOM and indirect buffers. */
- if (unlikely(capacity < 0)) {
- if (net_ratelimit()) {
- if (likely(capacity == -ENOMEM)) {
- dev_warn(&dev->dev,
- "TX queue failure: out of memory\n");
- } else {
- dev->stats.tx_fifo_errors++;
- dev_warn(&dev->dev,
- "Unexpected TX queue failure: %d\n",
- capacity);
- }
- }
+ ret = xmit_skb(vi, skb);
+
+ /* Failure to queue is unlikely. It's not a bug though: it might happen
+ * if we get an interrupt while the queue is still mostly full.
+ * We could stop the queue and re-enable callbacks (and possibly return
+ * TX_BUSY), but as this should be rare, we don't bother. */
+ if (unlikely(ret < 0)) {
+ if (net_ratelimit())
+ dev_info(&dev->dev, "TX queue failure: %d\n", ret);
dev->stats.tx_dropped++;
kfree_skb(skb);
return NETDEV_TX_OK;
@@ -603,15 +610,16 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
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 (capacity < 2+MAX_SKB_FRAGS) {
+ /* Apparently nice girls don't return TX_BUSY; check capacity and stop
+ * the queue before it gets out of hand.
+ * Naturally, this wastes entries. */
+ /* We transmit one skb, so try to free at least two pending skbs.
+ * This is so that we don't hog the skb memory unnecessarily. */
+ if (!likely(free_old_xmit_skbs(vi, 2))) {
netif_stop_queue(dev);
if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
- /* More just got used, free them then recheck. */
- free_old_xmit_skbs(vi);
- capacity = virtqueue_min_capacity(vi->svq);
- if (capacity >= 2+MAX_SKB_FRAGS) {
+ /* More just got used, free them and recheck. */
+ if (!likely(free_old_xmit_skbs(vi, 0))) {
netif_start_queue(dev);
virtqueue_disable_cb(vi->svq);
}
--
1.7.5.53.gc233e

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