[PATCHv2 10/14] virtio_net: limit xmit polling

Michael S. Tsirkin
2011-05-19T23:11:56Z

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-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
drivers/net/virtio_net.c | 22 ++++++++++++++--------
1 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f33c92b..42935cb 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -509,17 +509,25 @@ again:
return received;
}

-static void free_old_xmit_skbs(struct virtnet_info *vi)
+static bool free_old_xmit_skbs(struct virtnet_info *vi, int capacity)
{
struct sk_buff *skb;
unsigned int len;
-
- while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
+ bool c;
+ int n;
+
+ /* We try to free up at least 2 skbs per one sent, so that we'll get
+ * all of the memory back if they are used fast enough. */
+ for (n = 0;
+ ((c = virtqueue_get_capacity(vi->svq) < capacity) || n < 2) &&
+ ((skb = virtqueue_get_buf(vi->svq, &len)));
+ ++n) {
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 !c;
}

static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
@@ -574,8 +582,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
struct virtnet_info *vi = netdev_priv(dev);
int capacity;

- /* Free up any pending old buffers before queueing new ones. */
- free_old_xmit_skbs(vi);
+ /* Free enough pending old buffers to enable queueing new ones. */
+ free_old_xmit_skbs(vi, 2+MAX_SKB_FRAGS);

/* Try to transmit */
capacity = xmit_skb(vi, skb);
@@ -609,9 +617,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
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_get_capacity(vi->svq);
- if (capacity >= 2+MAX_SKB_FRAGS) {
+ if (!likely(free_old_xmit_skbs(vi, 2+MAX_SKB_FRAGS))) {
netif_start_queue(dev);
virtqueue_disable_cb(vi->svq);
}
--
1.7.5.53.gc233e