Re: [PATCHv2 10/14] virtio_net: limit xmit polling

Rusty Russell
2011-05-21T02:19:59Z

On Fri, 20 May 2011 02:11:56 +0300, "Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> 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.

Do we have more than speculation to back that up, BTW?

This patch is pretty sloppy; the previous ones were better polished.

> -static void free_old_xmit_skbs(struct virtnet_info *vi)
> +static bool free_old_xmit_skbs(struct virtnet_info *vi, int capacity)
> {

A comment here indicating it returns true if it frees something?

> 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;

This is for() abuse :)

Why is the capacity check in there at all? Surely it's simpler to try
to free 2 skbs each time around?

for (n = 0; n < 2; n++) {
skb = virtqueue_get_buf(vi->svq, &len);
if (!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);
}

> 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))) {

This extra argument to free_old_xmit_skbs seems odd, unless you have
future plans?

Thanks,
Rusty.