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    SubjectRe: [PATCHv2 10/14] virtio_net: limit xmit polling
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    On Fri, 20 May 2011 02:11:56 +0300, "Michael S. Tsirkin" <mst@redhat.com> 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.


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