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    SubjectRe: [PATCHv2 10/14] virtio_net: limit xmit polling
    On Sat, May 21, 2011 at 11:49:59AM +0930, Rusty Russell wrote:
    > 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?

    Need to dig this up: I thought we saw some reports of this on the list?

    > 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?

    Agree.

    > > 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?

    This is in case we can't use indirect: we want to free up
    enough buffers for the following add_buf to succeed.


    > 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.

    I just wanted to localize the 2+MAX_SKB_FRAGS logic that tries to make
    sure we have enough space in the buffer. Another way to do
    that is with a define :).

    --
    MST


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