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@xxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html