Re: [PATCHv2 10/14] virtio_net: limit xmit polling
From: Rusty Russell <hidden>
Date: 2011-05-21 02:34:49
Also in:
kvm, lkml
On Fri, 20 May 2011 02:11:56 +0300, "Michael S. Tsirkin" [off-list ref] 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);
}
quoted hunk ↗ jump to hunk
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.