Re: [PATCH RFC 3/3] virtio_net: limit xmit polling
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2011-06-02 14:43:46
On Thu, Jun 02, 2011 at 07:47:48PM +0530, Krishna Kumar2 wrote:
quoted
OK, I have something very similar, but I still dislike the screw the latency part: this path is exactly what the IBM guys seem to hit. So I created two functions: one tries to free a constant number and another one up to capacity. I'll post that now.Please review this patch to see if it looks reasonable:
Hmm, since you decided to work on top of my patch, I'd appreciate split-up fixes.
1. Picked comments/code from MST's code and Rusty's review. 2. virtqueue_min_capacity() needs to be called only if it returned empty the last time it was called. 3. Fix return value bug in free_old_xmit_skbs (hangs guest). 4. Stop queue only if capacity is not enough for next xmit.
That's what we always did ...
5. Fix/clean some likely/unlikely checks (hopefully). I have done some minimal netperf tests with this. With this patch, add_buf returning capacity seems to be useful - it allows less virtio API calls.
Why bother? It's cheap ...
quoted hunk
Signed-off-by: Krishna Kumar <redacted> --- drivers/net/virtio_net.c | 105 ++++++++++++++++++++++--------------- 1 file changed, 64 insertions(+), 41 deletions(-) diff -ruNp org/drivers/net/virtio_net.c new/drivers/net/virtio_net.c--- org/drivers/net/virtio_net.c 2011-06-02 15:49:25.000000000 +0530 +++ new/drivers/net/virtio_net.c 2011-06-02 19:13:02.000000000 +0530@@ -509,27 +509,43 @@ again: return received; } -/* Check capacity and try to free enough pending old buffers to enable queueing - * new ones. If min_skbs > 0, try to free at least the specified number of skbs - * even if the ring already has sufficient capacity. Return true if we can - * guarantee that a following virtqueue_add_buf will succeed. */ -static bool free_old_xmit_skbs(struct virtnet_info *vi, int min_skbs) +/* Return true if freed a skb, else false */ +static inline bool free_one_old_xmit_skb(struct virtnet_info *vi) { struct sk_buff *skb; unsigned int len; - bool r; - while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) || - min_skbs-- > 0) { - skb = virtqueue_get_buf(vi->svq, &len); - if (unlikely(!skb)) + skb = virtqueue_get_buf(vi->svq, &len); + if (unlikely(!skb)) + return 0; + + 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 1; +} + +static bool free_old_xmit_skbs(struct virtnet_info *vi, int to_free) +{ + bool empty = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2; + + do { + if (!free_one_old_xmit_skb(vi)) { + /* No more skbs to free up */ 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); - } - return r; + } + + if (empty) { + /* Check again if there is enough space */ + empty = virtqueue_min_capacity(vi->svq) < + MAX_SKB_FRAGS + 2; + } else { + --to_free; + } + } while (to_free > 0); + + return !empty; }
Why bother doing the capacity check in this function?
quoted hunk
static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)@@ -582,46 +598,53 @@ static int xmit_skb(struct virtnet_info static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) { struct virtnet_info *vi = netdev_priv(dev); - int ret, n; + int capacity; - /* Free up space in the ring in case this is the first time we get - * woken up after ring full condition. Note: this might try to free - * more than strictly necessary if the skb has a small - * number of fragments, but keep it simple. */ - free_old_xmit_skbs(vi, 0); + /* Try to free 2 buffers for every 1 xmit, to stay ahead. */ + free_old_xmit_skbs(vi, 2); /* Try to transmit */ - ret = xmit_skb(vi, skb); + capacity = xmit_skb(vi, skb); - /* Failure to queue is unlikely. It's not a bug though: it might happen - * if we get an interrupt while the queue is still mostly full. - * We could stop the queue and re-enable callbacks (and possibly return - * TX_BUSY), but as this should be rare, we don't bother. */ - if (unlikely(ret < 0)) { + if (unlikely(capacity < 0)) { + /* + * Failure to queue should be impossible. The only way to + * reach here is if we got a cb before 3/4th of space was + * available. We could stop the queue and re-enable + * callbacks (and possibly return TX_BUSY), but we don't + * bother since this is impossible.
It's far from impossible. The 3/4 thing is only a hint, and old devices don't support it anyway.
+ */
if (net_ratelimit())
- dev_info(&dev->dev, "TX queue failure: %d\n", ret);
+ dev_info(&dev->dev, "TX queue failure: %d\n", capacity);
dev->stats.tx_dropped++;
kfree_skb(skb);
return NETDEV_TX_OK;
}
+
virtqueue_kick(vi->svq);
/* Don't wait up for transmitted skbs to be freed. */
skb_orphan(skb);
nf_reset(skb);
- /* Apparently nice girls don't return TX_BUSY; check capacity and stop
- * the queue before it gets out of hand.
- * Naturally, this wastes entries. */
- /* We transmit one skb, so try to free at least two pending skbs.
- * This is so that we don't hog the skb memory unnecessarily. */
- if (!likely(free_old_xmit_skbs(vi, 2))) {
- netif_stop_queue(dev);
- if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
- /* More just got used, free them and recheck. */
- if (!likely(free_old_xmit_skbs(vi, 0))) {
- netif_start_queue(dev);
- virtqueue_disable_cb(vi->svq);
+ /*
+ * Apparently nice girls don't return TX_BUSY; check capacity and
+ * stop the queue before it gets out of hand. Naturally, this wastes
+ * entries.
+ */
+ if (capacity < 2+MAX_SKB_FRAGS) {
+ /*
+ * We don't have enough space for the next packet. Try
+ * freeing more.
+ */
+ if (likely(!free_old_xmit_skbs(vi, UINT_MAX))) {
+ netif_stop_queue(dev);
+ if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
+ /* More just got used, free them and recheck. */
+ if (likely(free_old_xmit_skbs(vi, UINT_MAX))) {Is this where the bug was?
+ netif_start_queue(dev); + virtqueue_disable_cb(vi->svq); + } } } }