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SubjectRe: [PATCH RFC 3/3] virtio_net: limit xmit polling
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> 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 (inline and
attachment):

1. Picked comments/code from Michael'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.
5. Fix/clean some likely/unlikely checks (hopefully).
6. I think xmit_skb cannot return error since
virtqueue_enable_cb_delayed() can return false only if
3/4th space became available, which is what we check.
6. The comments for free_old_xmit_skbs needs to be more
clear (not done).

I have done some minimal netperf tests with this.

With this patch, add_buf returning capacity seems to be useful - it
allows using fewer virtio API calls.

(See attached file: patch)

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
---
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;
}

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.
+ */
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))) {
+ netif_start_queue(dev);
+ virtqueue_disable_cb(vi->svq);
+ }
}
}
}[unhandled content-type:application/octet-stream]
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