Re: [PATCH net-next RFC 3/3] virtio-net: conditionally enable tx interrupt

On 10/15/2014 05:51 AM, Michael S. Tsirkin wrote:

On Sat, Oct 11, 2014 at 03:16:46PM +0800, Jason Wang wrote:

quoted

quoted

We free transmitted packets in ndo_start_xmit() in the past to get better
performance in the past. One side effect is that skb_orphan() needs to be
called in ndo_start_xmit() which makes sk_wmem_alloc not accurate in
fact. For TCP protocol, this means several optimization could not work well
such as TCP small queue and auto corking. This can lead extra low
throughput of small packets stream.

Thanks to the urgent descriptor support. This patch tries to solve this
issue by enable the tx interrupt selectively for stream packets. This means
we don't need to orphan TCP stream packets in ndo_start_xmit() but enable
tx interrupt for those packets. After we get tx interrupt, a tx napi was
scheduled to free those packets.

With this method, sk_wmem_alloc of TCP socket were more accurate than in
the past which let TCP can batch more through TSQ and auto corking.

Signed-off-by: Jason Wang <redacted>
---
 drivers/net/virtio_net.c | 164 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 128 insertions(+), 36 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 5810841..b450fc4 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -72,6 +72,8 @@ struct send_queue {
 
 	/* Name of the send queue: output.$index */
 	char name[40];
+
+	struct napi_struct napi;
 };
 
 /* Internal representation of a receive virtqueue */

@@ -217,15 +219,40 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
 	return p;
 }
 
+static int free_old_xmit_skbs(struct send_queue *sq, int budget)
+{
+	struct sk_buff *skb;
+	unsigned int len;
+	struct virtnet_info *vi = sq->vq->vdev->priv;
+	struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
+	int sent = 0;
+
+	while (sent < budget &&
+	       (skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
+		pr_debug("Sent skb %p\n", skb);
+
+		u64_stats_update_begin(&stats->tx_syncp);
+		stats->tx_bytes += skb->len;
+		stats->tx_packets++;
+		u64_stats_update_end(&stats->tx_syncp);
+
+		dev_kfree_skb_any(skb);
+		sent++;
+	}
+
+	return sent;
+}
+
 static void skb_xmit_done(struct virtqueue *vq)
 {
 	struct virtnet_info *vi = vq->vdev->priv;
+	struct send_queue *sq = &vi->sq[vq2txq(vq)];
 
-	/* Suppress further interrupts. */
-	virtqueue_disable_cb(vq);
-
-	/* We were probably waiting for more output buffers. */
-	netif_wake_subqueue(vi->dev, vq2txq(vq));
+	if (napi_schedule_prep(&sq->napi)) {
+		virtqueue_disable_cb(vq);
+		virtqueue_disable_cb_urgent(vq);

This disable_cb is no longer safe in xmit_done callback,
since queue can be running at the same time.

You must do it under tx lock. And yes, this likely will not work
work well without event_idx. We'll probably need extra
synchronization for such old hosts.

Yes, and the virtqueue_enable_cb_prepare() in virtnet_poll_tx() needs to
be synced with virtqueue_enabled_cb_dealyed(). Otherwise an old idx will
be published and we may still see tx interrupt storm.