Re: [PATCH netdev] virtio-net: support XDP_TX when not more queues

On Wed, 10 Feb 2021 16:40:41 -0500
"Michael S. Tsirkin" [off-list ref] wrote:

On Wed, Jan 13, 2021 at 04:08:57PM +0800, Xuan Zhuo wrote:

quoted

The number of queues implemented by many virtio backends is limited,
especially some machines have a large number of CPUs. In this case, it
is often impossible to allocate a separate queue for XDP_TX.

This patch allows XDP_TX to run by reuse the existing SQ with
__netif_tx_lock() hold when there are not enough queues.

I'm a little puzzled about the choice of using the netdevice TXQ
lock __netif_tx_lock() / __netif_tx_unlock().
Can you explain more about this choice?

quoted

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>  

I'd like to get some advice on whether this is ok from some
XDP experts - previously my understanding was that it is
preferable to disable XDP for such devices than
use locks on XDP fast path.

I think it is acceptable, because the ndo_xdp_xmit / virtnet_xdp_xmit
takes a bulk of packets (currently 16).

Some drivers already does this.

It would have been nice if we could set a feature flag, that allow
users to see that this driver uses locking in the XDP transmit
(ndo_xdp_xmit) function call... but it seems like a pipe dream :-P

Code related to the locking

quoted

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ba8e637..7a3b2a7 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

[...]

quoted

@@ -481,14 +484,34 @@ static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
 	return 0;
 }
 
-static struct send_queue *virtnet_xdp_sq(struct virtnet_info *vi)
+static struct send_queue *virtnet_get_xdp_sq(struct virtnet_info *vi)
 {
 	unsigned int qp;
+	struct netdev_queue *txq;
+
+	if (vi->curr_queue_pairs > nr_cpu_ids) {
+		qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
+	} else {
+		qp = smp_processor_id() % vi->curr_queue_pairs;
+		txq = netdev_get_tx_queue(vi->dev, qp);
+		__netif_tx_lock(txq, raw_smp_processor_id());
+	}
 
-	qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
 	return &vi->sq[qp];
 }
 
+static void virtnet_put_xdp_sq(struct virtnet_info *vi)
+{
+	unsigned int qp;
+	struct netdev_queue *txq;
+
+	if (vi->curr_queue_pairs <= nr_cpu_ids) {
+		qp = smp_processor_id() % vi->curr_queue_pairs;
+		txq = netdev_get_tx_queue(vi->dev, qp);
+		__netif_tx_unlock(txq);
+	}
+}
+
 static int virtnet_xdp_xmit(struct net_device *dev,
 			    int n, struct xdp_frame **frames, u32 flags)
 {

@@ -512,7 +535,7 @@ static int virtnet_xdp_xmit(struct net_device *dev,
 	if (!xdp_prog)
 		return -ENXIO;
 
-	sq = virtnet_xdp_sq(vi);
+	sq = virtnet_get_xdp_sq(vi);
 
 	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK)) {
 		ret = -EINVAL;

@@ -560,12 +583,13 @@ static int virtnet_xdp_xmit(struct net_device *dev,
 	sq->stats.kicks += kicks;
 	u64_stats_update_end(&sq->stats.syncp);
 
+	virtnet_put_xdp_sq(vi);
 	return ret;
 }
 

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

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