vsock/virtio: avoid queuing packets when intermediate queue is empty

When the driver needs to send new packets to the device, it always
queues the new sk_buffs into an intermediate queue (send_pkt_queue)
and schedules a worker (send_pkt_work) to then queue them into the
virtqueue exposed to the device.

This increases the chance of batching, but also introduces a lot of
latency into the communication. So we can optimize this path by
adding a fast path to be taken when there is no element in the
intermediate queue, there is space available in the virtqueue,
and no other process that is sending packets (tx_lock held).

The following benchmarks were run to check improvements in latency and
throughput. The test bed is a host with Intel i7-10700KF CPU @ 3.80GHz
and L1 guest running on QEMU/KVM with vhost process and all vCPUs
pinned individually to pCPUs.

- Latency
   Tool: Fio version 3.37-56
   Mode: pingpong (h-g-h)
   Test runs: 50
   Runtime-per-test: 50s
   Type: SOCK_STREAM

In the following fio benchmark (pingpong mode) the host sends
a payload to the guest and waits for the same payload back.

fio process pinned both inside the host and the guest system.

Before: Linux 6.9.8

Payload 64B:

	1st perc.	overall		99th perc.
Before	12.91		16.78		42.24		us
After	9.77		13.57		39.17		us

Payload 512B:

	1st perc.	overall		99th perc.
Before	13.35		17.35		41.52		us
After	10.25		14.11		39.58		us

Payload 4K:

	1st perc.	overall		99th perc.
Before	14.71		19.87		41.52		us
After	10.51		14.96		40.81		us

- Throughput
   Tool: iperf-vsock

The size represents the buffer length (-l) to read/write
P represents the number of parallel streams

P=1
	4K	64K	128K
Before	6.87	29.3	29.5 Gb/s
After	10.5	39.4	39.9 Gb/s

P=2
	4K	64K	128K
Before	10.5	32.8	33.2 Gb/s
After	17.8	47.7	48.5 Gb/s

P=4
	4K	64K	128K
Before	12.7	33.6	34.2 Gb/s
After	16.9	48.1	50.5 Gb/s

The performance improvement is related to this optimization,
I used a ebpf kretprobe on virtio_transport_send_skb to check
that each packet was sent directly to the virtqueue

Co-developed-by: Marco Pinna <marco.pinn95@gmail.com>
Signed-off-by: Marco Pinna <marco.pinn95@gmail.com>
Signed-off-by: Luigi Leonardi <luigi.leonardi@outlook.com>
Message-Id: <20240730-pinna-v4-2-5c9179164db5@outlook.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

net/vmw_vsock/virtio_transport.c

@@ -208,6 +208,28 @@ virtio_transport_send_pkt_work(struct work_struct *work)
 		queue_work(virtio_vsock_workqueue, &vsock->rx_work);
 }
 
+/* Caller need to hold RCU for vsock.
+ * Returns 0 if the packet is successfully put on the vq.
+ */
+static int virtio_transport_send_skb_fast_path(struct virtio_vsock *vsock, struct sk_buff *skb)
+{
+	struct virtqueue *vq = vsock->vqs[VSOCK_VQ_TX];
+	int ret;
+
+	/* Inside RCU, can't sleep! */
+	ret = mutex_trylock(&vsock->tx_lock);
+	if (unlikely(ret == 0))
+		return -EBUSY;
+
+	ret = virtio_transport_send_skb(skb, vq, vsock);
+	if (ret == 0)
+		virtqueue_kick(vq);
+
+	mutex_unlock(&vsock->tx_lock);
+
+	return ret;
+}
+
 static int
 virtio_transport_send_pkt(struct sk_buff *skb)
 {
@@ -231,11 +253,20 @@ virtio_transport_send_pkt(struct sk_buff *skb)
 		goto out_rcu;
 	}
 
-	if (virtio_vsock_skb_reply(skb))
-		atomic_inc(&vsock->queued_replies);
+	/* If send_pkt_queue is empty, we can safely bypass this queue
+	 * because packet order is maintained and (try) to put the packet
+	 * on the virtqueue using virtio_transport_send_skb_fast_path.
+	 * If this fails we simply put the packet on the intermediate
+	 * queue and schedule the worker.
+	 */
+	if (!skb_queue_empty_lockless(&vsock->send_pkt_queue) ||
+	    virtio_transport_send_skb_fast_path(vsock, skb)) {
+		if (virtio_vsock_skb_reply(skb))
+			atomic_inc(&vsock->queued_replies);
 
-	virtio_vsock_skb_queue_tail(&vsock->send_pkt_queue, skb);
-	queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work);
+		virtio_vsock_skb_queue_tail(&vsock->send_pkt_queue, skb);
+		queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work);
+	}
 
 out_rcu:
 	rcu_read_unlock();