[PATCH net-next v4 2/2] vsock/virtio: avoid queuing packets when intermediate queue is empty

Luigi Leonardi via B4 Relay
2024-07-30T19:47:35Z

From: Luigi Leonardi <luigi.leonardi@outlook.com>

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>
---
net/vmw_vsock/virtio_transport.c | 39 +++++++++++++++++++++++++++++++++++----
1 file changed, 35 insertions(+), 4 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index f641e906f351..f992f9a216f0 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/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();

--
2.45.2