From efcd71af38be403fa52223092f79ada446e121ba Mon Sep 17 00:00:00 2001 From: Luigi Leonardi Date: Tue, 30 Jul 2024 21:47:32 +0200 Subject: [PATCH] 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 Signed-off-by: Marco Pinna Signed-off-by: Luigi Leonardi Message-Id: <20240730-pinna-v4-2-5c9179164db5@outlook.com> Signed-off-by: Michael S. Tsirkin Reviewed-by: Stefano Garzarella --- 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 @@ out: 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();