Re: [PATCH net-next v3 2/4] vsock/virtio: amortize RX socket locking for stream packets

[netdev-bot+sashiko@kernel.org]

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Low priority / nit picks for your consideration:

- [Low] The patch is presented as a pure performance optimization
  ("amortize RX socket locking for stream packets").

--

vsock/virtio: amortize RX socket locking for stream packets

The virtio-vsock RX worker now keeps the socket lock and the lookup
reference held across a bounded run of STREAM/RW packets for one socket.
A batch is capped at 64 packets or 64KB of payload. It ends on control
packets, a flow change, or a change in socket state or protocol. Two new
exported functions are added, virtio_transport_recv_pkt_batch() and
virtio_transport_rx_batch_finish().

> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 8a930bc42fdc3..bfe40ecf0b2f0 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c

[ ... ]

> @@ -1909,6 +1945,128 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,

[ ... ]

> +	if (batch->sk) {
> +		vsk = vsock_sk(batch->sk);
> +		if (vsock_addr_equals_addr(&src, &vsk->remote_addr) &&
> +		    dst.svm_port == vsk->local_addr.svm_port &&
> +		    vsock_net_check_mode(sock_net(batch->sk), net) &&
> +		    virtio_transport_recv_pkt_batchable(t, batch->sk)) {

[ ... ]

> +		}
> +
> +		virtio_transport_rx_batch_finish(batch);
> +	}
> +
> +	sk = virtio_transport_find_socket(skb, &src, &dst, net);

[ ... ]

> +	lock_sock(sk);
> +	/*
> +	 * Sockmap removal restores the native protocol under sk_callback_lock.
> +	 * Serialize this initial eligibility check with that update.
> +	 */
> +	read_lock_bh(&sk->sk_callback_lock);
> +	start_batch = virtio_transport_recv_pkt_batchable(t, sk);
> +	read_unlock_bh(&sk->sk_callback_lock);
> +
> +	free_pkt = virtio_transport_recv_pkt_locked(t, skb, sk, &src, &dst,
> +						    net,
> +						    start_batch ? &batchable : NULL);
> +	if (start_batch && batchable) {
> +		/* Keep the lookup reference until the batch is released. */
> +		batch->sk = sk;
> +		return;
> +	}

[Severity: Low]
Is there data showing this is a net win? The subject says it will
"amortize RX socket locking for stream packets". The commit message
gives no throughput, latency or CPU-cost numbers.

Take interleaved RW traffic from two or more stream flows (A, B, A, B,
...). The cached-flow check above fails on every packet, so each packet
goes through:

virtio_transport_recv_pkt_batch()
    virtio_transport_rx_batch_finish()   /* release_sock + sock_put */
    virtio_transport_find_socket()
    lock_sock()
    read_lock_bh(&sk->sk_callback_lock)
    virtio_transport_recv_pkt_batchable()
    read_unlock_bh(&sk->sk_callback_lock)
    virtio_transport_recv_pkt_locked()
        virtio_transport_recv_pkt_batchable()

Every step of the old per-packet path is still done. On top of that,
each packet pays for the read_lock_bh()/read_unlock_bh() pair, two
predicate checks, and the batch bookkeeping in
virtio_transport_rx_work(). No batch ever grows past one packet.

The socket lock from the previous flow also stays held while
virtio_transport_rx_work() runs virtqueue_get_buf(), tap delivery and
header parsing for the next packet.

The added cost is small next to lock_sock() and the socket lookup. Could
the commit message include measurements, and mention this
interleaved-flow trade-off?

-- 
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