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Tsirkin" To: Alexander Graf Cc: netdev@vger.kernel.org, Stefano Garzarella , Stefan Hajnoczi , linux-kernel@vger.kernel.org, virtualization@lists.linux.dev, kvm@vger.kernel.org, Asias He , Paolo Abeni , Jakub Kicinski , Eric Dumazet , "David S . Miller" , nh-open-source@amazon.com, Simon Horman Subject: Re: [PATCH v3] vsock/virtio: Remove queued_replies pushback logic Message-ID: <20250402110955-mutt-send-email-mst@kernel.org> References: <20250402150646.42855-1-graf@amazon.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250402150646.42855-1-graf@amazon.com> On Wed, Apr 02, 2025 at 03:06:46PM +0000, Alexander Graf wrote: > Ever since the introduction of the virtio vsock driver, it included > pushback logic that blocks it from taking any new RX packets until the > TX queue backlog becomes shallower than the virtqueue size. > > This logic works fine when you connect a user space application on the > hypervisor with a virtio-vsock target, because the guest will stop > receiving data until the host pulled all outstanding data from the VM. > > With Nitro Enclaves however, we connect 2 VMs directly via vsock: > > Parent Enclave > > RX -------- TX > TX -------- RX > > This means we now have 2 virtio-vsock backends that both have the pushback > logic. If the parent's TX queue runs full at the same time as the > Enclave's, both virtio-vsock drivers fall into the pushback path and > no longer accept RX traffic. However, that RX traffic is TX traffic on > the other side which blocks that driver from making any forward > progress. We're now in a deadlock. > > To resolve this, let's remove that pushback logic altogether and rely on > higher levels (like credits) to ensure we do not consume unbounded > memory. > > RX and TX queues share the same work queue. To prevent starvation of TX > by an RX flood and vice versa now that the pushback logic is gone, let's > deliberately reschedule RX and TX work after a fixed threshold (256) of > packets to process. > > Fixes: 0ea9e1d3a9e3 ("VSOCK: Introduce virtio_transport.ko") > Signed-off-by: Alexander Graf Acked-by: Michael S. Tsirkin > --- > > v1 -> v2: > > - Rework to use fixed threshold > > v2 -> v3: > > - Remove superfluous reply variable > --- > net/vmw_vsock/virtio_transport.c | 73 +++++++++----------------------- > 1 file changed, 19 insertions(+), 54 deletions(-) > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c > index f0e48e6911fc..6ae30bf8c85c 100644 > --- a/net/vmw_vsock/virtio_transport.c > +++ b/net/vmw_vsock/virtio_transport.c > @@ -26,6 +26,12 @@ static struct virtio_vsock __rcu *the_virtio_vsock; > static DEFINE_MUTEX(the_virtio_vsock_mutex); /* protects the_virtio_vsock */ > static struct virtio_transport virtio_transport; /* forward declaration */ > > +/* > + * Max number of RX packets transferred before requeueing so we do > + * not starve TX traffic because they share the same work queue. > + */ > +#define VSOCK_MAX_PKTS_PER_WORK 256 > + > struct virtio_vsock { > struct virtio_device *vdev; > struct virtqueue *vqs[VSOCK_VQ_MAX]; > @@ -44,8 +50,6 @@ struct virtio_vsock { > struct work_struct send_pkt_work; > struct sk_buff_head send_pkt_queue; > > - atomic_t queued_replies; > - > /* The following fields are protected by rx_lock. vqs[VSOCK_VQ_RX] > * must be accessed with rx_lock held. > */ > @@ -158,7 +162,7 @@ virtio_transport_send_pkt_work(struct work_struct *work) > container_of(work, struct virtio_vsock, send_pkt_work); > struct virtqueue *vq; > bool added = false; > - bool restart_rx = false; > + int pkts = 0; > > mutex_lock(&vsock->tx_lock); > > @@ -169,32 +173,24 @@ virtio_transport_send_pkt_work(struct work_struct *work) > > for (;;) { > struct sk_buff *skb; > - bool reply; > int ret; > > + if (++pkts > VSOCK_MAX_PKTS_PER_WORK) { > + /* Allow other works on the same queue to run */ > + queue_work(virtio_vsock_workqueue, work); > + break; > + } > + > skb = virtio_vsock_skb_dequeue(&vsock->send_pkt_queue); > if (!skb) > break; > > - reply = virtio_vsock_skb_reply(skb); > - > ret = virtio_transport_send_skb(skb, vq, vsock, GFP_KERNEL); > if (ret < 0) { > virtio_vsock_skb_queue_head(&vsock->send_pkt_queue, skb); > break; > } > > - if (reply) { > - struct virtqueue *rx_vq = vsock->vqs[VSOCK_VQ_RX]; > - int val; > - > - val = atomic_dec_return(&vsock->queued_replies); > - > - /* Do we now have resources to resume rx processing? */ > - if (val + 1 == virtqueue_get_vring_size(rx_vq)) > - restart_rx = true; > - } > - > added = true; > } > > @@ -203,9 +199,6 @@ virtio_transport_send_pkt_work(struct work_struct *work) > > out: > mutex_unlock(&vsock->tx_lock); > - > - if (restart_rx) > - queue_work(virtio_vsock_workqueue, &vsock->rx_work); > } > > /* Caller need to hold RCU for vsock. > @@ -261,9 +254,6 @@ virtio_transport_send_pkt(struct sk_buff *skb) > */ > 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); > } > @@ -277,7 +267,7 @@ static int > virtio_transport_cancel_pkt(struct vsock_sock *vsk) > { > struct virtio_vsock *vsock; > - int cnt = 0, ret; > + int ret; > > rcu_read_lock(); > vsock = rcu_dereference(the_virtio_vsock); > @@ -286,17 +276,7 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk) > goto out_rcu; > } > > - cnt = virtio_transport_purge_skbs(vsk, &vsock->send_pkt_queue); > - > - if (cnt) { > - struct virtqueue *rx_vq = vsock->vqs[VSOCK_VQ_RX]; > - int new_cnt; > - > - new_cnt = atomic_sub_return(cnt, &vsock->queued_replies); > - if (new_cnt + cnt >= virtqueue_get_vring_size(rx_vq) && > - new_cnt < virtqueue_get_vring_size(rx_vq)) > - queue_work(virtio_vsock_workqueue, &vsock->rx_work); > - } > + virtio_transport_purge_skbs(vsk, &vsock->send_pkt_queue); > > ret = 0; > > @@ -367,18 +347,6 @@ static void virtio_transport_tx_work(struct work_struct *work) > queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work); > } > > -/* Is there space left for replies to rx packets? */ > -static bool virtio_transport_more_replies(struct virtio_vsock *vsock) > -{ > - struct virtqueue *vq = vsock->vqs[VSOCK_VQ_RX]; > - int val; > - > - smp_rmb(); /* paired with atomic_inc() and atomic_dec_return() */ > - val = atomic_read(&vsock->queued_replies); > - > - return val < virtqueue_get_vring_size(vq); > -} > - > /* event_lock must be held */ > static int virtio_vsock_event_fill_one(struct virtio_vsock *vsock, > struct virtio_vsock_event *event) > @@ -613,6 +581,7 @@ static void virtio_transport_rx_work(struct work_struct *work) > struct virtio_vsock *vsock = > container_of(work, struct virtio_vsock, rx_work); > struct virtqueue *vq; > + int pkts = 0; > > vq = vsock->vqs[VSOCK_VQ_RX]; > > @@ -627,11 +596,9 @@ static void virtio_transport_rx_work(struct work_struct *work) > struct sk_buff *skb; > unsigned int len; > > - if (!virtio_transport_more_replies(vsock)) { > - /* Stop rx until the device processes already > - * pending replies. Leave rx virtqueue > - * callbacks disabled. > - */ > + if (++pkts > VSOCK_MAX_PKTS_PER_WORK) { > + /* Allow other works on the same queue to run */ > + queue_work(virtio_vsock_workqueue, work); > goto out; > } > > @@ -675,8 +642,6 @@ static int virtio_vsock_vqs_init(struct virtio_vsock *vsock) > vsock->rx_buf_max_nr = 0; > mutex_unlock(&vsock->rx_lock); > > - atomic_set(&vsock->queued_replies, 0); > - > ret = virtio_find_vqs(vdev, VSOCK_VQ_MAX, vsock->vqs, vqs_info, NULL); > if (ret < 0) > return ret; > -- > 2.47.1