| Message ID | 20250401201349.23867-1-graf@amazon.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [v2] vsock/virtio: Remove queued_replies pushback logic | expand |
On Tue, Apr 01, 2025 at 08:13:49PM +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 <graf@amazon.com> > --- > net/vmw_vsock/virtio_transport.c | 70 +++++++++----------------------- > 1 file changed, 19 insertions(+), 51 deletions(-) > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c ... > @@ -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); > > @@ -172,6 +176,12 @@ virtio_transport_send_pkt_work(struct work_struct *work) > 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; Hi Alexander, The next non-blank line of code looks like this: reply = virtio_vsock_skb_reply(skb); But with this patch reply is assigned but otherwise unused. So perhaps the line above, and the declaration of reply, can be removed? Flagged by W=1 builds. > @@ -184,17 +194,6 @@ virtio_transport_send_pkt_work(struct work_struct *work) > 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 +202,6 @@ 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. ...
On Wed, Apr 02, 2025 at 10:26:05AM +0100, Simon Horman wrote: >On Tue, Apr 01, 2025 at 08:13:49PM +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 <graf@amazon.com> >> --- >> net/vmw_vsock/virtio_transport.c | 70 +++++++++----------------------- >> 1 file changed, 19 insertions(+), 51 deletions(-) >> >> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c > >... > >> @@ -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); >> >> @@ -172,6 +176,12 @@ virtio_transport_send_pkt_work(struct work_struct *work) >> 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; > >Hi Alexander, > >The next non-blank line of code looks like this: > > reply = virtio_vsock_skb_reply(skb); > >But with this patch reply is assigned but otherwise unused. Thanks for the report! >So perhaps the line above, and the declaration of reply, can be removed? @Alex: yes, please remove it. A part of that the rest LGTM! I've been running some tests for a while and everything seems okay. I guess we can do something similar also in vhost-vsock, where we already have "vhost weight" support. IIUC it was added later by commit e79b431fb901 ("vhost: vsock: add weight support"), but we never removed "queued_replies" stuff, that IMO after that commit is pretty much useless. I'm not asking to that in this series, if you don't have time I can do it separately ;-) Thanks, Stefano > >Flagged by W=1 builds. > >> @@ -184,17 +194,6 @@ virtio_transport_send_pkt_work(struct work_struct *work) >> 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 +202,6 @@ 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. > >... >
On Tue, Apr 01, 2025 at 08:13:49PM +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. The reason for queued_replies is that rx packet processing may emit tx packets. Therefore tx virtqueue space is required in order to process the rx virtqueue. queued_replies puts a bound on the amount of tx packets that can be queued in memory so the other side cannot consume unlimited memory. Once that bound has been reached, rx processing stops until the other side frees up tx virtqueue space. It's been a while since I looked at this problem, so I don't have a solution ready. In fact, last time I thought about it I wondered if the design of virtio-vsock fundamentally suffers from deadlocks. I don't think removing queued_replies is possible without a replacement for the bounded memory and virtqueue exhaustion issue though. Credits are not a solution - they are about socket buffer space, not about virtqueue space, which includes control packets that are not accounted by socket buffer space. > > 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 <graf@amazon.com> > --- > net/vmw_vsock/virtio_transport.c | 70 +++++++++----------------------- > 1 file changed, 19 insertions(+), 51 deletions(-) > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c > index f0e48e6911fc..54030c729767 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); > > @@ -172,6 +176,12 @@ virtio_transport_send_pkt_work(struct work_struct *work) > 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; > @@ -184,17 +194,6 @@ virtio_transport_send_pkt_work(struct work_struct *work) > 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 +202,6 @@ 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 +257,6 @@ 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 +270,7 @@ 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 +279,7 @@ 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 +350,6 @@ 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 +584,7 @@ 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 +599,9 @@ 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 +645,6 @@ 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 >
diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c index f0e48e6911fc..54030c729767 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); @@ -172,6 +176,12 @@ virtio_transport_send_pkt_work(struct work_struct *work) 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; @@ -184,17 +194,6 @@ virtio_transport_send_pkt_work(struct work_struct *work) 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 +202,6 @@ 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 +257,6 @@ 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 +270,7 @@ 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 +279,7 @@ 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 +350,6 @@ 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 +584,7 @@ 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 +599,9 @@ 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 +645,6 @@ 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;
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 <graf@amazon.com> --- net/vmw_vsock/virtio_transport.c | 70 +++++++++----------------------- 1 file changed, 19 insertions(+), 51 deletions(-)