On Wed, 2 Jul 2025 17:50:34 +0200 Ilya Maximets <i.maxim...@ovn.org> wrote:
> When a packet enters OVS datapath and there is no flow to handle it, > packet goes to userspace through a MISS upcall. With per-CPU upcall > dispatch mechanism, we're using the current CPU id to select the > Netlink PID on which to send this packet. This allows us to send > packets from the same traffic flow through the same handler. > > The handler will process the packet, install required flow into the > kernel and re-inject the original packet via OVS_PACKET_CMD_EXECUTE. > > While handling OVS_PACKET_CMD_EXECUTE, however, we may hit a > recirculation action that will pass the (likely modified) packet > through the flow lookup again. And if the flow is not found, the > packet will be sent to userspace again through another MISS upcall. > > However, the handler thread in userspace is likely running on a > different CPU core, and the OVS_PACKET_CMD_EXECUTE request is handled > in the syscall context of that thread. So, when the time comes to > send the packet through another upcall, the per-CPU dispatch will > choose a different Netlink PID, and this packet will end up processed > by a different handler thread on a different CPU. > > The process continues as long as there are new recirculations, each > time the packet goes to a different handler thread before it is sent > out of the OVS datapath to the destination port. In real setups the > number of recirculations can go up to 4 or 5, sometimes more. > > There is always a chance to re-order packets while processing upcalls, > because userspace will first install the flow and then re-inject the > original packet. So, there is a race window when the flow is already > installed and the second packet can match it and be forwarded to the > destination before the first packet is re-injected. But the fact that > packets are going through multiple upcalls handled by different > userspace threads makes the reordering noticeably more likely, because > we not only have a race between the kernel and a userspace handler > (which is hard to avoid), but also between multiple userspace handlers. > > For example, let's assume that 10 packets got enqueued through a MISS > upcall for handler-1, it will start processing them, will install the > flow into the kernel and start re-injecting packets back, from where > they will go through another MISS to handler-2. Handler-2 will install > the flow into the kernel and start re-injecting the packets, while > handler-1 continues to re-inject the last of the 10 packets, they will > hit the flow installed by handler-2 and be forwarded without going to > the handler-2, while handler-2 still re-injects the first of these 10 > packets. Given multiple recirculations and misses, these 10 packets > may end up completely mixed up on the output from the datapath. > > Let's allow userspace to specify on which Netlink PID the packets > should be upcalled while processing OVS_PACKET_CMD_EXECUTE. > This makes it possible to ensure that all the packets are processed > by the same handler thread in the userspace even with them being > upcalled multiple times in the process. Packets will remain in order > since they will be enqueued to the same socket and re-injected in the > same order. This doesn't eliminate re-ordering as stated above, since > we still have a race between kernel and the userspace thread, but it > allows to eliminate races between multiple userspace threads. > > Userspace knows the PID of the socket on which the original upcall is > received, so there is no need to send it up from the kernel. > > Solution requires storing the value somewhere for the duration of the > packet processing. There are two potential places for this: our skb > extension or the per-CPU storage. It's not clear which is better, > so just following currently used scheme of storing this kind of things > along the skb. We still have a decent amount of space in the cb. > > Signed-off-by: Ilya Maximets <i.maxim...@ovn.org> Acked-by: Flavio Leitner <f...@sysclose.org> Thanks! > --- > > Version 2: > * Converted to use nla_get_u32_default() instead of plain-coding it. > > Version 1: > * > https://lore.kernel.org/netdev/20250627220219.1504221-1-i.maxim...@ovn.org/ > > include/uapi/linux/openvswitch.h | 6 ++++++ > net/openvswitch/actions.c | 6 ++++-- > net/openvswitch/datapath.c | 8 +++++++- > net/openvswitch/datapath.h | 3 +++ > net/openvswitch/vport.c | 1 + > 5 files changed, 21 insertions(+), 3 deletions(-) > > diff --git a/include/uapi/linux/openvswitch.h > b/include/uapi/linux/openvswitch.h index 3a701bd1f31b..3092c2c6f1d2 100644 > --- a/include/uapi/linux/openvswitch.h > +++ b/include/uapi/linux/openvswitch.h > @@ -186,6 +186,11 @@ enum ovs_packet_cmd { > * %OVS_PACKET_ATTR_USERSPACE action specify the Maximum received fragment > * size. > * @OVS_PACKET_ATTR_HASH: Packet hash info (e.g. hash, sw_hash and l4_hash > in skb). > + * @OVS_PACKET_ATTR_UPCALL_PID: Netlink PID to use for upcalls while > + * processing %OVS_PACKET_CMD_EXECUTE. Takes precedence over all other > ways > + * to determine the Netlink PID including %OVS_USERSPACE_ATTR_PID, > + * %OVS_DP_ATTR_UPCALL_PID, %OVS_DP_ATTR_PER_CPU_PIDS and the > + * %OVS_VPORT_ATTR_UPCALL_PID. > * > * These attributes follow the &struct ovs_header within the Generic > Netlink > * payload for %OVS_PACKET_* commands. > @@ -205,6 +210,7 @@ enum ovs_packet_attr { > OVS_PACKET_ATTR_MRU, /* Maximum received IP fragment > size. */ OVS_PACKET_ATTR_LEN, /* Packet size before truncation. > */ OVS_PACKET_ATTR_HASH, /* Packet hash. */ > + OVS_PACKET_ATTR_UPCALL_PID, /* u32 Netlink PID. */ > __OVS_PACKET_ATTR_MAX > }; > > diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c > index 3add108340bf..2832e0794197 100644 > --- a/net/openvswitch/actions.c > +++ b/net/openvswitch/actions.c > @@ -941,8 +941,10 @@ static int output_userspace(struct datapath *dp, > struct sk_buff *skb, break; > > case OVS_USERSPACE_ATTR_PID: > - if (dp->user_features & > - OVS_DP_F_DISPATCH_UPCALL_PER_CPU) > + if (OVS_CB(skb)->upcall_pid) > + upcall.portid = OVS_CB(skb)->upcall_pid; > + else if (dp->user_features & > + OVS_DP_F_DISPATCH_UPCALL_PER_CPU) > upcall.portid = > ovs_dp_get_upcall_portid(dp, > smp_processor_id()); > diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c > index b990dc83504f..d5b6e2002bc1 100644 > --- a/net/openvswitch/datapath.c > +++ b/net/openvswitch/datapath.c > @@ -267,7 +267,9 @@ void ovs_dp_process_packet(struct sk_buff *skb, struct > sw_flow_key *key) memset(&upcall, 0, sizeof(upcall)); > upcall.cmd = OVS_PACKET_CMD_MISS; > > - if (dp->user_features & OVS_DP_F_DISPATCH_UPCALL_PER_CPU) > + if (OVS_CB(skb)->upcall_pid) > + upcall.portid = OVS_CB(skb)->upcall_pid; > + else if (dp->user_features & > OVS_DP_F_DISPATCH_UPCALL_PER_CPU) upcall.portid = > ovs_dp_get_upcall_portid(dp, > smp_processor_id()); else > @@ -651,6 +653,9 @@ static int ovs_packet_cmd_execute(struct sk_buff *skb, > struct genl_info *info) !!(hash & OVS_PACKET_HASH_L4_BIT)); > } > > + OVS_CB(packet)->upcall_pid = > + nla_get_u32_default(a[OVS_PACKET_ATTR_UPCALL_PID], 0); > + > /* Build an sw_flow for sending this packet. */ > flow = ovs_flow_alloc(); > err = PTR_ERR(flow); > @@ -719,6 +724,7 @@ static const struct nla_policy > packet_policy[OVS_PACKET_ATTR_MAX + 1] = { [OVS_PACKET_ATTR_PROBE] = { > .type = NLA_FLAG }, [OVS_PACKET_ATTR_MRU] = { .type = NLA_U16 }, > [OVS_PACKET_ATTR_HASH] = { .type = NLA_U64 }, > + [OVS_PACKET_ATTR_UPCALL_PID] = { .type = NLA_U32 }, > }; > > static const struct genl_small_ops dp_packet_genl_ops[] = { > diff --git a/net/openvswitch/datapath.h b/net/openvswitch/datapath.h > index cfeb817a1889..db0c3e69d66c 100644 > --- a/net/openvswitch/datapath.h > +++ b/net/openvswitch/datapath.h > @@ -121,6 +121,8 @@ struct datapath { > * @cutlen: The number of bytes from the packet end to be removed. > * @probability: The sampling probability that was applied to this skb; 0 > means > * no sampling has occurred; U32_MAX means 100% probability. > + * @upcall_pid: Netlink socket PID to use for sending this packet to > userspace; > + * 0 means "not set" and default per-CPU or per-vport dispatch should be > used. */ > struct ovs_skb_cb { > struct vport *input_vport; > @@ -128,6 +130,7 @@ struct ovs_skb_cb { > u16 acts_origlen; > u32 cutlen; > u32 probability; > + u32 upcall_pid; > }; > #define OVS_CB(skb) ((struct ovs_skb_cb *)(skb)->cb) > > diff --git a/net/openvswitch/vport.c b/net/openvswitch/vport.c > index 8732f6e51ae5..6bbbc16ab778 100644 > --- a/net/openvswitch/vport.c > +++ b/net/openvswitch/vport.c > @@ -501,6 +501,7 @@ int ovs_vport_receive(struct vport *vport, struct > sk_buff *skb, OVS_CB(skb)->mru = 0; > OVS_CB(skb)->cutlen = 0; > OVS_CB(skb)->probability = 0; > + OVS_CB(skb)->upcall_pid = 0; > if (unlikely(dev_net(skb->dev) != ovs_dp_get_net(vport->dp))) { > u32 mark; > _______________________________________________ dev mailing list d...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-dev