On 7/2/25 3:53 PM, Flavio Leitner wrote:
> On Sat, 28 Jun 2025 00:01:33 +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 per-CPU dispatch mode is supposed to rely on the CPU context,
> which according with what you said above, it is working okay on
> the first MISS. However, when we hit a recirculation action and
> there is another MISS, another thread from another CPU context
> is selected, why?
Because the second miss is happening while processing OVS_PACKET_CMD_EXECUTE,
which is happening in a syscall context of a userspace handler thread, which
is running on a different CPU.
>
> Thanks,
> Flavio
>
>>
>> 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.
>>
>> Signed-off-by: Ilya Maximets <i.maxim...@ovn.org>
>> ---
>> include/uapi/linux/openvswitch.h | 6 ++++++
>> net/openvswitch/actions.c | 6 ++++--
>> net/openvswitch/datapath.c | 10 +++++++++-
>> net/openvswitch/datapath.h | 3 +++
>> net/openvswitch/vport.c | 1 +
>> 5 files changed, 23 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..ec08ce72f439 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
>> @@ -616,6 +618,7 @@ static int ovs_packet_cmd_execute(struct sk_buff
>> *skb, struct genl_info *info) struct sw_flow_actions *sf_acts;
>> struct datapath *dp;
>> struct vport *input_vport;
>> + u32 upcall_pid = 0;
>> u16 mru = 0;
>> u64 hash;
>> int len;
>> @@ -651,6 +654,10 @@ static int ovs_packet_cmd_execute(struct sk_buff
>> *skb, struct genl_info *info) !!(hash & OVS_PACKET_HASH_L4_BIT));
>> }
>>
>> + if (a[OVS_PACKET_ATTR_UPCALL_PID])
>> + upcall_pid =
>> nla_get_u32(a[OVS_PACKET_ATTR_UPCALL_PID]);
>> + OVS_CB(packet)->upcall_pid = upcall_pid;
>> +
>> /* Build an sw_flow for sending this packet. */
>> flow = ovs_flow_alloc();
>> err = PTR_ERR(flow);
>> @@ -719,6 +726,7 @@ 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;
>>
>
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