Message-ID: <00067667-0329-4d8c-9c9a-a6660806b137@ovn.org>
Date: Wed, 2 Jul 2025 16:41:19 +0200
From: Ilya Maximets <i.maximets@....org>
To: Flavio Leitner <fbl@...close.org>
Cc: i.maximets@....org, netdev@...r.kernel.org, dev@...nvswitch.org,
 linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
 Simon Horman <horms@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [ovs-dev] [PATCH net-next] net: openvswitch: allow providing
 upcall pid for the 'execute' command

On 7/2/25 3:53 PM, Flavio Leitner wrote:
> On Sat, 28 Jun 2025 00:01:33 +0200
> Ilya Maximets <i.maximets@....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.maximets@....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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