Message-ID: <b450a7f2-cab4-4b71-aec8-ef867403663a@ovn.org>
Date: Wed, 2 Jul 2025 14:37:05 +0200
From: Ilya Maximets <i.maximets@....org>
To: Aaron Conole <aconole@...hat.com>
Cc: i.maximets@....org, netdev@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, dev@...nvswitch.org,
 linux-kernel@...r.kernel.org, Eelco Chaudron <echaudro@...hat.com>
Subject: Re: [PATCH net-next] net: openvswitch: allow providing upcall pid for
 the 'execute' command

On 7/2/25 2:14 PM, Aaron Conole wrote:
> Ilya Maximets <i.maximets@....org> writes:
> 
>> 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.
> 
> Just wondering but why can't we choose the existing core handler when
> running the packet_cmd_execute?  For example, when looking into the
> per-cpu table we know what the current core is, can we just queue to
> that one?  I actually thought that's what the PER_CPU dispatch mode was
> supposed to do.

This is exactly how it works today and it is the problem, because our
userspace handler is running on a different CPU and so the 'current CPU'
during the packet_cmd_execute is different from the one where kernel
was processing the original upcall.

> Or is it that we want to make sure we keep the
> association between the skbuff for re-injection always?

We want the same packet to be enqueued to the same upcall socket after
each recirculation, so it gets handled by the same userspace thread.

> 
>> 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.
> 
> Is it because the userspace handler threads are being rescheduled across
> CPUs?

Yes.  Userspace handlers are not pinned to a specific core in most cases,
so they will be running on different CPUs and will float around.

> Do we still see this behavior if we pinned each handler thread to
> a specific CPU rather than letting the scheduler make the decision?

If you pin each userspace thread to a core that is specified in the
PCPU_UPCALL_PIDS for the socket that it is listening on, then the
problem will go away, as the packet_cmd_execute syscall will be executed
on the same core where the kernel received the original packet.
However, that's not possible in many cases (reserved CPUs, different
CPU affinity for IRQs and userspace applications, etc.) and not desired
as may impact performance of the system, because the kernel and userspace
will compete for the same core.

> 
>> 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.
> 
> With this change we're almost full on the OVS sk_buff control block.
> Might be good to mention it in the commit message if you're respinning.

Are we full?  The skb->cb size is 48 bytes and we're only using 24
with this change, unless I'm missing something.

Best regards, Ilya Maximets.

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