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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , dev@openvswitch.org, linux-kernel@vger.kernel.org, Eelco Chaudron Subject: Re: [PATCH net-next] net: openvswitch: allow providing upcall pid for the 'execute' command In-Reply-To: (Ilya Maximets's message of "Wed, 2 Jul 2025 14:37:05 +0200") References: <20250627220219.1504221-1-i.maximets@ovn.org> Date: Wed, 02 Jul 2025 08:46:26 -0400 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Ilya Maximets writes: > On 7/2/25 2:14 PM, Aaron Conole wrote: >> Ilya Maximets 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. Just wanted to make sure I was understanding the problem. Okay, this makes sense. >> >>> 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. Hrrm... I guess I miscounted. Yes I agree, with this change we're at 24 bytes in-use on 64-bit platform. Okay no worries. > Best regards, Ilya Maximets.