On Wed, 11 Mar 2020 at 16:59, Gaul, Maximilian <maximilian.gaul@xxxxxx> wrote: > > Hello everyone, > Hi! I'm moving this to the XDP newbies list, which is a more proper place for these kind of discussions! > > I am not sure if this is the correct address for my question / problem but I was forwarded to this e-mail from the libbpf github-issue section, so this is my excuse. > > > Just a few information at the start of this e-mail: My program is largely based on: https://github.com/xdp-project/xdp-tutorial/tree/master/advanced03-AF_XDP and I am using libbpf: https://github.com/libbpf/libbpf > > > I am currently trying to build an application that enables me to process multiple udp-multicast streams at once in parallel (each with up to several ten-thousands of packets per second). > > > My first solution was to steer each multicast-stream on a separate RX-Queue on my NIC via `ethtool -N <if> flow-type udp4 ...` and to spawn as much user-space processes (each with a separate AF-XDP socket connected to one of the RX-Queues) as there are streams to process. > > > But because this solution is limited to the amount of RX-Queues the NIC has and I wanted to build something hardware-independent, I looked around a bit and found a feature called `XDP_SHARED_UMEM`. > Let's start with defining what shared-umem is: The idea is to share the same umem, fill ring, and completion ring for multiple sockets. The sockets sharing that umem/fr/cr are tied (bound) to one hardware ring. It's a mechanism to load-balance a HW queue over multiple sockets. If I'm reading you correctly, you'd like a solution: hw_q0, xsk_q0_0, xsk_q0_1, xsk_q0_2, instead of: hw_q0, hw_q1, hw_q2, xsk_q0_0, xsk_q1_0, xsk_q2_0, In the first case you'll need to mux the flows in the XDP program using an XSKMAP. Is this what you're trying to do? > > > As far as I understand (please correct me if I am wrong), at the moment libbpf only supports shared umem between threads of a process but not between processes - right? > Yes, that is correct, and for a reason! :-) Note that if you'd like to do a multi-*process* setup with shared umem, you: need to have a control process that manages the fill/completion rings, and synchronize between the processes, OR re-mmap the fill/completetion ring from the socket owning the umem in multiple processes *and* synchronize the access to them. Neither is pleasant. Honestly, not a setup I'd recommend. > I ran unto the problem, that `struct xsk_umem` is hidden in `xsk.c`. This prevents me from copying the content from the original socket / umem into shared memory. I am not sure, what information the sub-process (the one which is using the umem from another process) needs so I figured the simplest solution would be to just copy the whole umem struct. > Just for completeness; To setup shared umem: 1. create socket 0 and register the umem to this. 2. mmap the fr/cr using socket 0 3. create socket 1, 2, n and refer to socket 0 for the umem. So, in a multiprocess solution step 3 would be done in separate processes, and step 2 depending on your application. You'd need to pass socket 0 to the other processes *and* share the umem memory from the process where socket 0 was created. This is pretty much a threaded solution, given all the shared state. I advice not taking this path. > > > So I went with the "quick-fix" to just move the definition of `struct xsk_umem` into `xsk.h` and to copy the umem-information from the original process into a shared memory. This process then calls `fork()` thus spawning a sub-process. This sub-process then reads the previously written umem-information from shared memory and passes it into `xsk_configure_socket` (af_xdp_user.c) which then eventually calls `xsk_socket__create` in `xsk.c`. This function then checks for `umem->refcount` and sets the flags for shared umem accordingly. > > > > After returning from `xsk_socket__create` (we are still in `xsk_configure_socket` in af_xdp_user.c), `bpf_get_link_xdp_id` is called (I don't know if that's necessary). But after that call I exit the function `xsk_socket__create` in the sub-process because I figured it is probably bad to configure the umem a second time by calling `xsk_ring_prod__reserve` after that: > > > > > static struct xsk_socket_info *xsk_configure_socket(struct config *cfg, struct xsk_umem_info *umem) { > > struct xsk_socket_config xsk_cfg; > struct xsk_socket_info *xsk_info; > uint32_t idx; > uint32_t prog_id = 0; > int i; > int ret; > > xsk_info = calloc(1, sizeof(*xsk_info)); > if (!xsk_info) > return NULL; > > xsk_info->umem = umem; > xsk_cfg.rx_size = XSK_RING_CONS__DEFAULT_NUM_DESCS; > xsk_cfg.tx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS; > xsk_cfg.libbpf_flags = 0; > xsk_cfg.xdp_flags = cfg->xdp_flags; > xsk_cfg.bind_flags = cfg->xsk_bind_flags; > ret = xsk_socket__create(&xsk_info->xsk, cfg->ifname, cfg->xsk_if_queue, umem->umem, &xsk_info->rx, &xsk_info->tx, &xsk_cfg); > > if (ret) { > fprintf(stderr, "FAIL 1\n"); > goto error_exit; > } > > ret = bpf_get_link_xdp_id(cfg->ifindex, &prog_id, cfg->xdp_flags); > if (ret) { > fprintf(stderr, "FAIL 2\n"); > goto error_exit; > } > > /* Initialize umem frame allocation */ > for (i = 0; i < NUM_FRAMES; i++) > xsk_info->umem_frame_addr[i] = i * FRAME_SIZE; > > xsk_info->umem_frame_free = NUM_FRAMES; > > if(cfg->use_shrd_umem) { > return xsk_info; > } > ... > } > > Somehow what I am doing doesn't work because my sub-process dies in `xsk_configure_socket`. I am not able to debug it properly with GDB though. Another point I don't understand is the statement: > > However, note that you need to supply the XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD libbpf_flag with the xsk_socket__create calls and load your own XDP program as there is no built in one in libbpf that will route the traffic for you. > > from https://www.kernel.org/doc/html/latest/networking/af_xdp.html#xdp-shared-umem-bind-flag > > I didn't know that libbpf loads a XDP-program? Why would it do that? I am using my own af-xdp program which filters for udp-packets. If I set `xsk_cfg.libbpf_flags = XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD;` in `xsk_configure_socket`, the af-xdp-socket fd is not put into the kernel `xsks-map` which basically means that I don't receive any packets. > > As you probably already noticed, I am overstrained with the concept of Shared Umem and I have to say, there is no documentation about it besides the two sentences in https://www.kernel.org/doc/html/latest/networking/af_xdp.html#xdp-shared-umem-bind-flag and a mail in a linux mailbox from Nov. 2019 stating that this feature is now implemented. > > Can you please help? > XDP sockets always use an XDP program, it just that a default one is provided if the use doesn't explicitly add one. Have a look at tools/lib/bpf/xsk.c:xsk_load_xdp_prog. So, for shared umem you need to explicitly have a program that muxes over the sockets. A naïve variant can be found in samples/bpf/xdpsock_kern.c Cheers, Björn > Best regards > > Max
