On Thu, 12 Mar 2020 at 09:20, Gaul, Maximilian <maximilian.gaul@xxxxxx> wrote:
>
> I don't know if this reply works but I will try.
>
It worked! :-)
> On Thu, 12 Mar 2020 at 08:55, Björn Töpel <bjorn.topel@xxxxxxxxx> wrote:
> >>
> >> Hello everyone,
> >>
> >
> > Hi! I'm moving this to the XDP newbies list, which is a more proper
> > place for these kind of discussions!
> >
> Sure, no problem. Thank you.
> >>
> >> 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?
> >
> Yes it is. But I had the problem that I couldn't create multiple sockets (no sharing, everyone with its own umem and rx/tx queues) tied to the same RX-Queue. Maybe I did something wrong. But is this possible?
No; one socket, one umem, one queue. Unless you're using shared umem,
then multiple sockets, one umem, one queue.
> >>
> >>
> >> 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.
> >
> This indeed sounds very unpleasent. So instead, if I understand correctly, you would go with the version above (the XDP program distributing the packets on the sockets via a XSKMAP). Is there something I have to watch out for? As I said, I wasn't able to create multiple sockets for the same RX-Queue.
I would probably go for the first option, without shared umem, but
that's really up to you! If you're going for the shared umem, I'd do
it single process.
> >> 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.
> >
> I am not entirely sure what you mean with *passing socket 0* is this just the fd of the socket? What's about the `struct xsk_umem`? Do I need that? I guess so because `xsk_socket__create()` has a parameter `struct xsk_umem`.
> >>
> >>
> >> 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
>
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