AW: Shared Umem between processes

["Gaul, Maximilian" <maximilian.gaul@hm.edu>]

Björn Töpel <bjorn.topel@gmail.com> wrote:

>On Thu, 12 Mar 2020 at 09:20, Gaul, Maximilian <maximilian.gaul@hm.edu> 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@gmail.com> 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 am sorry but I am confused, you just said *No; one socket, one umem, one queue.*. How would I be able to follow your rough sketch of

                    hw_q0
xsk_q0_0, xsk_q0_1, xsk_q0_2

I don't have deep knowledge about XDP and the pipeline, maybe there is something I am missing. I am sorry.

>> >> 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
>>