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From: "Björn Töpel" <bjorn.topel@gmail.com>
To: "Gaul, Maximilian" <maximilian.gaul@hm.edu>,
	Xdp <xdp-newbies@vger.kernel.org>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>
Subject: Re: Shared Umem between processes
Date: Thu, 12 Mar 2020 08:55:22 +0100	[thread overview]
Message-ID: <CAJ+HfNghFctg3L=3QdeoWyqDdj4wP4EKWjYyF01=SmCO5+=32g@mail.gmail.com> (raw)
In-Reply-To: <fd5e40efd5c1426cb4a5942682407ea2@hm.edu>

On Wed, 11 Mar 2020 at 16:59, Gaul, Maximilian <maximilian.gaul@hm.edu> 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:

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


> Best regards
> Max

  reply	other threads:[~2020-03-12  7:55 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-11 15:58 Shared Umem between processes Gaul, Maximilian
2020-03-12  7:55 ` Björn Töpel [this message]
2020-03-12  8:20   ` AW: " Gaul, Maximilian

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