net: add devmem TCP documentation
Add documentation outlining the usage and details of devmem TCP. Signed-off-by: Mina Almasry <almasrymina@google.com> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> Reviewed-by: Donald Hunter <donald.hunter@gmail.com> Link: https://patch.msgid.link/20240910171458.219195-12-almasrymina@google.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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Mina Almasry
Jakub Kicinski
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Documentation/networking/devmem.rst
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269
Documentation/networking/devmem.rst
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.. SPDX-License-Identifier: GPL-2.0
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=================
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Device Memory TCP
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=================
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Intro
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=====
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Device memory TCP (devmem TCP) enables receiving data directly into device
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memory (dmabuf). The feature is currently implemented for TCP sockets.
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Opportunity
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-----------
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A large number of data transfers have device memory as the source and/or
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destination. Accelerators drastically increased the prevalence of such
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transfers. Some examples include:
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- Distributed training, where ML accelerators, such as GPUs on different hosts,
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exchange data.
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- Distributed raw block storage applications transfer large amounts of data with
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remote SSDs. Much of this data does not require host processing.
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Typically the Device-to-Device data transfers in the network are implemented as
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the following low-level operations: Device-to-Host copy, Host-to-Host network
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transfer, and Host-to-Device copy.
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The flow involving host copies is suboptimal, especially for bulk data transfers,
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and can put significant strains on system resources such as host memory
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bandwidth and PCIe bandwidth.
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Devmem TCP optimizes this use case by implementing socket APIs that enable
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the user to receive incoming network packets directly into device memory.
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Packet payloads go directly from the NIC to device memory.
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Packet headers go to host memory and are processed by the TCP/IP stack
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normally. The NIC must support header split to achieve this.
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Advantages:
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- Alleviate host memory bandwidth pressure, compared to existing
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network-transfer + device-copy semantics.
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- Alleviate PCIe bandwidth pressure, by limiting data transfer to the lowest
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level of the PCIe tree, compared to the traditional path which sends data
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through the root complex.
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More Info
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---------
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slides, video
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https://netdevconf.org/0x17/sessions/talk/device-memory-tcp.html
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patchset
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[PATCH net-next v24 00/13] Device Memory TCP
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https://lore.kernel.org/netdev/20240831004313.3713467-1-almasrymina@google.com/
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Interface
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=========
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Example
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-------
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tools/testing/selftests/net/ncdevmem.c:do_server shows an example of setting up
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the RX path of this API.
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NIC Setup
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---------
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Header split, flow steering, & RSS are required features for devmem TCP.
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Header split is used to split incoming packets into a header buffer in host
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memory, and a payload buffer in device memory.
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Flow steering & RSS are used to ensure that only flows targeting devmem land on
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an RX queue bound to devmem.
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Enable header split & flow steering::
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# enable header split
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ethtool -G eth1 tcp-data-split on
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# enable flow steering
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ethtool -K eth1 ntuple on
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Configure RSS to steer all traffic away from the target RX queue (queue 15 in
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this example)::
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ethtool --set-rxfh-indir eth1 equal 15
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The user must bind a dmabuf to any number of RX queues on a given NIC using
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the netlink API::
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/* Bind dmabuf to NIC RX queue 15 */
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struct netdev_queue *queues;
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queues = malloc(sizeof(*queues) * 1);
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queues[0]._present.type = 1;
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queues[0]._present.idx = 1;
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queues[0].type = NETDEV_RX_QUEUE_TYPE_RX;
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queues[0].idx = 15;
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*ys = ynl_sock_create(&ynl_netdev_family, &yerr);
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req = netdev_bind_rx_req_alloc();
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netdev_bind_rx_req_set_ifindex(req, 1 /* ifindex */);
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netdev_bind_rx_req_set_dmabuf_fd(req, dmabuf_fd);
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__netdev_bind_rx_req_set_queues(req, queues, n_queue_index);
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rsp = netdev_bind_rx(*ys, req);
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dmabuf_id = rsp->dmabuf_id;
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The netlink API returns a dmabuf_id: a unique ID that refers to this dmabuf
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that has been bound.
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The user can unbind the dmabuf from the netdevice by closing the netlink socket
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that established the binding. We do this so that the binding is automatically
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unbound even if the userspace process crashes.
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Note that any reasonably well-behaved dmabuf from any exporter should work with
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devmem TCP, even if the dmabuf is not actually backed by devmem. An example of
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this is udmabuf, which wraps user memory (non-devmem) in a dmabuf.
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Socket Setup
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------------
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The socket must be flow steered to the dmabuf bound RX queue::
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ethtool -N eth1 flow-type tcp4 ... queue 15
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Receiving data
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--------------
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The user application must signal to the kernel that it is capable of receiving
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devmem data by passing the MSG_SOCK_DEVMEM flag to recvmsg::
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ret = recvmsg(fd, &msg, MSG_SOCK_DEVMEM);
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Applications that do not specify the MSG_SOCK_DEVMEM flag will receive an EFAULT
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on devmem data.
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Devmem data is received directly into the dmabuf bound to the NIC in 'NIC
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Setup', and the kernel signals such to the user via the SCM_DEVMEM_* cmsgs::
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for (cm = CMSG_FIRSTHDR(&msg); cm; cm = CMSG_NXTHDR(&msg, cm)) {
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if (cm->cmsg_level != SOL_SOCKET ||
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(cm->cmsg_type != SCM_DEVMEM_DMABUF &&
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cm->cmsg_type != SCM_DEVMEM_LINEAR))
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continue;
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dmabuf_cmsg = (struct dmabuf_cmsg *)CMSG_DATA(cm);
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if (cm->cmsg_type == SCM_DEVMEM_DMABUF) {
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/* Frag landed in dmabuf.
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*
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* dmabuf_cmsg->dmabuf_id is the dmabuf the
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* frag landed on.
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*
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* dmabuf_cmsg->frag_offset is the offset into
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* the dmabuf where the frag starts.
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*
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* dmabuf_cmsg->frag_size is the size of the
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* frag.
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*
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* dmabuf_cmsg->frag_token is a token used to
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* refer to this frag for later freeing.
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*/
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struct dmabuf_token token;
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token.token_start = dmabuf_cmsg->frag_token;
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token.token_count = 1;
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continue;
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}
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if (cm->cmsg_type == SCM_DEVMEM_LINEAR)
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/* Frag landed in linear buffer.
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*
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* dmabuf_cmsg->frag_size is the size of the
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* frag.
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*/
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continue;
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}
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Applications may receive 2 cmsgs:
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- SCM_DEVMEM_DMABUF: this indicates the fragment landed in the dmabuf indicated
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by dmabuf_id.
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- SCM_DEVMEM_LINEAR: this indicates the fragment landed in the linear buffer.
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This typically happens when the NIC is unable to split the packet at the
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header boundary, such that part (or all) of the payload landed in host
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memory.
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Applications may receive no SO_DEVMEM_* cmsgs. That indicates non-devmem,
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regular TCP data that landed on an RX queue not bound to a dmabuf.
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Freeing frags
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-------------
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Frags received via SCM_DEVMEM_DMABUF are pinned by the kernel while the user
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processes the frag. The user must return the frag to the kernel via
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SO_DEVMEM_DONTNEED::
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ret = setsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_DONTNEED, &token,
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sizeof(token));
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The user must ensure the tokens are returned to the kernel in a timely manner.
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Failure to do so will exhaust the limited dmabuf that is bound to the RX queue
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and will lead to packet drops.
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Implementation & Caveats
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========================
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Unreadable skbs
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---------------
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Devmem payloads are inaccessible to the kernel processing the packets. This
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results in a few quirks for payloads of devmem skbs:
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- Loopback is not functional. Loopback relies on copying the payload, which is
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not possible with devmem skbs.
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- Software checksum calculation fails.
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- TCP Dump and bpf can't access devmem packet payloads.
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Testing
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=======
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More realistic example code can be found in the kernel source under
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``tools/testing/selftests/net/ncdevmem.c``
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ncdevmem is a devmem TCP netcat. It works very similarly to netcat, but
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receives data directly into a udmabuf.
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To run ncdevmem, you need to run it on a server on the machine under test, and
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you need to run netcat on a peer to provide the TX data.
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ncdevmem has a validation mode as well that expects a repeating pattern of
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incoming data and validates it as such. For example, you can launch
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ncdevmem on the server by::
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ncdevmem -s <server IP> -c <client IP> -f eth1 -d 3 -n 0000:06:00.0 -l \
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-p 5201 -v 7
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On client side, use regular netcat to send TX data to ncdevmem process
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on the server::
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yes $(echo -e \\x01\\x02\\x03\\x04\\x05\\x06) | \
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tr \\n \\0 | head -c 5G | nc <server IP> 5201 -p 5201
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@ -49,6 +49,7 @@ Contents:
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cdc_mbim
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dccp
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dctcp
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devmem
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dns_resolver
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driver
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eql
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