On Sun, Aug 13, 2023 at 6:12 PM David Ahern <dsahern@xxxxxxxxxx> wrote: > > On 8/9/23 7:57 PM, Mina Almasry wrote: > > Changes in RFC v2: > > ------------------ > > > > The sticking point in RFC v1[1] was the dma-buf pages approach we used to > > deliver the device memory to the TCP stack. RFC v2 is a proof-of-concept > > that attempts to resolve this by implementing scatterlist support in the > > networking stack, such that we can import the dma-buf scatterlist > > directly. This is the approach proposed at a high level here[2]. > > > > Detailed changes: > > 1. Replaced dma-buf pages approach with importing scatterlist into the > > page pool. > > 2. Replace the dma-buf pages centric API with a netlink API. > > 3. Removed the TX path implementation - there is no issue with > > implementing the TX path with scatterlist approach, but leaving > > out the TX path makes it easier to review. > > 4. Functionality is tested with this proposal, but I have not conducted > > perf testing yet. I'm not sure there are regressions, but I removed > > perf claims from the cover letter until they can be re-confirmed. > > 5. Added Signed-off-by: contributors to the implementation. > > 6. Fixed some bugs with the RX path since RFC v1. > > > > Any feedback welcome, but specifically the biggest pending questions > > needing feedback IMO are: > > > > 1. Feedback on the scatterlist-based approach in general. > > 2. Netlink API (Patch 1 & 2). > > 3. Approach to handle all the drivers that expect to receive pages from > > the page pool (Patch 6). > > > > [1] https://lore.kernel.org/netdev/dfe4bae7-13a0-3c5d-d671-f61b375cb0b4@xxxxxxxxx/T/ > > [2] https://lore.kernel.org/netdev/CAHS8izPm6XRS54LdCDZVd0C75tA1zHSu6jLVO8nzTLXCc=H7Nw@xxxxxxxxxxxxxx/ > > > > ---------------------- > > > > * TL;DR: > > > > Device memory TCP (devmem TCP) is a proposal for transferring data to and/or > > from device memory efficiently, without bouncing the data to a host memory > > buffer. > > > > * Problem: > > > > A large amount of data transfers have device memory as the source and/or > > destination. Accelerators drastically increased the volume of such transfers. > > Some examples include: > > - ML accelerators transferring large amounts of training data from storage into > > GPU/TPU memory. In some cases ML training setup time can be as long as 50% of > > TPU compute time, improving data transfer throughput & efficiency can help > > improving GPU/TPU utilization. > > > > - Distributed training, where ML accelerators, such as GPUs on different hosts, > > exchange data among them. > > > > - Distributed raw block storage applications transfer large amounts of data with > > remote SSDs, much of this data does not require host processing. > > > > Today, the majority of the Device-to-Device data transfers the network are > > implemented as the following low level operations: Device-to-Host copy, > > Host-to-Host network transfer, and Host-to-Device copy. > > > > The implementation is suboptimal, especially for bulk data transfers, and can > > put significant strains on system resources, such as host memory bandwidth, > > PCIe bandwidth, etc. One important reason behind the current state is the > > kernel’s lack of semantics to express device to network transfers. > > > > * Proposal: > > > > In this patch series we attempt to optimize this use case by implementing > > socket APIs that enable the user to: > > > > 1. send device memory across the network directly, and > > 2. receive incoming network packets directly into device memory. > > > > Packet _payloads_ go directly from the NIC to device memory for receive and from > > device memory to NIC for transmit. > > Packet _headers_ go to/from host memory and are processed by the TCP/IP stack > > normally. The NIC _must_ support header split to achieve this. > > > > Advantages: > > > > - Alleviate host memory bandwidth pressure, compared to existing > > network-transfer + device-copy semantics. > > > > - Alleviate PCIe BW pressure, by limiting data transfer to the lowest level > > of the PCIe tree, compared to traditional path which sends data through the > > root complex. > > > > * Patch overview: > > > > ** Part 1: netlink API > > > > Gives user ability to bind dma-buf to an RX queue. > > > > ** Part 2: scatterlist support > > > > Currently the standard for device memory sharing is DMABUF, which doesn't > > generate struct pages. On the other hand, networking stack (skbs, drivers, and > > page pool) operate on pages. We have 2 options: > > > > 1. Generate struct pages for dmabuf device memory, or, > > 2. Modify the networking stack to process scatterlist. > > > > Approach #1 was attempted in RFC v1. RFC v2 implements approach #2. > > > > ** part 3: page pool support > > > > We piggy back on page pool memory providers proposal: > > https://github.com/kuba-moo/linux/tree/pp-providers > > > > It allows the page pool to define a memory provider that provides the > > page allocation and freeing. It helps abstract most of the device memory > > TCP changes from the driver. > > > > ** part 4: support for unreadable skb frags > > > > Page pool iovs are not accessible by the host; we implement changes > > throughput the networking stack to correctly handle skbs with unreadable > > frags. > > > > ** Part 5: recvmsg() APIs > > > > We define user APIs for the user to send and receive device memory. > > > > Not included with this RFC is the GVE devmem TCP support, just to > > simplify the review. Code available here if desired: > > https://github.com/mina/linux/tree/tcpdevmem > > > > This RFC is built on top of net-next with Jakub's pp-providers changes > > cherry-picked. > > > > * NIC dependencies: > > > > 1. (strict) Devmem TCP require the NIC to support header split, i.e. the > > capability to split incoming packets into a header + payload and to put > > each into a separate buffer. Devmem TCP works by using device memory > > for the packet payload, and host memory for the packet headers. > > > > 2. (optional) Devmem TCP works better with flow steering support & RSS support, > > i.e. the NIC's ability to steer flows into certain rx queues. This allows the > > sysadmin to enable devmem TCP on a subset of the rx queues, and steer > > devmem TCP traffic onto these queues and non devmem TCP elsewhere. > > > > The NIC I have access to with these properties is the GVE with DQO support > > running in Google Cloud, but any NIC that supports these features would suffice. > > I may be able to help reviewers bring up devmem TCP on their NICs. > > > > * Testing: > > > > The series includes a udmabuf kselftest that show a simple use case of > > devmem TCP and validates the entire data path end to end without > > a dependency on a specific dmabuf provider. > > > > ** Test Setup > > > > Kernel: net-next with this RFC and memory provider API cherry-picked > > locally. > > > > Hardware: Google Cloud A3 VMs. > > > > NIC: GVE with header split & RSS & flow steering support. > > This set seems to depend on Jakub's memory provider patches and a netdev > driver change which is not included. For the testing mentioned here, you > must have a tree + branch with all of the patches. Is it publicly available? > Yes, the net-next based branch is right here: https://github.com/mina/linux/tree/tcpdevmem Here is the git log of that branch: https://github.com/mina/linux/commits/tcpdevmem FWIW, it's already linked from the (long) cover letter, at the end of the '* Patch overview:' section. The branch includes all you mentioned above. The netdev driver I'm using in the GVE. It also includes patches to implement header split & flow steering for GVE (being upstreamed separately), and some debug changes. > It would be interesting to see how well (easy) this integrates with > io_uring. Besides avoiding all of the syscalls for receiving the iov and > releasing the buffers back to the pool, io_uring also brings in the > ability to seed a page_pool with registered buffers which provides a > means to get simpler Rx ZC for host memory. > > Overall I like the intent and possibilities for extensions, but a lot of > details are missing - perhaps some are answered by seeing an end-to-end > implementation. -- Thanks, Mina
