[RFC PATCH v2 00/11] Device Memory TCP
Christian König
christian.koenig at amd.com
Thu Aug 10 10:29:08 UTC 2023
Am 10.08.23 um 03:57 schrieb Mina Almasry:
> 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.
Impressive work, I didn't thought that this would be possible that "easily".
Please note that we have considered replacing scatterlists with simple
arrays of DMA-addresses in the DMA-buf framework to avoid people trying
to access the struct page inside the scatterlist.
It might be a good idea to push for that first before this here is
finally implemented.
GPU drivers already convert the scatterlist used to arrays of
DMA-addresses as soon as they get them. This leaves RDMA and V4L as the
other two main users which would need to be converted.
> 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.
As far as I can see this sounds like the right thing to do in general.
Question is rather if we should stick with scatterlist, use array of
DMA-addresses or maybe even come up with a completely new structure.
> 2. Netlink API (Patch 1 & 2).
How does netlink manage the lifetime of objects?
> 3. Approach to handle all the drivers that expect to receive pages from
> the page pool (Patch 6).
Can't say anything about that. I know TCP/IP inside out, but I'm a GPU
and not a network driver author.
Regards,
Christian.
>
> [1] https://lore.kernel.org/netdev/dfe4bae7-13a0-3c5d-d671-f61b375cb0b4@gmail.com/T/
> [2] https://lore.kernel.org/netdev/CAHS8izPm6XRS54LdCDZVd0C75tA1zHSu6jLVO8nzTLXCc=H7Nw@mail.gmail.com/
>
> ----------------------
>
> * 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.
>
> Mina Almasry (11):
> net: add netdev netlink api to bind dma-buf to a net device
> netdev: implement netlink api to bind dma-buf to netdevice
> netdev: implement netdevice devmem allocator
> memory-provider: updates to core provider API for devmem TCP
> memory-provider: implement dmabuf devmem memory provider
> page-pool: add device memory support
> net: support non paged skb frags
> net: add support for skbs with unreadable frags
> tcp: implement recvmsg() RX path for devmem TCP
> net: add SO_DEVMEM_DONTNEED setsockopt to release RX pages
> selftests: add ncdevmem, netcat for devmem TCP
>
> Documentation/netlink/specs/netdev.yaml | 27 ++
> include/linux/netdevice.h | 61 +++
> include/linux/skbuff.h | 54 ++-
> include/linux/socket.h | 1 +
> include/net/page_pool.h | 186 ++++++++-
> include/net/sock.h | 2 +
> include/net/tcp.h | 5 +-
> include/uapi/asm-generic/socket.h | 6 +
> include/uapi/linux/netdev.h | 10 +
> include/uapi/linux/uio.h | 10 +
> net/core/datagram.c | 6 +
> net/core/dev.c | 214 ++++++++++
> net/core/gro.c | 2 +-
> net/core/netdev-genl-gen.c | 14 +
> net/core/netdev-genl-gen.h | 1 +
> net/core/netdev-genl.c | 103 +++++
> net/core/page_pool.c | 171 ++++++--
> net/core/skbuff.c | 80 +++-
> net/core/sock.c | 36 ++
> net/ipv4/tcp.c | 196 ++++++++-
> net/ipv4/tcp_input.c | 13 +-
> net/ipv4/tcp_ipv4.c | 7 +
> net/ipv4/tcp_output.c | 5 +-
> net/packet/af_packet.c | 4 +-
> tools/include/uapi/linux/netdev.h | 10 +
> tools/net/ynl/generated/netdev-user.c | 41 ++
> tools/net/ynl/generated/netdev-user.h | 46 ++
> tools/testing/selftests/net/.gitignore | 1 +
> tools/testing/selftests/net/Makefile | 5 +
> tools/testing/selftests/net/ncdevmem.c | 534 ++++++++++++++++++++++++
> 30 files changed, 1787 insertions(+), 64 deletions(-)
> create mode 100644 tools/testing/selftests/net/ncdevmem.c
>
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