Re: [RFC PATCH net-next v3 0/9] net/smc: Introduce SMC-D-based OS internal communication acceleration

[Wen Gu]

On 2023/2/16 00:18, Wen Gu wrote:

> Hi, all
>
> # Background
>
> The background and previous discussion can be referred from [1].
>
> We found SMC-D can be used to accelerate OS internal communication, such as
> loopback or between two containers within the same OS instance. So this patch
> set provides a kind of SMC-D dummy device (we call it the SMC-D loopback device)
> to emulate an ISM device, so that SMC-D can also be used on architectures
> other than s390. The SMC-D loopback device are designed as a system global
> device, visible to all containers.
>
> This version is implemented based on the generalized interface provided by [2].
> And there is an open issue of this version, which will be mentioned later.
>
> # Design
>
> This patch set basically follows the design of the previous version.
>
> Patch #1/9 ~ #3/9 attempt to decouple ISM-related structures from the SMC-D
> generalized code and extract some helpers to make SMC-D protocol compatible
> with devices other than s390 ISM device.
>
> Patch #4/9 introduces a kind of loopback device, which is defined as SMC-D v2
> device and designed to provide communication between SMC sockets in the same OS
> instance.
>
>   +-------------------------------------------+
>   |  +--------------+       +--------------+  |
>   |  | SMC socket A |       | SMC socket B |  |
>   |  +--------------+       +--------------+  |
>   |       ^                         ^         |
>   |       |    +----------------+   |         |
>   |       |    |   SMC stack    |   |         |
>   |       +--->| +------------+ |<--|         |
>   |            | |   dummy    | |             |
>   |            | |   device   | |             |
>   |            +-+------------+-+             |
>   |                   OS                      |
>   +-------------------------------------------+
>
> Patch #5/9 ~ #8/9 expand SMC-D protocol interface (smcd_ops) for scenarios where
> SMC-D is used to communicate within VM (loopback here) or between VMs on the same
> host (based on virtio-ism device, see [3]). What these scenarios have in common
> is that the local sndbuf and peer RMB can be mapped to same physical memory region,
> so the data copy between the local sndbuf and peer RMB can be omitted. Performance
> improvement brought by this extension can be found in # Benchmark Test.
>
>   +----------+                     +----------+
>   | socket A |                     | socket B |
>   +----------+                     +----------+
>         |                               ^
>         |         +---------+           |
>    regard as      |         | ----------|
>    local sndbuf   |  B's    |     regard as
>         |         |  RMB    |     local RMB
>         |-------> |         |
>                   +---------+
>
> Patch #9/9 realizes the support of loopback device for the above-mentioned expanded
> SMC-D protocol interface.
>
> # Benchmark Test
>
>   * Test environments:
>        - VM with Intel Xeon Platinum 8 core 2.50GHz, 16 GiB mem.
>        - SMC sndbuf/RMB size 1MB.
>
>   * Test object:
>        - TCP lo: run on TCP loopback.
>        - domain: run on UNIX domain.
>        - SMC lo: run on SMC loopback device with patch #1/9 ~ #4/9.
>        - SMC lo-nocpy: run on SMC loopback device with patch #1/9 ~ #9/9.
>
> 1. ipc-benchmark (see [4])
>
>   - ./<foo> -c 1000000 -s 100
>
>                      TCP-lo              domain              SMC-lo          SMC-lo-nocpy
> Message
> rate (msg/s)         79025      115736(+46.45%)    146760(+85.71%)       149800(+89.56%)
>
> 2. sockperf
>
>   - serv: <smc_run> taskset -c <cpu> sockperf sr --tcp
>   - clnt: <smc_run> taskset -c <cpu> sockperf { tp | pp } --tcp --msg-size={ 64000 for tp | 14 for pp } -i 127.0.0.1 -t 30
>
>                      TCP-lo                  SMC-lo             SMC-lo-nocpy
> Bandwidth(MBps)   4822.388        4940.918(+2.56%)         8086.67(+67.69%)
> Latency(us)          6.298          3.352(-46.78%)            3.35(-46.81%)
>
> 3. iperf3
>
>   - serv: <smc_run> taskset -c <cpu> iperf3 -s
>   - clnt: <smc_run> taskset -c <cpu> iperf3 -c 127.0.0.1 -t 15
>
>                      TCP-lo                  SMC-lo             SMC-lo-nocpy
> Bitrate(Gb/s)         40.7            40.5(-0.49%)            72.4(+77.89%)
>
> 4. nginx/wrk
>
>   - serv: <smc_run> nginx
>   - clnt: <smc_run> wrk -t 8 -c 500 -d 30 http://127.0.0.1:80
>
>                      TCP-lo                  SMC-lo             SMC-lo-nocpy
> Requests/s       155994.57      214544.79(+37.53%)       215538.55(+38.17%)
>
>
> # Open issue
>
> The open issue has not been resolved now is about how to detect that the source
> and target of CLC proposal are within the same OS instance and can communicate
> through the SMC-D loopback device. Similar issue also exists when using virtio-ism
> devices (the background and details of virtio-ism device can be referred from [3]).
> In previous discussions, multiple options were proposed (see [5]). Thanks again for
> the help of the community. cc Alexandra Winter :)
>
> But as we discussed, these solutions have some imperfection. So this version of RFC
> continues to use previous workaround, that is, a 64-bit random GID is generated for
> SMC-D loopback device. If the GIDs of the devices found by two peers are the same,
> then they are considered to be in the same OS instance and can communicate with each
> other by the loopback device.
>
> This approach has very small risk. Assume the following situations:
>
> (1) Assume that the SMC-D loopback devices of the two OS instances happen to
>      generate the same 64-bit GID.
>
>      For the convenience of description, we refer to the sockets on these two
>      different OS instance as server A and client B.
>
>      A will misjudge that the two are on the same OS instance because the same GID
>      in CLC proposal message. Then A creates its RMB and sends 64-bit token-A to B
>      in CLC accept message.
>
>      B receives the CLC accept message. And according to patch #7/9, B tries to
>      attach its sndbuf to A's RMB by token-A.
>
> (2) Assume that the OS instance where B is located happens to have an unattached
>      RMB whose 64-bit token is same as token-A.
>
>      Then B successfully attaches its sndbuf to the wrong RMB, and creates its RMB,
>      sends token-B to A in CLC confirm message.
>
>      Similarly, A receives the message and tries to attach its sndbuf to B's RMB by
>      token-B.
>
> (3) Similar to (2), assume that the OS instance where A is located happens to have
>      an unattached RMB whose 64-bit token is same as token-B.
>
>      Then A successfully attach its sndbuf to the wrong RMB. Both sides mistakenly
>      believe that an SMC-D connection based on the loopback device is established
>      between them.
>
> If the above 3 coincidences all happen, that is, 64-bit random number conflicts occur
> 3 times, then an unreachable SMC-D connection will be established, which is nasty.
> If one of above is not satisfied, it will safely fallback to TCP.
>
> Since the chances of these happening are very small, I wonder if this risk of 1/2^(64*3)
> probability can be tolerated ?

Hi,

Any comments about this open issue or other parts of this RFC patch set? :)

Thanks,
Wen Gu

> Another way to solve this open issue is using a 128-bit UUID to identify SMC-D loopback
> device or virtio-ism device, because the probability of a 128-bit UUID collision is
> considered negligible. But it may need to extend the CLC message to carry a longer GID,
> which is the last option.
>
> v3->v2
>   1. Adapt new generalized interface provided by [2];
>   2. Select loopback device through SMC-D v2 protocol;
>   3. Split the loopback-related implementation and generic implementation into different
>      patches more reasonably.
>
> v1->v2
>   1. Fix some build WARNINGs complained by kernel test rebot
>      Reported-by: kernel test robot <lkp@xxxxxxxxx>
>   2. Add iperf3 test data.
>
> [1] https://lore.kernel.org/netdev/1671506505-104676-1-git-send-email-guwen@xxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/netdev/20230123181752.1068-1-jaka@xxxxxxxxxxxxx/
> [3] https://lists.oasis-open.org/archives/virtio-comment/202302/msg00148.html
> [4] https://github.com/goldsborough/ipc-bench
> [5] https://lore.kernel.org/netdev/b9867c7d-bb2b-16fc-feda-b79579aa833d@xxxxxxxxxxxxx/
>
> Wen Gu (9):
>    net/smc: Decouple ism_dev from SMC-D device dump
>    net/smc: Decouple ism_dev from SMC-D DMB registration
>    net/smc: Extract v2 check helper from SMC-D device registration
>    net/smc: Introduce SMC-D loopback device
>    net/smc: Introduce an interface for getting DMB attribute
>    net/smc: Introudce interfaces for DMB attach and detach
>    net/smc: Avoid data copy from sndbuf to peer RMB in SMC-D
>    net/smc: Modify cursor update logic when using mappable DMB
>    net/smc: Add interface implementation of loopback device
>
>   drivers/s390/net/ism_drv.c |   5 +-
>   include/net/smc.h          |  18 +-
>   net/smc/Makefile           |   2 +-
>   net/smc/af_smc.c           |  26 ++-
>   net/smc/smc_cdc.c          |  59 ++++--
>   net/smc/smc_cdc.h          |   1 +
>   net/smc/smc_core.c         |  70 ++++++-
>   net/smc/smc_core.h         |   1 +
>   net/smc/smc_ism.c          |  79 ++++++--
>   net/smc/smc_ism.h          |   4 +
>   net/smc/smc_loopback.c     | 442 +++++++++++++++++++++++++++++++++++++++++++++
>   net/smc/smc_loopback.h     |  55 ++++++
>   12 files changed, 725 insertions(+), 37 deletions(-)
>   create mode 100644 net/smc/smc_loopback.c
>   create mode 100644 net/smc/smc_loopback.h