Date:   Wed, 21 Dec 2022 21:14:35 +0800
From:   Wen Gu <guwen@...ux.alibaba.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>, kgraul@...ux.ibm.com,
        wenjia@...ux.ibm.com, jaka@...ux.ibm.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next v2 0/5] net/smc:Introduce SMC-D based
 loopback acceleration



On 2022/12/20 22:02, Niklas Schnelle wrote:

> On Tue, 2022-12-20 at 11:21 +0800, Wen Gu wrote:
>> Hi, all
>>
>> # Background
>>
>> As previously mentioned in [1], we (Alibaba Cloud) are trying to use SMC
>> to accelerate TCP applications in cloud environment, improving inter-host
>> or inter-VM communication.
>>
>> In addition of these, we also found the value of SMC-D in scenario of local
>> inter-process communication, such as accelerate communication between containers
>> within the same host. So this RFC tries to provide a SMC-D loopback solution
>> in such scenario, to bring a significant improvement in latency and throughput
>> compared to TCP loopback.
>>
>> # Design
>>
>> This patch set provides a kind of SMC-D loopback solution.
>>
>> Patch #1/5 and #2/5 provide an SMC-D based dummy device, preparing for the
>> inter-process communication acceleration. Except for loopback acceleration,
>> the dummy device can also meet the requirements mentioned in [2], which is
>> providing a way to test SMC-D logic for broad community without ISM device.
>>
>>   +------------------------------------------+
>>   |  +-----------+           +-----------+   |
>>   |  | process A |           | process B |   |
>>   |  +-----------+           +-----------+   |
>>   |       ^                        ^         |
>>   |       |    +---------------+   |         |
>>   |       |    |   SMC stack   |   |         |
>>   |       +--->| +-----------+ |<--|         |
>>   |            | |   dummy   | |             |
>>   |            | |   device  | |             |
>>   |            +-+-----------+-+             |
>>   |                   VM                     |
>>   +------------------------------------------+
>>
>> Patch #3/5, #4/5, #5/5 provides a way to avoid data copy from sndbuf to RMB
>> and improve SMC-D loopback performance. Through extending smcd_ops with two
>> new semantic: attach_dmb and detach_dmb, sender's sndbuf shares the same
>> physical memory region with receiver's RMB. The data copied from userspace
>> to sender's sndbuf directly reaches the receiver's RMB without unnecessary
>> memory copy in the same kernel.
>>
>>   +----------+                     +----------+
>>   | socket A |                     | socket B |
>>   +----------+                     +----------+
>>         |                               ^
>>         |         +---------+           |
>>    regard as      |         | ----------|
>>    local sndbuf   |  B's    |     regard as
>>         |         |  RMB    |     local RMB
>>         |-------> |         |
>>                   +---------+
> 
> Hi Wen Gu,
> 
> I maintain the s390 specific PCI support in Linux and would like to
> provide a bit of background on this. You're surely wondering why we
> even have a copy in there for our ISM virtual PCI device. To understand
> why this copy operation exists and why we need to keep it working, one
> needs a bit of s390 aka mainframe background.
> 
> On s390 all (currently supported) native machines have a mandatory
> machine level hypervisor. All OSs whether z/OS or Linux run either on
> this machine level hypervisor as so called Logical Partitions (LPARs)
> or as second/third/… level guests on e.g. a KVM or z/VM hypervisor that
> in turn runs in an LPAR. Now, in terms of memory this machine level
> hypervisor sometimes called PR/SM unlike KVM, z/VM, or VMWare is a
> partitioning hypervisor without paging. This is one of the main reasons
> for the very-near-native performance of the machine hypervisor as the
> memory of its guests acts just like native RAM on other systems. It is
> never paged out and always accessible to IOMMU translated DMA from
> devices without the need for pinning pages and besides a trivial
> offset/limit adjustment an LPAR's MMU does the same amount of work as
> an MMU on a bare metal x86_64/ARM64 box.
> 
> It also means however that when SMC-D is used to communicate between
> LPARs via an ISM device there is  no way of mapping the DMBs to the
> same physical memory as there exists no MMU-like layer spanning
> partitions that could do such a mapping. Meanwhile for machine level
> firmware including the ISM virtual PCI device it is still possible to
> _copy_ memory between different memory partitions. So yeah while I do
> see the appeal of skipping the memcpy() for loopback or even between
> guests of a paging hypervisor such as KVM, which can map the DMBs on
> the same physical memory, we must keep in mind this original use case
> requiring a copy operation.
> 
> Thanks,
> Niklas
> 

Hi Niklas,

Thank you so much for the complete and detailed explanation! This provides
me a brand new perspective of s390 device that we hadn't dabbled in before.
Now I understand why shared memory is unavailable between different LPARs.

Our original intention of proposing loopback device and the incoming device
(virtio-ism) for inter-VM is to use SMC-D to accelerate communication in the
case with no existing s390 ISM devices. In our conception, s390 ISM device,
loopback device and virtio-ism device are parallel and are abstracted by smcd_ops.

  +------------------------+
  |          SMC-D         |
  +------------------------+
  -------- smcd_ops ---------
  +------+ +------+ +------+
  | s390 | | loop | |virtio|
  | ISM  | | back | | -ism |
  | dev  | | dev  | | dev  |
  +------+ +------+ +------+

We also believe that keeping the existing design and behavior of s390 ISM
device is unshaken. What we want to get support for is some smcd_ops extension
for devices with optional beneficial capability, such as nocopy here (Let's call
it this for now), which is really helpful for us in inter-process and inter-VM
scenario.

And coincided with IBM's intention to add APIs between SMC-D and devices to
support various devices for SMC-D, as mentioned in [2], we send out this RFC and
the incoming virio-ism RFC, to provide some examples.

>>
>> # Benchmark Test
>>
>>   * Test environments:
>>        - VM with Intel Xeon Platinum 8 core 2.50GHz, 16 GiB mem.
>>        - SMC sndbuf/RMB size 1MB.
>>
>>   * Test object:
>>        - TCP: run on TCP loopback.
>>        - domain: run on UNIX domain.
>>        - SMC lo: run on SMC loopback device with patch #1/5 ~ #2/5.
>>        - SMC lo-nocpy: run on SMC loopback device with patch #1/5 ~ #5/5.
>>
>> 1. ipc-benchmark (see [3])
>>
>>   - ./<foo> -c 1000000 -s 100
>>
>>                         TCP              domain              SMC-lo             SMC-lo-nocpy
>> Message
>> rate (msg/s)         75140      129548(+72.41)    152266(+102.64%)         151914(+102.17%)
> 
> Interesting that it does beat UNIX domain sockets. Also, see my below
> comment for nginx/wrk as this seems very similar.
> 
>>
>> 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                  SMC-lo             SMC-lo-nocpy
>> Bandwidth(MBps)   4943.359        4936.096(-0.15%)        8239.624(+66.68%)
>> Latency(us)          6.372          3.359(-47.28%)            3.25(-49.00%)
>>
>> 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                  SMC-lo             SMC-lo-nocpy
>> Bitrate(Gb/s)         40.5            41.4(+2.22%)            76.4(+88.64%)
>>
>> 4. nginx/wrk
>>
>>   - serv: <smc_run> nginx
>>   - clnt: <smc_run> wrk -t 8 -c 500 -d 30 http://127.0.0.1:80
>>
>>                         TCP                  SMC-lo             SMC-lo-nocpy
>> Requests/s       154643.22      220894.03(+42.84%)        226754.3(+46.63%)
> 
> 
> This result is very interesting indeed. So with the much more realistic
> nginx/wrk workload it seems to copy hurts much less than the
> iperf3/sockperf would suggest while SMC-D itself seems to help more.
> I'd hope that this translates to actual applications as well. Maybe
> this makes SMC-D based loopback interesting even while keeping the
> copy, at least until we can come up with a sane way to work a no-copy
> variant into SMC-D?
> 

I agree, nginx/wrk workload is much more realistic for many applications.

But we also encounter many other cases similar to sockperf on the cloud, which
requires high throughput, such as AI training and big data.

So avoidance of copying between DMBs can help these cases a lot :)

>>
>>
>> # Discussion
>>
>> 1. API between SMC-D and ISM device
>>
>> As Jan mentioned in [2], IBM are working on placing an API between SMC-D
>> and the ISM device for easier use of different "devices" for SMC-D.
>>
>> So, considering that the introduction of attach_dmb or detach_dmb can
>> effectively avoid data copying from sndbuf to RMB and brings obvious
>> throughput advantages in inter-VM or inter-process scenarios, can the
>> attach/detach semantics be taken into consideration when designing the
>> API to make it a standard ISM device behavior?
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> Due to the reasons explained above this behavior can't be emulated by
> ISM devices at least not when crossing partitions. Not sure if we can
> still incorporate it in the API and allow for both copying and
> remapping SMC-D like devices, it definitely needs careful consideration
> and I think also a better understanding of the benefit for real world
> workloads.
> 

Here I am not rigorous.

Nocopy shouldn't be a standard ISM device behavior indeed. Actually we hope it be a
standard optional _SMC-D_ device behavior and defined by smcd_ops.

For devices don't support these options, like ISM device on s390 architecture,
.attach_dmb/.detach_dmb and other reasonable extensions (which will be proposed to
discuss in incoming virtio-ism RFC) can be set to NULL or return invalid. And for
devices do support, they may be used for improving performance in some cases.

In addition, can I know more latest news about the API design? :) , like its scale, will
it be a almost refactor of existing interface or incremental patching? and its object,
will it be tailored for exact ISM behavior or to reserve some options for other devices,
like nocopy here? From my understanding of [2], it might be the latter?

>>
>> Maybe our RFC of SMC-D based inter-process acceleration (this one) and
>> inter-VM acceleration (will coming soon, which is the update of [1])
>> can provide some examples for new API design. And we are very glad to
>> discuss this on the mail list.
>>
>> 2. Way to select different ISM-like devices
>>
>> With the proposal of SMC-D loopback 'device' (this RFC) and incoming
>> device used for inter-VM acceleration as update of [1], SMC-D has more
>> options to choose from. So we need to consider that how to indicate
>> supported devices, how to determine which one to use, and their priority...
> 
> Agree on this part, though it is for the SMC maintainers to decide, I
> think we would definitely want to be able to use any upcoming inter-VM
> devices on s390 possibly also in conjunction with ISM devices for
> communication across partitions.
> 

Yes, this part needs to be discussed with SMC maintainers. And thank you, we are very glad
if our devices can be applied on s390 through the efforts.


Best Regards,
Wen Gu

>>
>> IMHO, this may require an update of CLC message and negotiation mechanism.
>> Again, we are very glad to discuss this with you on the mailing list.
>>
>> [1] https://lore.kernel.org/netdev/20220720170048.20806-1-tonylu@linux.alibaba.com/
>> [2] https://lore.kernel.org/netdev/35d14144-28f7-6129-d6d3-ba16dae7a646@linux.ibm.com/
>> [3] https://github.com/goldsborough/ipc-bench
>>
>> v1->v2
>>   1. Fix some build WARNINGs complained by kernel test rebot
>>      Reported-by: kernel test robot <lkp@...el.com>
>>   2. Add iperf3 test data.
>>
>> Wen Gu (5):
>>    net/smc: introduce SMC-D loopback device
>>    net/smc: choose loopback device in SMC-D communication
>>    net/smc: add dmb attach and detach interface
>>    net/smc: avoid data copy from sndbuf to peer RMB in SMC-D loopback
>>    net/smc: logic of cursors update in SMC-D loopback connections
>>
>>   include/net/smc.h      |   3 +
>>   net/smc/Makefile       |   2 +-
>>   net/smc/af_smc.c       |  88 +++++++++++-
>>   net/smc/smc_cdc.c      |  59 ++++++--
>>   net/smc/smc_cdc.h      |   1 +
>>   net/smc/smc_clc.c      |   4 +-
>>   net/smc/smc_core.c     |  62 +++++++++
>>   net/smc/smc_core.h     |   2 +
>>   net/smc/smc_ism.c      |  39 +++++-
>>   net/smc/smc_ism.h      |   2 +
>>   net/smc/smc_loopback.c | 358 +++++++++++++++++++++++++++++++++++++++++++++++++
>>   net/smc/smc_loopback.h |  63 +++++++++
>>   12 files changed, 662 insertions(+), 21 deletions(-)
>>   create mode 100644 net/smc/smc_loopback.c
>>   create mode 100644 net/smc/smc_loopback.h
>>

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