Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs

Haakon Bugge
2021-04-12T18:35:15Z

> On 10 Apr 2021, at 15:30, David Laight <David.Laight@aculab.com> wrote:
>
> From: Tom Talpey
>> Sent: 09 April 2021 18:49
>> On 4/9/2021 12:27 PM, Haakon Bugge wrote:
>>>
>>>
>>>> On 9 Apr 2021, at 17:32, Tom Talpey <tom@talpey.com> wrote:
>>>>
>>>> On 4/9/2021 10:45 AM, Chuck Lever III wrote:
>>>>>> On Apr 9, 2021, at 10:26 AM, Tom Talpey <tom@talpey.com> wrote:
>>>>>>
>>>>>> On 4/6/2021 7:49 AM, Jason Gunthorpe wrote:
>>>>>>> On Mon, Apr 05, 2021 at 11:42:31PM +0000, Chuck Lever III wrote:
>>>>>>>
>>>>>>>> We need to get a better idea what correctness testing has been done,
>>>>>>>> and whether positive correctness testing results can be replicated
>>>>>>>> on a variety of platforms.
>>>>>>> RO has been rolling out slowly on mlx5 over a few years and storage
>>>>>>> ULPs are the last to change. eg the mlx5 ethernet driver has had RO
>>>>>>> turned on for a long time, userspace HPC applications have been using
>>>>>>> it for a while now too.
>>>>>>
>>>>>> I'd love to see RO be used more, it was always something the RDMA
>>>>>> specs supported and carefully architected for. My only concern is
>>>>>> that it's difficult to get right, especially when the platforms
>>>>>> have been running strictly-ordered for so long. The ULPs need
>>>>>> testing, and a lot of it.
>>>>>>
>>>>>>> We know there are platforms with broken RO implementations (like
>>>>>>> Haswell) but the kernel is supposed to globally turn off RO on all
>>>>>>> those cases. I'd be a bit surprised if we discover any more from this
>>>>>>> series.
>>>>>>> On the other hand there are platforms that get huge speed ups from
>>>>>>> turning this on, AMD is one example, there are a bunch in the ARM
>>>>>>> world too.
>>>>>>
>>>>>> My belief is that the biggest risk is from situations where completions
>>>>>> are batched, and therefore polling is used to detect them without
>>>>>> interrupts (which explicitly). The RO pipeline will completely reorder
>>>>>> DMA writes, and consumers which infer ordering from memory contents may
>>>>>> break. This can even apply within the provider code, which may attempt
>>>>>> to poll WR and CQ structures, and be tripped up.
>>>>> You are referring specifically to RPC/RDMA depending on Receive
>>>>> completions to guarantee that previous RDMA Writes have been
>>>>> retired? Or is there a particular implementation practice in
>>>>> the Linux RPC/RDMA code that worries you?
>>>>
>>>> Nothing in the RPC/RDMA code, which is IMO correct. The worry, which
>>>> is hopefully unfounded, is that the RO pipeline might not have flushed
>>>> when a completion is posted *after* posting an interrupt.
>>>>
>>>> Something like this...
>>>>
>>>> RDMA Write arrives
>>>> PCIe RO Write for data
>>>> PCIe RO Write for data
>>>> ...
>>>> RDMA Write arrives
>>>> PCIe RO Write for data
>>>> ...
>>>> RDMA Send arrives
>>>> PCIe RO Write for receive data
>>>> PCIe RO Write for receive descriptor
>>>
>>> Do you mean the Write of the CQE? It has to be Strongly Ordered for a correct implementation. Then
>> it will shure prior written RO date has global visibility when the CQE can be observed.
>>
>> I wasn't aware that a strongly-ordered PCIe Write will ensure that
>> prior relaxed-ordered writes went first. If that's the case, I'm
>> fine with it - as long as the providers are correctly coded!!

The PCIe spec (Table Ordering Rules Summary) is quite clear here (A Posted request is Memory Write Request in this context):

A Posted Request must not pass another Posted Request unless A2b applies.

A2b: A Posted Request with RO Set is permitted to pass another Posted Request.

Thxs, Håkon

>
> I remember trying to read the relevant section of the PCIe spec.
> (Possibly in a book that was trying to make it easier to understand!)
> It is about as clear as mud.
>
> I presume this is all about allowing PCIe targets (eg ethernet cards)
> to use relaxed ordering on write requests to host memory.
> And that such writes can be completed out of order?
>
> It isn't entirely clear that you aren't talking of letting the
> cpu do 'relaxed order' writes to PCIe targets!
>
> For a typical ethernet driver the receive interrupt just means
> 'go and look at the receive descriptor ring'.
> So there is an absolute requirement that the writes for data
> buffer complete before the write to the receive descriptor.
> There is no requirement for the interrupt (requested after the
> descriptor write) to have been seen by the cpu.
>
> Quite often the driver will find the 'receive complete'
> descriptor when processing frames from an earlier interrupt
> (and nothing to do in response to the interrupt itself).
>
> So the write to the receive descriptor would have to have RO clear
> to ensure that all the buffer writes complete first.
>
> (The furthest I've got into PCIe internals was fixing the bug
> in some vendor-supplied FPGA logic that failed to correctly
> handle multiple data TLP responses to a single read TLP.
> Fortunately it wasn't in the hard-IP bit.)
>
> David
>
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