Date:   Fri, 9 Apr 2021 16:27:10 +0000
From:   Haakon Bugge <haakon.bugge@...cle.com>
To:     Tom Talpey <tom@...pey.com>
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Subject: Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs



> On 9 Apr 2021, at 17:32, Tom Talpey <tom@...pey.com> wrote:
> 
> On 4/9/2021 10:45 AM, Chuck Lever III wrote:
>>> On Apr 9, 2021, at 10:26 AM, Tom Talpey <tom@...pey.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.



> 	PCIe interrupt (flushes RO pipeline for all three ops above)

Before the interrupt, the HCA will write the EQE (Event Queue Entry). This has to be a Strongly Ordered write to "push" prior written CQEs so that when the EQE is observed, the prior writes of CQEs have global visibility.

And the MSI-X write likewise, to avoid spurious interrupts.



Thxs, Håkon

> 
> RPC/RDMA polls CQ
> 	Reaps receive completion
> 
> RDMA Send arrives
> 	PCIe RO Write for receive data
> 	PCIe RO write for receive descriptor
> 	Does *not* interrupt, since CQ not armed
> 
> RPC/RDMA continues to poll CQ
> 	Reaps receive completion
> 	PCIe RO writes not yet flushed
> 	Processes incomplete in-memory data
> 	Bzzzt
> 
> Hopefully, the adapter performs a PCIe flushing read, or something
> to avoid this when an interrupt is not generated. Alternatively, I'm
> overly paranoid.
> 
> Tom.
> 
>>> The Mellanox adapter, itself, historically has strict in-order DMA
>>> semantics, and while it's great to relax that, changing it by default
>>> for all consumers is something to consider very cautiously.
>>> 
>>>> Still, obviously people should test on the platforms they have.
>>> 
>>> Yes, and "test" be taken seriously with focus on ULP data integrity.
>>> Speedups will mean nothing if the data is ever damaged.
>> I agree that data integrity comes first.
>> Since I currently don't have facilities to test RO in my lab, the
>> community will have to agree on a set of tests and expected results
>> that specifically exercise the corner cases you are concerned about.
>> --
>> Chuck Lever

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