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From: Haakon Bugge <haakon.bugge@oracle.com>
To: Tom Talpey <tom@talpey.com>
Cc: Chuck Lever III <chuck.lever@oracle.com>,
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Subject: Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs
Date: Fri, 9 Apr 2021 16:27:10 +0000
Message-ID: <1FA38618-E245-4C53-BF49-6688CA93C660@oracle.com> (raw)
In-Reply-To: <4b02d1b2-be0e-0d1d-7ac3-38d32e44e77e@talpey.com>



> 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.



> 	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


  reply index

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-05  5:23 Leon Romanovsky
2021-04-05  5:23 ` [PATCH rdma-next 01/10] RDMA: Add access flags to ib_alloc_mr() and ib_mr_pool_init() Leon Romanovsky
2021-04-05 13:46   ` Christoph Hellwig
2021-04-06  5:24     ` Leon Romanovsky
2021-04-05 15:27   ` Bart Van Assche
2021-04-06  5:23     ` Leon Romanovsky
2021-04-06  5:27       ` Christoph Hellwig
2021-04-06  5:58         ` Leon Romanovsky
2021-04-06 12:13           ` Jason Gunthorpe
2021-04-06 12:30             ` Christoph Hellwig
2021-04-06 14:04               ` Jason Gunthorpe
2021-04-06 14:15                 ` Christoph Hellwig
2021-04-06 14:40                   ` Jason Gunthorpe
2021-04-06 14:54                     ` Christoph Hellwig
2021-04-06 15:03                       ` Christoph Hellwig
2021-04-07 18:28                       ` Jason Gunthorpe
2021-04-05  5:23 ` [PATCH rdma-next 02/10] RDMA/core: Enable Relaxed Ordering in __ib_alloc_pd() Leon Romanovsky
2021-04-05 18:01   ` Tom Talpey
2021-04-05 20:40     ` Adit Ranadive
2021-04-06  6:28     ` Leon Romanovsky
2021-04-05  5:23 ` [PATCH rdma-next 03/10] RDMA/iser: Enable Relaxed Ordering Leon Romanovsky
2021-04-05  5:23 ` [PATCH rdma-next 04/10] RDMA/rtrs: " Leon Romanovsky
2021-04-05  5:23 ` [PATCH rdma-next 05/10] RDMA/srp: " Leon Romanovsky
2021-04-05  5:24 ` [PATCH rdma-next 06/10] nvme-rdma: " Leon Romanovsky
2021-04-05  5:24 ` [PATCH rdma-next 07/10] cifs: smbd: " Leon Romanovsky
2021-04-05  5:24 ` [PATCH rdma-next 08/10] net/rds: " Leon Romanovsky
2021-04-05  5:24 ` [PATCH rdma-next 09/10] net/smc: " Leon Romanovsky
2021-04-05  5:24 ` [PATCH rdma-next 10/10] xprtrdma: " Leon Romanovsky
2021-04-05 13:41 ` [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs Christoph Hellwig
2021-04-05 14:08   ` Leon Romanovsky
2021-04-05 16:11     ` Santosh Shilimkar
2021-04-05 17:54     ` Tom Talpey
2021-04-05 20:07   ` Jason Gunthorpe
2021-04-05 23:42     ` Chuck Lever III
2021-04-05 23:50       ` Keith Busch
2021-04-06  5:12       ` Leon Romanovsky
2021-04-06 11:49       ` Jason Gunthorpe
2021-04-09 14:26         ` Tom Talpey
2021-04-09 14:45           ` Chuck Lever III
2021-04-09 15:32             ` Tom Talpey
2021-04-09 16:27               ` Haakon Bugge [this message]
2021-04-09 17:49                 ` Tom Talpey
2021-04-10 13:30                   ` David Laight
2021-04-12 18:32                     ` Haakon Bugge
2021-04-12 20:20                       ` Tom Talpey
2021-04-12 22:48                         ` Jason Gunthorpe
2021-04-14 14:16                           ` Tom Talpey
2021-04-14 14:41                             ` David Laight
2021-04-14 14:49                               ` Jason Gunthorpe
2021-04-14 14:44                             ` Jason Gunthorpe
2021-04-09 16:40           ` Jason Gunthorpe
2021-04-09 17:44             ` Tom Talpey
2021-04-06  2:37 ` Honggang LI
2021-04-06  5:09   ` Leon Romanovsky
2021-04-06 11:53     ` Jason Gunthorpe
2021-04-11 10:09       ` Max Gurtovoy

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