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From: Tom Talpey <tom@talpey.com>
To: Haakon Bugge <haakon.bugge@oracle.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 13:49:15 -0400
Message-ID: <7b9e7d9c-13d7-0d18-23b4-0d94409c7741@talpey.com> (raw)
In-Reply-To: <1FA38618-E245-4C53-BF49-6688CA93C660@oracle.com>

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!!

>> 	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.

Ok, and yes agreed the same principle would apply.

Is there any implication if a PCIe switch were present on the
motherboard? The switch is allowed to do some creative routing
if the operation is relaxed, correct?

Tom.

> 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
2021-04-09 17:49                 ` Tom Talpey [this message]
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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