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

Tom Talpey
2021-04-09T14:26:35Z

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.

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.

Tom.