linux-kernel - Re: [PATCH rdma-next] RDMA/rdmavt: Decouple QP and SGE lists allocations

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Date:   Thu, 13 May 2021 15:31:48 -0400
From:   Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Leon Romanovsky <leon@...nel.org>,
        "Marciniszyn, Mike" <mike.marciniszyn@...nelisnetworks.com>,
        Doug Ledford <dledford@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH rdma-next] RDMA/rdmavt: Decouple QP and SGE lists
 allocations

On 5/13/21 3:15 PM, Jason Gunthorpe wrote:
> On Thu, May 13, 2021 at 03:03:43PM -0400, Dennis Dalessandro wrote:
>> On 5/12/21 8:50 AM, Leon Romanovsky wrote:
>>> On Wed, May 12, 2021 at 12:25:15PM +0000, Marciniszyn, Mike wrote:
>>>>>> Thanks Leon, we'll get this put through our testing.
>>>>>
>>>>> Thanks a lot.
>>>>>
>>>>>>
>>>>
>>>> The patch as is passed all our functional testing.
>>>
>>> Thanks Mike,
>>>
>>> Can I ask you to perform a performance comparison between this patch and
>>> the following?
>>
>> We have years of performance data with the code the way it is. Please
>> maintain the original functionality of the code when moving things into the
>> core unless there is a compelling reason to change. That is not the case
>> here.
> 
> Well, making the core do node allocations for metadata on every driver
> is a pretty big thing to ask for with no data.

Can't you just make the call into the core take a flag for this? You are 
  looking to make a change to key behavior without any clear reason that 
I can see for why it needs to be that way. If there is a good reason, 
please explain so we can understand.

I would think the person authoring the patch should be responsible to 
prove their patch doesn't cause a regression. We are telling you it did 
make a difference when the code was first written, probably like 6 years 
ago. At the very least no one had an issue with this code 4 years ago 
the last time it was touched (by Leon btw). Nor issues with the other 
uses of the call.

We can have our performance experts look at it but it's going to take 
some time as it's nothing that has been on anyone's radar.

-Denny