Re: [RFC 1/8] Introduce Peer-to-Peer memory (p2pmem) device

[Sinan Kaya <okaya@codeaurora.org>]

On 3/31/2017 6:42 PM, Logan Gunthorpe wrote:
> 
> 
> On 31/03/17 03:38 PM, Sinan Kaya wrote:
>> On 3/31/2017 5:23 PM, Logan Gunthorpe wrote:
>>> What exactly would you white/black list? It can't be the NIC or the
>>> disk. If it's going to be a white/black list on the switch or root port
>>> then you'd need essentially the same code to ensure they are all behind
>>> the same switch or root port.
>>
>> What is so special about being connected to the same switch?
>>
>> Why don't we allow the feature by default and blacklist by the root ports
>> that don't work with a quirk.
> 
> Well root ports have the same issue here. There may be more than one
> root port or other buses (ie QPI) between the devices in question. So
> you can't just say "this system has root port X therefore we can always
> use p2pmem". 

We only care about devices on the data path between two devices.

> In the end, if you want to do any kind of restrictions
> you're going to have to walk the tree, as the code currently does, and
> figure out what's between the devices being used and black or white list
> accordingly. Then seeing there's just such a vast number of devices out
> there you'd almost certainly have to use some kind of white list and not
> a black list. Then the question becomes which devices will be white
> listed? 

How about a combination of blacklist + time bomb + peer-to-peer feature?

You can put a restriction with DMI/SMBIOS such that all devices from 2016
work else they belong to blacklist.

> The first to be listed would be switches seeing they will always
> work. This is pretty much what we have (though it doesn't currently
> cover multiple levels of switches). The next step, if someone wanted to
> test with specific hardware, might be to allow the case where all the
> devices are behind the same root port which Intel Ivy Bridge or newer.

Sorry, I'm not familiar with Intel architecture. Based on what you just
wrote, I think I see your point. 

I'm trying to generalize what you are doing to a little
bigger context so that I can use it on another architecture like arm64
where I may or may not have a switch.

This text below is sort of repeating what you are writing above. 

How about this:

The goal is to find a common parent between any two devices that need to
use your code. 

- all bridges/switches on the data need to support peer-to-peer, otherwise
stop.

- Make sure that all devices on the data path are not blacklisted via your
code.

- If there is at least somebody blacklisted, we stop and the feature is
not allowed.

- If we find a common parent and no errors, you are good to go.

- We don't care about devices above the common parent whether they have
some feature X, Y, Z or not. 

Maybe, a little bit less code than what you have but it is flexible and
not that too hard to implement.

Well, the code is in RFC. I don't see why we can't remove some restrictions
and still have your code move forward. 

> However, I don't think a comprehensive white list should be a
> requirement for this work to go forward and I don't think anything
> you've suggested will remove any of the "ugliness".

I don't think the ask above is a very big deal. If you feel like
addressing this on another patchset like you suggested in your cover letter,
I'm fine with that too.

> 
> What we discussed at LSF was that only allowing cases with a switch was
> the simplest way to be sure any given setup would actually work.
> 
>> I'm looking at this from portability perspective to be honest.
> 
> I'm looking at this from the fact that there's a vast number of
> topologies and devices involved, and figuring out which will work is
> very complicated and could require a lot of hardware testing. The LSF
> folks were primarily concerned with not having users enable the feature
> and see breakage or terrible performance.
> 
>> I'd rather see the feature enabled by default without any assumptions.
>> Using it with a switch is just a use case that you happened to test.
>> It can allow new architectures to use your code tomorrow.
> 
> That's why I was advocating for letting userspace decide such that if
> you're setting up a system with this you say to use a specific p2pmem
> device and then you are responsible to test and benchmark it and decide
> to use it in going forward. However, this has received a lot of push back.

Yeah, we shouldn't trust the userspace for such things.

> 
> Logan
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
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