linux-kernel - Re: [PATCH V3 0/4] Define coherent device memory node

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Message-ID: <a0271d52-c60c-782a-5d0d-33c1d6d5508b@gmail.com>
Date:   Wed, 1 Mar 2017 13:42:40 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
        vbabka@...e.cz, minchan@...nel.org,
        aneesh.kumar@...ux.vnet.ibm.com, srikar@...ux.vnet.ibm.com,
        haren@...ux.vnet.ibm.com, jglisse@...hat.com,
        dave.hansen@...el.com, dan.j.williams@...el.com
Subject: Re: [PATCH V3 0/4] Define coherent device memory node

>>> The idea of this patchset was to introduce
>>> the concept of memory that is not necessarily system memory, but is coherent
>>> in terms of visibility/access with some restrictions
>>>
>>
>> Which should be done without special casing the page allocator, cpusets and
>> special casing how cpusets are handled. It's not necessary for any other
>> mechanism used to restrict access to portions of memory such as cpusets,
>> mempolicies or even memblock reservations.
>
> Agreed, I mentioned a limitation that we see a cpusets. I do agree that
> we should reuse any infrastructure we have, but cpusets are more static
> in nature and inheritence compared to the requirements of CDM.
>

Mel, I went back and looked at cpusets and found some limitations that
I mentioned earlier, isolating a particular node requires some amount
of laborious work in terms of isolating all tasks away from the root cpuset
and then creating a hierarchy where the root cpuset is empty and now
belong to a child cpuset that has everything but the node we intend to
ioslate. Even with hardwalling, it does not prevent allocations from
the parent cpuset.

I am trying to understand the concerns that you/Michal/Vlastimil have
so that Anshuman/I/other stake holders can respond to the concerns
in one place if that makes sense. Here are the concerns I have heard
so far

1. Lets not add any overhead to the page allocator path
2. Lets try and keep the allocator changes easy to read/parse
3. Why do we need a NUMA interface?
4. How does this compare with HMM?
5. Why can't we use cpusets?

Would that be a fair set of concerns to address?

@Anshuman/@...kar/@...esh anything else you'd like to add in terms
of concerns/issues? I think it will also make a good discussion thread
for those attending LSF/MM (I am not there) on this topic.

Balbir Singh.