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

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

On 02/27/2017 07:26 AM, Bob Liu wrote:
> On 2017/2/24 12:53, Jerome Glisse wrote:
>> On Fri, Feb 24, 2017 at 09:06:19AM +0800, Bob Liu wrote:
>>> On 2017/2/21 21:39, Anshuman Khandual wrote:
>>>> On 02/21/2017 04:41 PM, Michal Hocko wrote:
>>>>> On Fri 17-02-17 17:11:57, Anshuman Khandual wrote:
>>>>> [...]
>>>>>> * User space using mbind() to get CDM memory is an additional benefit
>>>>>>   we get by making the CDM plug in as a node and be part of the buddy
>>>>>>   allocator. But the over all idea from the user space point of view
>>>>>>   is that the application can allocate any generic buffer and try to
>>>>>>   use the buffer either from the CPU side or from the device without
>>>>>>   knowing about where the buffer is really mapped physically. That
>>>>>>   gives a seamless and transparent view to the user space where CPU
>>>>>>   compute and possible device based compute can work together. This
>>>>>>   is not possible through a driver allocated buffer.
>>>>>
>>>>> But how are you going to define any policy around that. Who is allowed
>>>>
>>>> The user space VMA can define the policy with a mbind(MPOL_BIND) call
>>>> with CDM/CDMs in the nodemask.
>>>>
>>>>> to allocate and how much of this "special memory". Is it possible that
>>>>
>>>> Any user space application with mbind(MPOL_BIND) call with CDM/CDMs in
>>>> the nodemask can allocate from the CDM memory. "How much" gets controlled
>>>> by how we fault from CPU and the default behavior of the buddy allocator.
>>>>
>>>>> we will eventually need some access control mechanism? If yes then mbind
>>>>
>>>> No access control mechanism is needed. If an application wants to use
>>>> CDM memory by specifying in the mbind() it can. Nothing prevents it
>>>> from using the CDM memory.
>>>>
>>>>> is really not suitable interface to (ab)use. Also what should happen if
>>>>> the mbind mentions only CDM memory and that is depleted?
>>>>
>>>> IIUC *only CDM* cannot be requested from user space as there are no user
>>>> visible interface which can translate to __GFP_THISNODE. MPOL_BIND with
>>>> CDM in the nodemask will eventually pick a FALLBACK zonelist which will
>>>> have zones of the system including CDM ones. If the resultant CDM zones
>>>> run out of memory, we fail the allocation request as usual.
>>>>
>>>>>
>>>>> Could you also explain why the transparent view is really better than
>>>>> using a device specific mmap (aka CDM awareness)?
>>>>
>>>> Okay with a transparent view, we can achieve a control flow of application
>>>> like the following.
>>>>
>>>> (1) Allocate a buffer:		alloc_buffer(buf, size)
>>>> (2) CPU compute on buffer:	cpu_compute(buf, size)
>>>> (3) Device compute on buffer:	device_compute(buf, size)
>>>> (4) CPU compute on buffer:	cpu_compute(buf, size)
>>>> (5) Release the buffer:		release_buffer(buf, size)
>>>>
>>>> With assistance from a device specific driver, the actual page mapping of
>>>> the buffer can change between system RAM and device memory depending on
>>>> which side is accessing at a given point. This will be achieved through
>>>> driver initiated migrations.
>>>>
>>>
>>> Sorry, I'm a bit confused here.
>>> What's the difference with the Heterogeneous memory management?
>>> Which also "allows to use device memory transparently inside any process
>>> without any modifications to process program code."
>>
>> HMM is first and foremost for platform (like Intel) where CPU can not
>> access device memory in cache coherent way or at all. CDM is for more
>> advance platform with a system bus that allow the CPU to access device
>> memory in cache coherent way.
>>
>> Hence CDM was design to integrate more closely in existing concept like
>> NUMA. From my point of view it is like another level in the memory
>> hierarchy. Nowaday you have local node memory and other node memory.
>> In not too distant future you will have fast CPU on die memory, local
>> memory (you beloved DDR3/DDR4), slightly slower but gigantic persistant
>> memory and also device memory (all those local to a node).
>>
>> On top of that you will still have the regular NUMA hierarchy between
>> nodes. But each node will have its own local hierarchy of memory.
>>
>> CDM wants to integrate with existing memory hinting API and i believe
>> this is needed to get some experience with how end user might want to
>> use this to fine tune their application.
>>
>> Some bit of HMM are generic and will be reuse by CDM, for instance the
>> DMA capable memory migration helpers. Wether they can also share HMM
>> approach of using ZONE_DEVICE is yet to be proven but it comes with
>> limitations (can't be on lru or have device lru) that might hinder a
>> closer integration of CDM memory with many aspect of kernel mm.
>>
>>
>> This is my own view and it likely differ in some way from the view of
>> the people behind CDM :)
>>
> 
> Got it, thank you for the kindly explanation.
> And also thank you, John.

Thanks Jerome and John for helping out with the detailed explanation.

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