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

[Bob Liu]

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.

Regards,
Bob