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

[Jerome Glisse]

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 :)

Cheers,
Jérôme