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

[Bob Liu]

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."

Thanks,
-Bob

>>  
>>> * The placement of the memory on the buffer can happen on system memory
>>>   when the CPU faults while accessing it. But a driver can manage the
>>>   migration between system RAM and CDM memory once the buffer is being
>>>   used from CPU and the device interchangeably. As you have mentioned
>>>   driver will have more information about where which part of the buffer
>>>   should be placed at any point of time and it can make it happen with
>>>   migration. So both allocation and placement are decided by the driver
>>>   during runtime. CDM provides the framework for this can kind device
>>>   assisted compute and driver managed memory placements.
>>>
>>> * If any application is not using CDM memory for along time placed on
>>>   its buffer and another application is forced to fallback on system
>>>   RAM when it really wanted is CDM, the driver can detect these kind
>>>   of situations through memory access patterns on the device HW and
>>>   take necessary migration decisions.