Re: [PATCH 0/6] Cache coherent device memory (CDM) with HMM v5

[Bob Liu]

On 2017/7/21 9:41, Jerome Glisse wrote:
> On Fri, Jul 21, 2017 at 09:15:29AM +0800, Bob Liu wrote:
>> On 2017/7/20 23:03, Jerome Glisse wrote:
>>> On Wed, Jul 19, 2017 at 05:09:04PM +0800, Bob Liu wrote:
>>>> On 2017/7/19 10:25, Jerome Glisse wrote:
>>>>> On Wed, Jul 19, 2017 at 09:46:10AM +0800, Bob Liu wrote:
>>>>>> On 2017/7/18 23:38, Jerome Glisse wrote:
>>>>>>> On Tue, Jul 18, 2017 at 11:26:51AM +0800, Bob Liu wrote:
>>>>>>>> On 2017/7/14 5:15, Jérôme Glisse wrote:
> 
> [...]
> 
>>>> Then it's more like replace the numa node solution(CDM) with ZONE_DEVICE
>>>> (type MEMORY_DEVICE_PUBLIC). But the problem is the same, e.g how to make
>>>> sure the device memory say HBM won't be occupied by normal CPU allocation.
>>>> Things will be more complex if there are multi GPU connected by nvlink
>>>> (also cache coherent) in a system, each GPU has their own HBM.
>>>>
>>>> How to decide allocate physical memory from local HBM/DDR or remote HBM/
>>>> DDR? 
>>>>
>>>> If using numa(CDM) approach there are NUMA mempolicy and autonuma mechanism
>>>> at least.
>>>
>>> NUMA is not as easy as you think. First like i said we want the device
>>> memory to be isolated from most existing mm mechanism. Because memory
>>> is unreliable and also because device might need to be able to evict
>>> memory to make contiguous physical memory allocation for graphics.
>>>
>>
>> Right, but we need isolation any way.
>> For hmm-cdm, the isolation is not adding device memory to lru list, and many
>> if (is_device_public_page(page)) ...
>>
>> But how to evict device memory?
> 
> What you mean by evict ? Device driver can evict whenever they see the need
> to do so. CPU page fault will evict too. Process exit or munmap() will free
> the device memory.
> 
> Are you refering to evict in the sense of memory reclaim under pressure ?
> 
> So the way it flows for memory pressure is that if device driver want to
> make room it can evict stuff to system memory and if there is not enough

Yes, I mean this. 
So every driver have to maintain their own LRU-similar list instead of reuse what already in linux kernel.

> system memory than thing get reclaim as usual before device driver can
> make progress on device memory reclaim.
> 
> 
>>> Second device driver are not integrated that closely within mm and the
>>> scheduler kernel code to allow to efficiently plug in device access
>>> notification to page (ie to update struct page so that numa worker
>>> thread can migrate memory base on accurate informations).
>>>
>>> Third it can be hard to decide who win between CPU and device access
>>> when it comes to updating thing like last CPU id.
>>>
>>> Fourth there is no such thing like device id ie equivalent of CPU id.
>>> If we were to add something the CPU id field in flags of struct page
>>> would not be big enough so this can have repercusion on struct page
>>> size. This is not an easy sell.
>>>
>>> They are other issues i can't think of right now. I think for now it
>>
>> My opinion is most of the issues are the same no matter use CDM or HMM-CDM.
>> I just care about a more complete solution no matter CDM,HMM-CDM or other ways.
>> HMM or HMM-CDM depends on device driver, but haven't see a public/full driver to 
>> demonstrate the whole solution works fine.
> 
> I am working with NVidia close source driver team to make sure that it works
> well for them. I am also working on nouveau open source driver for same NVidia
> hardware thought it will be of less use as what is missing there is a solid
> open source userspace to leverage this. Nonetheless open source driver are in
> the work.
> 

Looking forward to see these drivers be public.

> The way i see it is start with HMM-CDM which isolate most of the changes in
> hmm code. Once we get more experience with real workload and not with device
> driver test suite then we can start revisiting NUMA and deeper integration
> with the linux kernel. I rather grow organicaly toward that than trying to
> design something that would make major changes all over the kernel without
> knowing for sure that we are going in the right direction. I hope that this
> make sense to others too.
> 

Make sense.

Thanks,
Bob Liu