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

[chetan L]

On Wed, Nov 15, 2017 at 7:23 PM, chetan L <loke.chetan@xxxxxxxxx> wrote:
> CC'ing : linux-accelerators@xxxxxxxxxxxxxxx
>

Sorry, CC'ing the correct list this time: linux-accelerators@xxxxxxxxxxxxxxxx



> On Wed, Nov 15, 2017 at 6:44 PM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>> On Wed, Nov 15, 2017 at 06:10:08PM -0800, chet l wrote:
>>> >> You may think it as a CCIX device or CAPI device.
>>> >> The requirement is eliminate any extra copy.
>>> >> A typical usecase/requirement is malloc() and madvise() allocate from
>>> >> device memory, then CPU write data to device memory directly and
>>> >> trigger device to read the data/do calculation.
>>> >
>>> > I suggest you rely on the device driver userspace API to do a migration after malloc
>>> > then. Something like:
>>> >   ptr = malloc(size);
>>> >   my_device_migrate(ptr, size);
>>> >
>>> > Which would call an ioctl of the device driver which itself would migrate memory or
>>> > allocate device memory for the range if pointer return by malloc is not yet back by
>>> > any pages.
>>> >
>>>
>>> So for CCIX, I don't think there is going to be an inline device
>>> driver that would allocate any memory for you. The expansion memory
>>> will become part of the system memory as part of the boot process. So,
>>> if the host DDR is 256GB and the CCIX expansion memory is 4GB, the
>>> total system mem will be 260GB.
>>>
>>> Assume that the 'mm' is taught to mark/anoint the ZONE_DEVICE(or
>>> ZONE_XXX) range from 256 to 260 GB. Then, for kmalloc it(mm) won't use
>>> the ZONE_DEV range. But for a malloc, it will/can use that range.
>>
>> HMM zone device memory would work with that, you just need to teach the
>> platform to identify this memory zone and not hotplug it. Again you
>> should rely on specific device driver API to allocate this memory.
>>
>
> @Jerome - a new linux-accelerator's list has just been created. I have
> CC'd that list since we have overlapping interests w.r.t CCIX.
>
> I cannot comment on surprise add/remove as of now ... will cross the
> bridge later.
>
>
>>> > There has been several discussions already about madvise/mbind/set_mempolicy/
>>> > move_pages and at this time i don't think we want to add or change any of them to
>>> > understand device memory. My personal opinion is that we first need to have enough
>>>
>>> We will visit these APIs when we are more closer to building exotic
>>> CCIX devices. And the plan is to present/express the CCIX proximity
>>> attributes just like a NUMA node-proximity attribute today. That way
>>> there would be minimal disruptions to the existing OS ecosystem.
>>
>> NUMA have been rejected previously see CDM/CAPI threads. So i don't see
>> it being accepted for CCIX either. My belief is that we want to hide this
>> inside device driver and only once we see multiple devices all doing the
>> same kind of thing we should move toward building something generic that
>> catter to CCIX devices.
>
>
> Thanks for pointing out the NUMA thingy. I will visit the CDM/CAPI
> threads to understand what was discussed before commenting further.
>