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

[chetan L <loke.chetan@gmail.com>]

CC'ing : linux-accelerators@vger.kernel.org

On Wed, Nov 15, 2017 at 6:44 PM, Jerome Glisse <jglisse@redhat.com> 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.


Chetan

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