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

[Dan Williams]

On Thu, Jul 20, 2017 at 6:41 PM, Jerome Glisse <jglisse@xxxxxxxxxx> 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, JÃrÃme 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
> 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.

Can you point to the nouveau patches? I still find these HMM patches
un-reviewable without an upstream consumer.