Implement svm without BO concept in xe driver

[Dave Airlie]

On Thu, 17 Aug 2023 at 12:13, Zeng, Oak <oak.zeng at intel.com> wrote:
>
> > -----Original Message-----
> > From: Dave Airlie <airlied at gmail.com>
> > Sent: August 16, 2023 6:52 PM
> > To: Felix Kuehling <felix.kuehling at amd.com>
> > Cc: Zeng, Oak <oak.zeng at intel.com>; Christian König
> > <christian.koenig at amd.com>; Thomas Hellström
> > <thomas.hellstrom at linux.intel.com>; Brost, Matthew
> > <matthew.brost at intel.com>; maarten.lankhorst at linux.intel.com;
> > Vishwanathapura, Niranjana <niranjana.vishwanathapura at intel.com>; Welty,
> > Brian <brian.welty at intel.com>; Philip Yang <Philip.Yang at amd.com>; intel-
> > xe at lists.freedesktop.org; dri-devel at lists.freedesktop.org
> > Subject: Re: Implement svm without BO concept in xe driver
> >
> > On Thu, 17 Aug 2023 at 08:15, Felix Kuehling <felix.kuehling at amd.com> wrote:
> > >
> > > On 2023-08-16 13:30, Zeng, Oak wrote:
> > > > I spoke with Thomas. We discussed two approaches:
> > > >
> > > > 1) make ttm_resource a central place for vram management functions such as
> > eviction, cgroup memory accounting. Both the BO-based driver and BO-less SVM
> > codes call into ttm_resource_alloc/free functions for vram allocation/free.
> > > >      *This way BO driver and SVM driver shares the eviction/cgroup logic, no
> > need to reimplment LRU eviction list in SVM driver. Cgroup logic should be in
> > ttm_resource layer. +Maarten.
> > > >      *ttm_resource is not a perfect match for SVM to allocate vram. It is still a
> > big overhead. The *bo* member of ttm_resource is not needed for SVM - this
> > might end up with invasive changes to ttm...need to look into more details
> > >
> > > Overhead is a problem. We'd want to be able to allocate, free and evict
> > > memory at a similar granularity as our preferred migration and page
> > > fault granularity, which defaults to 2MB in our SVM implementation.
> > >
> > >
> > > >
> > > > 2) svm code allocate memory directly from drm-buddy allocator, and expose
> > memory eviction functions from both ttm and svm so they can evict memory
> > from each other. For example, expose the ttm_mem_evict_first function from
> > ttm side so hmm/svm code can call it; expose a similar function from svm side so
> > ttm can evict hmm memory.
> > >
> > > I like this option. One thing that needs some thought with this is how
> > > to get some semblance of fairness between the two types of clients.
> > > Basically how to choose what to evict. And what share of the available
> > > memory does each side get to use on average. E.g. an idle client may get
> > > all its memory evicted while a busy client may get a bigger share of the
> > > available memory.
> >
> > I'd also like to suggest we try to write any management/generic code
> > in driver agnostic way as much as possible here. I don't really see
> > much hw difference should be influencing it.
> >
> > I do worry about having effectively 2 LRUs here, you can't really have
> > two "leasts".
> >
> > Like if we hit the shrinker paths who goes first? do we shrink one
> > object from each side in turn?
>
> One way to solve this fairness problem is to create a driver agnostic drm_vram_mgr. Maintain a single LRU in drm_vram_mgr. Move the memory eviction/cgroups memory accounting logic from ttm_resource manager to drm_vram_mgr. Both BO-based driver and SVM driver calls to drm_vram_mgr to allocate/free memory.
>
> I am not sure whether this meets the 2M allocate/free/evict granularity requirement Felix mentioned above. SVM can allocate 2M size blocks. But BO driver should be able to allocate any arbitrary sized blocks - So the eviction is also arbitrary size.
>
> >
> > Also will we have systems where we can expose system SVM but userspace
> > may choose to not use the fine grained SVM and use one of the older
> > modes, will that path get emulated on top of SVM or use the BO paths?
>
>
> If by "older modes" you meant the gem_bo_create (such as xe_gem_create or amdgpu_gem_create), then today both amd and intel implement those interfaces using BO path. We don't have a plan to emulate that old mode on tope of SVM, afaict.

I'm not sure how the older modes manifest in the kernel I assume as bo
creates (but they may use userptr), SVM isn't a specific thing, it's a
group of 3 things.

coarse-grained SVM which I think is BO
fine-grained SVM which is page level
fine-grained system SVM which is HMM

I suppose I'm asking about the previous versions and how they would
operate in a system SVM capable system.

Dave.
>
> Thanks,
> Oak
>
> >
> > Dave.