Implement svm without BO concept in xe driver

[Zeng, Oak]

> -----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.

Thanks,
Oak

> 
> Dave.