Implement svm without BO concept in xe driver

Wed Aug 16 17:30:32 UTC 2023

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

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.

Today we don't know which approach is better. I will work on some prove of concept codes, starting with #1 approach firstly.

Btw, I talked with application engineers and they said most applications actually use a mixture of gem_bo create and malloc, so we definitely need to solve this problem. 

Cheers,
Oak

> -----Original Message-----
> From: Christian König <christian.koenig at amd.com>
> Sent: August 16, 2023 2:06 AM
> To: Zeng, Oak <oak.zeng at intel.com>; Felix Kuehling <felix.kuehling at amd.com>;
> Thomas Hellström <thomas.hellstrom at linux.intel.com>; Brost, Matthew
> <matthew.brost at 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
> 
> Hi Oak,
> 
> yeah, I completely agree with you and Felix. The main problem here is
> getting the memory pressure visible on both sides.
> 
> At the moment I have absolutely no idea how to handle that, maybe
> something like the ttm_resource object shared between TTM and HMM?
> 
> Regards,
> Christian.
> 
> Am 16.08.23 um 05:47 schrieb Zeng, Oak:
> > Hi Felix,
> >
> > It is great to hear from you!
> >
> > When I implement the HMM-based SVM for intel devices, I found this
> interesting problem: HMM uses struct page based memory management scheme
> which is completely different against the BO/TTM style memory management
> philosophy. Writing SVM code upon the BO/TTM concept seems overkill and
> awkward. So I thought we better make the SVM code BO-less and TTM-less. But
> on the other hand, currently vram eviction and cgroup memory accounting are all
> hooked to the TTM layer, which means a TTM-less SVM driver won't be able to
> evict vram allocated through TTM/gpu_vram_mgr.
> >
> > Ideally HMM migration should use drm-buddy for vram allocation, but we need
> to solve this TTM/HMM mutual eviction problem as you pointed out (I am
> working with application engineers to figure out whether mutual eviction can
> truly benefit applications). Maybe we can implement a TTM-less vram
> management block which can be shared b/t the HMM-based driver and the BO-
> based driver:
> >     * allocate/free memory from drm-buddy, buddy-block based
> >     * memory eviction logics, allow driver to specify which allocation is evictable
> >     * memory accounting, cgroup logic
> >
> > Maybe such a block can be placed at drm layer (say, call it drm_vram_mgr for
> now), so it can be shared b/t amd and intel. So I involved amd folks. Today both
> amd and intel-xe driver implemented a TTM-based vram manager which doesn't
> serve above design goal. Once the drm_vram_mgr is implemented, both amd
> and intel's BO-based/TTM-based vram manager, and the HMM-based vram
> manager can call into this drm-vram-mgr.
> >
> > Thanks again,
> > Oak
> >
> >> -----Original Message-----
> >> From: Felix Kuehling <felix.kuehling at amd.com>
> >> Sent: August 15, 2023 6:17 PM
> >> To: Zeng, Oak <oak.zeng at intel.com>; Thomas Hellström
> >> <thomas.hellstrom at linux.intel.com>; Brost, Matthew
> >> <matthew.brost at intel.com>; Vishwanathapura, Niranjana
> >> <niranjana.vishwanathapura at intel.com>; Welty, Brian
> <brian.welty at intel.com>;
> >> Christian König <christian.koenig at amd.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
> >>
> >> Hi Oak,
> >>
> >> I'm not sure what you're looking for from AMD? Are we just CC'ed FYI? Or
> >> are you looking for comments about
> >>
> >>    * Our plans for VRAM management with HMM
> >>    * Our experience with BO-based VRAM management
> >>    * Something else?
> >>
> >> IMO, having separate memory pools for HMM and TTM is a non-starter for
> >> AMD. We need access to the full VRAM in either of the APIs for it to be
> >> useful. That also means, we need to handle memory pressure in both
> >> directions. That's one of the main reasons we went with the BO-based
> >> approach initially. I think in the long run, using the buddy allocator,
> >> or the amdgpu_vram_mgr directly for HMM migrations would be better,
> >> assuming we can handle memory pressure in both directions between HMM
> >> and TTM sharing the same pool of physical memory.
> >>
> >> Regards,
> >>     Felix
> >>
> >>
> >> On 2023-08-15 16:34, Zeng, Oak wrote:
> >>> Also + Christian
> >>>
> >>> Thanks,
> >>>
> >>> Oak
> >>>
> >>> *From:*Intel-xe <intel-xe-bounces at lists.freedesktop.org> *On Behalf Of
> >>> *Zeng, Oak
> >>> *Sent:* August 14, 2023 11:38 PM
> >>> *To:* Thomas Hellström <thomas.hellstrom at linux.intel.com>; Brost,
> >>> Matthew <matthew.brost at intel.com>; Vishwanathapura, Niranjana
> >>> <niranjana.vishwanathapura at intel.com>; Welty, Brian
> >>> <brian.welty at intel.com>; Felix Kuehling <felix.kuehling at amd.com>;
> >>> Philip Yang <Philip.Yang at amd.com>; intel-xe at lists.freedesktop.org;
> >>> dri-devel at lists.freedesktop.org
> >>> *Subject:* [Intel-xe] Implement svm without BO concept in xe driver
> >>>
> >>> Hi Thomas, Matt and all,
> >>>
> >>> This came up when I port i915 svm codes to xe driver. In i915
> >>> implementation, we have i915_buddy manage gpu vram and svm codes
> >>> directly call i915_buddy layer to allocate/free vram. There is no
> >>> gem_bo/ttm bo concept involved in the svm implementation.
> >>>
> >>> In xe driver,  we have drm_buddy, xe_ttm_vram_mgr and ttm layer to
> >>> manage vram. Drm_buddy is initialized during xe_ttm_vram_mgr
> >>> initialization. Vram allocation/free is done through xe_ttm_vram_mgr
> >>> functions which call into drm_buddy layer to allocate vram blocks.
> >>>
> >>> I plan to implement xe svm driver the same way as we did in i915,
> >>> which means there will not be bo concept in the svm implementation.
> >>> Drm_buddy will be passed to svm layer during vram initialization and
> >>> svm will allocate/free memory directly from drm_buddy, bypassing
> >>> ttm/xee vram manager. Here are a few considerations/things we are
> >>> aware of:
> >>>
> >>>   1. This approach seems match hmm design better than bo concept. Our
> >>>      svm implementation will be based on hmm. In hmm design, each vram
> >>>      page is backed by a struct page. It is very easy to perform page
> >>>      granularity migrations (b/t vram and system memory). If BO concept
> >>>      is involved, we will have to split/remerge BOs during page
> >>>      granularity migrations.
> >>>
> >>>   2. We have a prove of concept of this approach in i915, originally
> >>>      implemented by Niranjana. It seems work but it only has basic
> >>>      functionalities for now. We don’t have advanced features such as
> >>>      memory eviction etc.
> >>>
> >>>   3. With this approach, vram will divided into two separate pools: one
> >>>      for xe_gem_created BOs and one for vram used by svm. Those two
> >>>      pools are not connected: memory pressure from one pool won’t be
> >>>      able to evict vram from another pool. At this point, we don’t
> >>>      whether this aspect is good or not.
> >>>
> >>>   4. Amdkfd svm went different approach which is BO based. The benefit
> >>>      of this approach is a lot of existing driver facilities (such as
> >>>      memory eviction/cgroup/accounting) can be reused
> >>>
> >>> Do you have any comment to this approach? Should I come back with a
> >>> RFC of some POC codes?
> >>>
> >>> Thanks,
> >>>
> >>> Oak
> >>>