> -----Original Message----- > From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of Dave > Airlie > Sent: August 20, 2023 6:21 PM > To: Zeng, Oak <oak.zeng@xxxxxxxxx> > Cc: Brost, Matthew <matthew.brost@xxxxxxxxx>; Thomas Hellström > <thomas.hellstrom@xxxxxxxxxxxxxxx>; Philip Yang <Philip.Yang@xxxxxxx>; Felix > Kuehling <felix.kuehling@xxxxxxx>; Welty, Brian <brian.welty@xxxxxxxxx>; dri- > devel@xxxxxxxxxxxxxxxxxxxxx; intel-xe@xxxxxxxxxxxxxxxxxxxxx; Vishwanathapura, > Niranjana <niranjana.vishwanathapura@xxxxxxxxx>; Christian König > <christian.koenig@xxxxxxx> > Subject: Re: Implement svm without BO concept in xe driver > > On Thu, 17 Aug 2023 at 12:13, Zeng, Oak <oak.zeng@xxxxxxxxx> wrote: > > > > > -----Original Message----- > > > From: Dave Airlie <airlied@xxxxxxxxx> > > > Sent: August 16, 2023 6:52 PM > > > To: Felix Kuehling <felix.kuehling@xxxxxxx> > > > Cc: Zeng, Oak <oak.zeng@xxxxxxxxx>; Christian König > > > <christian.koenig@xxxxxxx>; Thomas Hellström > > > <thomas.hellstrom@xxxxxxxxxxxxxxx>; Brost, Matthew > > > <matthew.brost@xxxxxxxxx>; maarten.lankhorst@xxxxxxxxxxxxxxx; > > > Vishwanathapura, Niranjana <niranjana.vishwanathapura@xxxxxxxxx>; Welty, > > > Brian <brian.welty@xxxxxxxxx>; Philip Yang <Philip.Yang@xxxxxxx>; intel- > > > xe@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx > > > Subject: Re: Implement svm without BO concept in xe driver > > > > > > On Thu, 17 Aug 2023 at 08:15, Felix Kuehling <felix.kuehling@xxxxxxx> > 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. > > 1) coarse-grained SVM which I think is BO > 2) fine-grained SVM which is page level > 3) 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. I got your question now. As I understand it, the system SVM provides similar functionality as BO-based SVM (i.e., share virtual address space b/t cpu and gpu program, no explicit memory placement for gpu program), but they have different user interface (malloc, mmap vs bo create, vm bind). >From functionality perspective, on a system SVM capable system, we don't need #1/#2. Once #3 is implemented and turned out be as performant as #1/#2, we can ask user space to switch to #3. As far as I know, AMD doesn't have #1/#2 - their BO-based driver *requires* all valid GPU virtual address be mapped to GPU page table *before* GPU kernel submission, aka a GPU page fault is treated as fatal. Felix please fix me, as my AMD knowledge is fading away... >From interface perspective, i.e., to keep UMD which using #1/#2 continue to run without modification, we need #1/#2 to continue exist. Should we emulate #1/#2 on top of #3? I feel the BO-based memory management and the struct page/hmm based memory management are quite different design philosophy. Trying to emulate one on top of another can run into serious difficulty. For example, how do we emulate a vm_bind on top of #3? Remember for #1/#2 virtual address space is managed by user space while #3 virtual address space is managed by kernel core mm (vma struct...). It is a hard conflict here... Thanks again for the great question! Oak > > Dave. > > > > Thanks, > > Oak > > > > > > > > Dave.
