On Sun Sep 7, 2025 at 1:39 PM CEST, Alice Ryhl wrote: > On Sun, Sep 07, 2025 at 01:28:05PM +0200, Danilo Krummrich wrote: >> On Sun Sep 7, 2025 at 1:15 PM CEST, Alice Ryhl wrote: >> > On Sat, Sep 06, 2025 at 12:47:36AM +0200, Danilo Krummrich wrote: >> >> On Fri Sep 5, 2025 at 8:18 PM CEST, Alice Ryhl wrote: >> >> > On Fri, Sep 5, 2025 at 3:25 PM Boris Brezillon >> >> > <[email protected]> wrote: >> >> >> On Fri, 05 Sep 2025 12:11:28 +0000 >> >> >> Alice Ryhl <[email protected]> wrote: >> >> >> > +static bool >> >> >> > +drm_gpuvm_bo_is_dead(struct drm_gpuvm_bo *vm_bo) >> >> >> > +{ >> >> >> > + return !kref_read(&vm_bo->kref); >> >> >> >> >> >> I'm not too sure I like the idea of [ab]using vm_bo::kref to defer the >> >> >> vm_bo release. I get why it's done like that, but I'm wondering why we >> >> >> don't defer the release of drm_gpuva objects instead (which is really >> >> >> what's being released in va_unlink()). I can imagine drivers wanting to >> >> >> attach resources to the gpuva that can't be released in the >> >> >> dma-signalling path in the future, and if we're doing that at the gpuva >> >> >> level, we also get rid of this kref dance, since the va will hold a >> >> >> vm_bo ref until it's destroyed. >> >> >> >> >> >> Any particular reason you went for vm_bo destruction deferral instead >> >> >> of gpuva? >> >> > >> >> > All of the things that were unsafe to release in the signalling path >> >> > were tied to the vm_bo, so that is why I went for vm_bo cleanup. >> >> > Another advantage is that it lets us use the same deferred logic for >> >> > the vm_bo_put() call that drops the refcount from vm_bo_obtain(). >> >> > >> >> > Of course if gpuvas might have resources that need deferred cleanup, >> >> > that might change the situation somewhat. >> >> >> >> I think we want to track PT(E) allocations, or rather reference counts of >> >> page >> >> table structures carried by the drm_gpuva, but we don't need to release >> >> them on >> >> drm_gpuva_unlink(), which is where we drop the reference count of the >> >> vm_bo. >> >> >> >> Deferring drm_gpuva_unlink() isn't really an option I think, the GEMs >> >> list of >> >> VM_BOs and the VM_BOs list of VAs is usually used in >> >> ttm_device_funcs::move to >> >> map or unmap all VAs associated with a GEM object. >> >> >> >> I think PT(E) reference counts etc. should be rather released when the >> >> drm_gpuva >> >> is freed, i.e. page table allocations can be bound to the lifetime of a >> >> drm_gpuva. Given that, I think that eventually we'll need a cleanup list >> >> for >> >> those as well, since once they're removed from the VM tree (in the fence >> >> signalling critical path), we loose access otherwise. >> > >> > Hmm. Another more conceptual issue with deferring gpuva is that >> > "immediate mode" is defined as having the GPUVM match the GPU's actual >> > address space at all times, which deferred gpuva cleanup would go >> > against. >> >> Depends on what "deferred gpuva cleanup" means. >> >> What needs to happen in the run_job() is drm_gpuva_unlink() and >> drm_gpuva_unmap(). Freeing the drm_gpuva, inluding releasing the assoiciated >> driver specific resources, can be deferred. > > Yeah I guess we could have unlink remove the gpuva, but then allow the > end-user to attach the gpuva to a list of gpuvas to kfree deferred. That > way, the drm_gpuva_unlink() is not deferred but any resources it has can > be. > > Of course, this approach also makes deferred gpuva cleanup somewhat > orthogonal to this patch.
Yes, it is. > One annoying part is that we don't have an gpuvm ops operation for > freeing gpuva, and if we add one for this, it would *only* be used in > this case as most drivers explicitly kfree gpuvas, which could be > confusing for end-users. I think the reason why I left GPUVA alloc and free to drivers was that I was expecting them to use a dedicated kmemcache for that. However, we can still provide drm_gpuva_alloc(), drm_gpuva_free() and va_free(), va_alloc() callbacks for drivers to implement.
