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. > Deferring vm_bo cleanup doesn't have this issue because even though the > vm_bo isn't kfreed immediately, all GPUVM apis still treat it as-if it > isn't there anymore.
