On Fri, Jun 3, 2022 at 8:41 PM Christian König <christian.koenig@xxxxxxx> wrote: > > Am 03.06.22 um 19:50 schrieb Bas Nieuwenhuizen: > > [SNIP] > >>>> Yeah, but that's exactly the bubble we try to avoid. Isn't it? > >>> For this series, not really. To clarify there are two sides for > >>> getting GPU bubbles and no overlap: > >>> > >>> (1) VM operations implicitly wait for earlier CS submissions > >>> (2) CS submissions implicitly wait for earlier VM operations > >>> > >>> Together, these combine to ensure that you get a (potentially small) > >>> bubble any time VM work happens. > >>> > >>> Your series (and further ideas) tackles (2), and is a worthwhile thing > >>> to do. However, while writing the userspace for this I noticed this > >>> isn't enough to get rid of all our GPU bubbles. In particular when > >>> doing a non-sparse map of a new BO, that tends to need to be waited on > >>> for the next CS anyway for API semantics. Due to VM operations > >>> happening on a single timeline that means this high priority map can > >>> end up being blocked by earlier sparse maps and hence the bubble in > >>> that case still exists. > >>> > >>> So in this series I try to tackle (1) instead. Since GPU work > >>> typically lags behind CPU submissions and VM operations aren't that > >>> slow, we can typically execute VM operations early enough that any > >>> implicit syncs from (2) are less/no issue. > >> Ok, once more since you don't seem to understand what I want to say: It > >> isn't possible to fix #1 before you have fixed #2. > >> > >> The VM unmap operation here is a barrier which divides the CS operations > >> in a before and after. This is intentional design. > > Why is that barrier needed? The two barriers I got and understood and > > I think we can deal with: > > > > 1) the VM unmap is a barrier between prior CS and later memory free. > > 2) The TLB flush need to happen between a VM unmap and later CS. > > > > But why do we need the VM unmap to be a strict barrier between prior > > CS and later CS? > > Exactly because of the two reasons you mentioned. This is the part I'm not seeing. I get that removing #2 is a nightmare, which is why I did something that doesn't violate that constraint. Like if an explicit CS that was running before the VM operation runs till after the VM operation (and hence possibly till after the TLB flush, or otherwise have the TLB flush not apply due to lack of async TLB flush support), that is not an issue. It might see the state from before the unmap, or after the unmap, or some intermediate state and all of those would be okay. We still get the constraint that the TLB flush happens between the VM unmap and later CS and hence the unmap is certainly visible to them. > > #1 Is rather easy to fix, you just need to copy all dma_fences from the > page table dma_resv object over to the BOs dma_resv object in the gem > close handler. E.g. exactly what you suggested with the dma_resv_copy > function. > > #2 is a nightmare. > > We can't move the TLB flush at the end of the unmap operation because on > async TLB flushes are either a bit complicated (double flushes etc..) or > don't even work at all because of hw bugs. So to have a reliable TLB > flush we must make sure that nothing else is ongoing and that means > CS->VM->CS barrier. > > We try very hard to circumvent that already on maps by (for example) > using a completely new VMID for CS after the VM map operation. > > But for the unmap operation we would need some kind special dma_fence > implementation which would not only wait for all existing dma_fence but > also for the one added until the unmap operation is completed. Cause > otherwise our operation we do at #1 would simply not catch all > dma_fences which have access to the memory. > > That's certainly doable, but I think just using the drm_exec stuff I > already came up with is easier. > > When we can grab locks for all the BOs involved amdgpu_vm_clear_freed() > goes away and we can keep track of the unmap operations in the bo_va > structure. > > With that done you can make the explicit sync you noted in the bo_va > structure and implicit sync when the bo_va structure goes away. > > Then the only reason I can see why we would need a CS->VM dependency is > implicit synchronization, and that's what we are trying to avoid here in > the first place. > > Regards, > Christian. > > > > >> To get rid of this barrier you must first fix the part where CS > >> submissions wait for the VM operation to complete, e.g. the necessity of > >> the barrier. > >> > >> I'm working on this for a couple of years now and I'm really running out > >> of idea how to explain this restriction. > >> > >> Regards, > >> Christian. > >> >
