Danilo Krummrich
2023-Jun-22 15:27 UTC
[Nouveau] [PATCH drm-next v5 00/14] [RFC] DRM GPUVA Manager & Nouveau VM_BIND UAPI
On 6/22/23 17:19, Boris Brezillon wrote:> Hi Danilo, > > On Thu, 22 Jun 2023 15:58:23 +0200 > Danilo Krummrich <dakr at redhat.com> wrote: > >> Hi Boris, >> >> On 6/22/23 15:01, Boris Brezillon wrote: >>> Hi Danilo, >>> >>> On Tue, 20 Jun 2023 14:46:07 +0200 >>> Danilo Krummrich <dakr at redhat.com> wrote: >>> >>>>> The only thing I'm worried about is the 'sync mapping requests have to >>>>> go through the async path and wait for all previous async requests to >>>>> be processed' problem I mentioned in one of your previous submission, >>>>> but I'm happy leave that for later. >>>> >>>> Yes, I'm aware of this limitation. >>>> >>>> Let me quickly try to explain where this limitation comes from and how I >>>> intend to address it. >>>> >>>> In order to be able to allocate the required page tables for a mapping >>>> request and in order to free corresponding page tables once the (async) >>>> job finished I need to know the corresponding sequence of operations >>>> (drm_gpuva_ops) to fulfill the mapping request. >>>> >>>> This requires me to update the GPUVA space in the ioctl() rather than in >>>> the async stage, because otherwise I would need to wait for previous >>>> jobs to finish before being able to submit subsequent jobs to the job >>>> queue, since I need an up to date view of the GPUVA space in order to >>>> calculate the sequence of operations to fulfill a mapping request. >>>> >>>> As a consequence all jobs need to be processed in the order they were >>>> submitted, including synchronous jobs. >>>> >>>> @Matt: I think you will have the same limitation with synchronous jobs >>>> as your implementation in XE should be similar? >>>> >>>> In order to address it I want to switch to using callbacks rather than >>>> 'pre-allocated' drm_gpuva_ops and update the GPUVA space within the >>>> asynchronous stage. >>>> This would allow me to 'fit' synchronous jobs >>>> between jobs waiting in the async job queue. However, to do this I have >>>> to re-work how the page table handling in Nouveau is implemented, since >>>> this would require me to be able to manage the page tables without >>>> knowing the exact sequence of operations to fulfill a mapping request. >>> >>> Ok, so I think that's more or less what we're trying to do right >>> now in PowerVR. >>> >>> - First, we make sure we reserve enough MMU page tables for a given map >>> operation to succeed no matter the VM state in the VM_BIND job >>> submission path (our VM_BIND ioctl). That means we're always >>> over-provisioning and returning unused memory back when the operation >>> is done if we end up using less memory. >>> - We pre-allocate for the mapple-tree insertions. >>> - Then we map using drm_gpuva_sm_map() and the callbacks we provided in >>> the drm_sched::run_job() path. We guarantee that no memory is >>> allocated in that path thanks to the pre-allocation/reservation we've >>> done at VM_BIND job submission time. >>> >>> The problem I see with this v5 is that: >>> >>> 1/ We now have a dma_resv_lock_held() in drm_gpuva_{link,unlink}(), >>> which, in our case, is called in the async drm_sched::run_job() path, >>> and we don't hold the lock in that path (it's been released just >>> after the job submission). >> >> My solution to this, as by now, is to - in the same way we pre-allocate >> - to just pre-link and pre-unlink. And then fix things up in the cleanup >> path. >> >> However, depending on the driver, this might require you to set a flag >> in the driver specific structure (embedding struct drm_gpuva) whether >> the gpuva is actually mapped (as in has active page table entries). >> Maybe we could also just add such a flag to struct drm_gpuva. But yeah, >> doesn't sound too nice to be honest... >> >>> 2/ I'm worried that Liam's plan to only reserve what's actually needed >>> based on the mapple tree state is going to play against us, because >>> the mapple-tree is only modified at job exec time, and we might have >>> several unmaps happening between the moment we created and queued the >>> jobs, and the moment they actually get executed, meaning the >>> mapple-tree reservation might no longer fit the bill. >> >> Yes, I'm aware and I explained to Liam in detail why we need the >> mas_preallocate_worst_case() way of doing it. >> >> See this mail: >> https://lore.kernel.org/nouveau/68cd25de-e767-725e-2e7b-703217230bb0 at redhat.com/T/#ma326e200b1de1e3c9df4e9fcb3bf243061fee8b5 >> >> He hasn't answered yet, but I hope we can just get (or actually keep) >> such a function (hopefully with better naming), since it shouldn't >> interfere with anything else. > > My bad, I started reading your reply and got interrupted. Never got > back to it, which I should definitely have done before posting my > questions. Anyway, glad to hear we're on the same page regarding the > mas_preallocate_worst_case() thing.No worries, I should probably also reply to Liams patch introducing the change. I will do that in a minute.> >> >>> >>> For issue #1, it shouldn't be to problematic if we use a regular lock to >>> insert to/remove from the GEM gpuva list. >> >> Yes, that's why I had a separate GEM gpuva list lock in the first place. >> However, this doesn't really work when generating ops rather than using >> the callback interface. >> >> Have a look at drm_gpuva_gem_unmap_ops_create() requested by Matt for >> XE. This function generates drm_gpuva_ops to unmap all mappings of a >> given GEM. In order to do that the function must iterate the GEM's gpuva >> list and allocate operations for each mapping. As a consequence the >> gpuva list lock wouldn't be allowed to be taken in the fence signalling >> path (run_job()) any longer. Hence, we can just protect the list with >> the GEM's dma-resv lock. > > Yeah, I see why using dma_resv when pre-inserting the mapping is > useful, it just didn't really work with late mapping insertion. > >> >> However, I can understand that it might be inconvenient for the callback >> interface and admittedly my solution to that isn't that nice as well. >> Hence the following idea: >> >> For drivers to be able to use their own lock for that it would be enough >> to get rid of the lockdep checks. We could just add a flag to the GPUVA >> manager to let the driver indicate it wants to do it's own locking for >> the GPUVA list and skip the lockdep checks for the dma-resv lock in that >> case. > > Sounds good to me.I think it's way better than the pre-link / pre-unlink mess. I will add this to v6.> >> >>> >>> For issue #2, I can see a way out if, instead of freeing gpuva nodes, >>> we flag those as unused when we see that something happening later in >>> the queue is going to map a section being unmapped. All of this implies >>> keeping access to already queued VM_BIND jobs (using the spsc queue at >>> the entity level is not practical), and iterating over them every time >>> a new sync or async job is queued to flag what needs to be retained. It >>> would obviously be easier if we could tell the mapple-tree API >>> 'provision as if the tree was empty', so all we have to do is just >>> over-provision for both the page tables and mapple-tree insertion, and >>> free the unused mem when the operation is done. >>> >>> Don't know if you already thought about that and/or have solutions to >>> solve these issues. >> >> As already mentioned above, I'd just expect we can keep it the >> over-provision way, as you say. I think it's a legit use case to not >> know the state of the maple tree at the time the pre-allocated nodes >> will be used and keeping that should not interfere with Liams plan to >> (hopefully separately) optimize for the pre-allocation use case they >> have within -mm. >> >> But let's wait for his take on that. > > Sure. As I said, I'm fine getting this version merged, we can sort out > the changes needed for PowerVR later. Just thought I'd mention those > issues early, so you're not surprised when we come back with crazy > requests (which apparently are not that crazy ;-)).They're not crazy at all, in fact they entirely represent what the callback interface was designed for. :-) - Danilo> > Regards, > > Boris >