[PATCH drm-next v5 00/14] [RFC] DRM GPUVA Manager & Nouveau VM_BIND UAPI

[Boris Brezillon]

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@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.

> 
> > 
> > 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.

> 
> > 
> > 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 ;-)).

Regards,

Boris