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

Danilo Krummrich
2023-06-22T15:31:33Z

On 6/22/23 17:19, Boris Brezillon wrote:
> Hi Danilo,
>
> On Thu, 22 Jun 2023 15:58:23 +0200
> Danilo Krummrich <dakr@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@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.

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
>