[Mesa-dev] [RFC] Linux Graphics Next: Explicit fences everywhere and no BO fences - initial proposal

Tue Apr 27 13:26:49 UTC 2021

Ok. So that would only make the following use cases broken for now:
- amd render -> external gpu
- amd video encode -> network device

What about the case when we get a buffer from an external device and we're
supposed to make it "busy" when we are using it, and the external device
wants to wait until we stop using it? Is it something that can happen, thus
turning "external -> amd" into "external <-> amd"?

Marek

On Tue., Apr. 27, 2021, 08:50 Christian König, <
ckoenig.leichtzumerken at gmail.com> wrote:

> Only amd -> external.
>
> We can easily install something in an user queue which waits for a
> dma_fence in the kernel.
>
> But we can't easily wait for an user queue as dependency of a dma_fence.
>
> The good thing is we have this wait before signal case on Vulkan timeline
> semaphores which have the same problem in the kernel.
>
> The good news is I think we can relatively easily convert i915 and older
> amdgpu device to something which is compatible with user fences.
>
> So yes, getting that fixed case by case should work.
>
> Christian
>
> Am 27.04.21 um 14:46 schrieb Marek Olšák:
>
> I'll defer to Christian and Alex to decide whether dropping sync with
> non-amd devices (GPUs, cameras etc.) is acceptable.
>
> Rewriting those drivers to this new sync model could be done on a case by
> case basis.
>
> For now, would we only lose the "amd -> external" dependency? Or the
> "external -> amd" dependency too?
>
> Marek
>
> On Tue., Apr. 27, 2021, 08:15 Daniel Vetter, <daniel at ffwll.ch> wrote:
>
>> On Tue, Apr 27, 2021 at 2:11 PM Marek Olšák <maraeo at gmail.com> wrote:
>> > Ok. I'll interpret this as "yes, it will work, let's do it".
>>
>> It works if all you care about is drm/amdgpu. I'm not sure that's a
>> reasonable approach for upstream, but it definitely is an approach :-)
>>
>> We've already gone somewhat through the pain of drm/amdgpu redefining
>> how implicit sync works without sufficiently talking with other
>> people, maybe we should avoid a repeat of this ...
>> -Daniel
>>
>> >
>> > Marek
>> >
>> > On Tue., Apr. 27, 2021, 08:06 Christian König, <
>> ckoenig.leichtzumerken at gmail.com> wrote:
>> >>
>> >> Correct, we wouldn't have synchronization between device with and
>> without user queues any more.
>> >>
>> >> That could only be a problem for A+I Laptops.
>> >>
>> >> Memory management will just work with preemption fences which pause
>> the user queues of a process before evicting something. That will be a
>> dma_fence, but also a well known approach.
>> >>
>> >> Christian.
>> >>
>> >> Am 27.04.21 um 13:49 schrieb Marek Olšák:
>> >>
>> >> If we don't use future fences for DMA fences at all, e.g. we don't use
>> them for memory management, it can work, right? Memory management can
>> suspend user queues anytime. It doesn't need to use DMA fences. There might
>> be something that I'm missing here.
>> >>
>> >> What would we lose without DMA fences? Just inter-device
>> synchronization? I think that might be acceptable.
>> >>
>> >> The only case when the kernel will wait on a future fence is before a
>> page flip. Everything today already depends on userspace not hanging the
>> gpu, which makes everything a future fence.
>> >>
>> >> Marek
>> >>
>> >> On Tue., Apr. 27, 2021, 04:02 Daniel Vetter, <daniel at ffwll.ch> wrote:
>> >>>
>> >>> On Mon, Apr 26, 2021 at 04:59:28PM -0400, Marek Olšák wrote:
>> >>> > Thanks everybody. The initial proposal is dead. Here are some
>> thoughts on
>> >>> > how to do it differently.
>> >>> >
>> >>> > I think we can have direct command submission from userspace via
>> >>> > memory-mapped queues ("user queues") without changing window
>> systems.
>> >>> >
>> >>> > The memory management doesn't have to use GPU page faults like HMM.
>> >>> > Instead, it can wait for user queues of a specific process to go
>> idle and
>> >>> > then unmap the queues, so that userspace can't submit anything.
>> Buffer
>> >>> > evictions, pinning, etc. can be executed when all queues are
>> unmapped
>> >>> > (suspended). Thus, no BO fences and page faults are needed.
>> >>> >
>> >>> > Inter-process synchronization can use timeline semaphores.
>> Userspace will
>> >>> > query the wait and signal value for a shared buffer from the
>> kernel. The
>> >>> > kernel will keep a history of those queries to know which process is
>> >>> > responsible for signalling which buffer. There is only the
>> wait-timeout
>> >>> > issue and how to identify the culprit. One of the solutions is to
>> have the
>> >>> > GPU send all GPU signal commands and all timed out wait commands
>> via an
>> >>> > interrupt to the kernel driver to monitor and validate userspace
>> behavior.
>> >>> > With that, it can be identified whether the culprit is the waiting
>> process
>> >>> > or the signalling process and which one. Invalid signal/wait
>> parameters can
>> >>> > also be detected. The kernel can force-signal only the semaphores
>> that time
>> >>> > out, and punish the processes which caused the timeout or used
>> invalid
>> >>> > signal/wait parameters.
>> >>> >
>> >>> > The question is whether this synchronization solution is robust
>> enough for
>> >>> > dma_fence and whatever the kernel and window systems need.
>> >>>
>> >>> The proper model here is the preempt-ctx dma_fence that amdkfd uses
>> >>> (without page faults). That means dma_fence for synchronization is
>> doa, at
>> >>> least as-is, and we're back to figuring out the winsys problem.
>> >>>
>> >>> "We'll solve it with timeouts" is very tempting, but doesn't work.
>> It's
>> >>> akin to saying that we're solving deadlock issues in a locking design
>> by
>> >>> doing a global s/mutex_lock/mutex_lock_timeout/ in the kernel. Sure it
>> >>> avoids having to reach the reset button, but that's about it.
>> >>>
>> >>> And the fundamental problem is that once you throw in userspace
>> command
>> >>> submission (and syncing, at least within the userspace driver,
>> otherwise
>> >>> there's kinda no point if you still need the kernel for cross-engine
>> sync)
>> >>> means you get deadlocks if you still use dma_fence for sync under
>> >>> perfectly legit use-case. We've discussed that one ad nauseam last
>> summer:
>> >>>
>> >>>
>> https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_fence#indefinite-dma-fences
>> >>>
>> >>> See silly diagramm at the bottom.
>> >>>
>> >>> Now I think all isn't lost, because imo the first step to getting to
>> this
>> >>> brave new world is rebuilding the driver on top of userspace fences,
>> and
>> >>> with the adjusted cmd submit model. You probably don't want to use
>> amdkfd,
>> >>> but port that as a context flag or similar to render nodes for gl/vk.
>> Of
>> >>> course that means you can only use this mode in headless, without
>> >>> glx/wayland winsys support, but it's a start.
>> >>> -Daniel
>> >>>
>> >>> >
>> >>> > Marek
>> >>> >
>> >>> > On Tue, Apr 20, 2021 at 4:34 PM Daniel Stone <daniel at fooishbar.org>
>> wrote:
>> >>> >
>> >>> > > Hi,
>> >>> > >
>> >>> > > On Tue, 20 Apr 2021 at 20:30, Daniel Vetter <daniel at ffwll.ch>
>> wrote:
>> >>> > >
>> >>> > >> The thing is, you can't do this in drm/scheduler. At least not
>> without
>> >>> > >> splitting up the dma_fence in the kernel into separate memory
>> fences
>> >>> > >> and sync fences
>> >>> > >
>> >>> > >
>> >>> > > I'm starting to think this thread needs its own glossary ...
>> >>> > >
>> >>> > > I propose we use 'residency fence' for execution fences which
>> enact
>> >>> > > memory-residency operations, e.g. faulting in a page ultimately
>> depending
>> >>> > > on GPU work retiring.
>> >>> > >
>> >>> > > And 'value fence' for the pure-userspace model suggested by
>> timeline
>> >>> > > semaphores, i.e. fences being (*addr == val) rather than being
>> able to look
>> >>> > > at ctx seqno.
>> >>> > >
>> >>> > > Cheers,
>> >>> > > Daniel
>> >>> > > _______________________________________________
>> >>> > > mesa-dev mailing list
>> >>> > > mesa-dev at lists.freedesktop.org
>> >>> > > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>> >>> > >
>> >>>
>> >>> --
>> >>> Daniel Vetter
>> >>> Software Engineer, Intel Corporation
>> >>> http://blog.ffwll.ch
>> >>
>> >>
>> >> _______________________________________________
>> >> mesa-dev mailing list
>> >> mesa-dev at lists.freedesktop.org
>> >> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>> >>
>> >>
>>
>>
>> --
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch
>>
>
>
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