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

[Daniel Vetter]

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