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

Marek Olšák maraeo at gmail.com
Mon Apr 26 20:59:28 UTC 2021

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.


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
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