[Mesa-dev] Linux Graphics Next: Userspace submission update

Christian König ckoenig.leichtzumerken at gmail.com
Fri Jun 4 07:00:31 UTC 2021

Am 02.06.21 um 21:19 schrieb Daniel Vetter:
> On Wed, Jun 02, 2021 at 08:52:38PM +0200, Christian König wrote:
>> Am 02.06.21 um 20:48 schrieb Daniel Vetter:
>>> On Wed, Jun 02, 2021 at 05:38:51AM -0400, Marek Olšák wrote:
>>>> On Wed, Jun 2, 2021 at 5:34 AM Marek Olšák <maraeo at gmail.com> wrote:
>>>>> Yes, we can't break anything because we don't want to complicate things
>>>>> for us. It's pretty much all NAK'd already. We are trying to gather more
>>>>> knowledge and then make better decisions.
>>>>> The idea we are considering is that we'll expose memory-based sync objects
>>>>> to userspace for read only, and the kernel or hw will strictly control the
>>>>> memory writes to those sync objects. The hole in that idea is that
>>>>> userspace can decide not to signal a job, so even if userspace can't
>>>>> overwrite memory-based sync object states arbitrarily, it can still decide
>>>>> not to signal them, and then a future fence is born.
>>>> This would actually be treated as a GPU hang caused by that context, so it
>>>> should be fine.
>>> This is practically what I proposed already, except your not doing it with
>>> dma_fence. And on the memory fence side this also doesn't actually give
>>> what you want for that compute model.
>>> This seems like a bit a worst of both worlds approach to me? Tons of work
>>> in the kernel to hide these not-dma_fence-but-almost, and still pain to
>>> actually drive the hardware like it should be for compute or direct
>>> display.
>>> Also maybe I've missed it, but I didn't see any replies to my suggestion
>>> how to fake the entire dma_fence stuff on top of new hw. Would be
>>> interesting to know what doesn't work there instead of amd folks going of
>>> into internal again and then coming back with another rfc from out of
>>> nowhere :-)
>> Well to be honest I would just push back on our hardware/firmware guys that
>> we need to keep kernel queues forever before going down that route.
> I looked again, and you said the model wont work because preemption is way
> too slow, even when the context is idle.
> I guess at that point I got maybe too fed up and just figured "not my
> problem", but if preempt is too slow as the unload fence, you can do it
> with pte removal and tlb shootdown too (that is hopefully not too slow,
> otherwise your hw is just garbage and wont even be fast for direct submit
> compute workloads).

Have you seen that one here: 
https://www.spinics.net/lists/amd-gfx/msg63101.html :)

I've rejected it because I think polling for 6 seconds on a TLB flush 
which can block interrupts as well is just madness.

> The only thing that you need to do when you use pte clearing + tlb
> shootdown instad of preemption as the unload fence for buffers that get
> moved is that if you get any gpu page fault, you don't serve that, but
> instead treat it as a tdr and shot the context permanently.
> So summarizing the model I proposed:
> - you allow userspace to directly write into the ringbuffer, and also
>    write the fences directly
> - actual submit is done by the kernel, using drm/scheduler. The kernel
>    blindly trusts userspace to set up everything else, and even just wraps
>    dma_fences around the userspace memory fences.
> - the only check is tdr. If a fence doesn't complete an tdr fires, a) the
>    kernel shot the entire context and b) userspace recovers by setting up a
>    new ringbuffer
> - memory management is done using ttm only, you still need to supply the
>    buffer list (ofc that list includes the always present ones, so CS will
>    only get the list of special buffers like today). If you hw can't trun
>    gpu page faults and you ever get one we pull up the same old solution:
>    Kernel shots the entire context.
>    The important thing is that from the gpu pov memory management works
>    exactly like compute workload with direct submit, except that you just
>    terminate the context on _any_ page fault, instead of only those that go
>    somewhere where there's really no mapping and repair the others.
>    Also I guess from reading the old thread this means you'd disable page
>    fault retry because that is apparently also way too slow for anything.
> - memory management uses an unload fence. That unload fences waits for all
>    userspace memory fences (represented as dma_fence) to complete, with
>    maybe some fudge to busy-spin until we've reached the actual end of the
>    ringbuffer (maybe you have a IB tail there after the memory fence write,
>    we have that on intel hw), and it waits for the memory to get
>    "unloaded". This is either preemption, or pte clearing + tlb shootdown,
>    or whatever else your hw provides which is a) used for dynamic memory
>    management b) fast enough for actual memory management.
> - any time a context dies we force-complete all it's pending fences,
>    in-order ofc
> So from hw pov this looks 99% like direct userspace submit, with the exact
> same mappings, command sequences and everything else. The only difference
> is that the rinbuffer head/tail updates happen from drm/scheduler, instead
> of directly from userspace.
> None of this stuff needs funny tricks where the kernel controls the
> writes to memory fences, or where you need kernel ringbuffers, or anything
> like thist. Userspace is allowed to do anything stupid, the rules are
> guaranteed with:
> - we rely on the hw isolation features to work, but _exactly_ like compute
>    direct submit would too
> - dying on any page fault captures memory management issues
> - dying (without kernel recover, this is up to userspace if it cares) on
>    any tdr makes sure fences complete still
>> That syncfile and all that Android stuff isn't working out of the box with
>> the new shiny user queue submission model (which in turn is mostly because
>> of Windows) already raised some eyebrows here.
> I think if you really want to make sure the current linux stack doesn't
> break the _only_ option you have is provide a ctx mode that allows
> dma_fence and drm/scheduler to be used like today.

Yeah, but I still can just tell our hw/fw guys that we really really 
need to keep kernel queues or the whole Linux/Android infrastructure 
needs to get a compatibility layer like you describe above.

> For everything else it sounds you're a few years too late, because even
> just huge kernel changes wont happen in time. Much less rewriting
> userspace protocols.

Seconded, question is rather if we are going to start migrating at some 
point or if we should keep pushing on our hw/fw guys.


> -Daniel

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