Re: [RFC PATCH 08/10] dma-buf/dma-fence: Introduce long-running completion fences

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On 4/4/23 15:10, Christian König wrote:
Am 04.04.23 um 14:54 schrieb Thomas Hellström:
Hi, Christian,

On 4/4/23 11:09, Christian König wrote:
Am 04.04.23 um 02:22 schrieb Matthew Brost:
From: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>

For long-running workloads, drivers either need to open-code completion
waits, invent their own synchronization primitives or internally use
dma-fences that do not obey the cross-driver dma-fence protocol, but
without any lockdep annotation all these approaches are error prone.

So since for example the drm scheduler uses dma-fences it is desirable for a driver to be able to use it for throttling and error handling also with internal dma-fences tha do not obey the cros-driver dma-fence protocol.

Introduce long-running completion fences in form of dma-fences, and add
lockdep annotation for them. In particular:

* Do not allow waiting under any memory management locks.
* Do not allow to attach them to a dma-resv object.
* Introduce a new interface for adding callbacks making the helper adding
   a callback sign off on that it is aware that the dma-fence may not
   complete anytime soon. Typically this will be the scheduler chaining
   a new long-running fence on another one.

Well that's pretty much what I tried before: https://lwn.net/Articles/893704/

And the reasons why it was rejected haven't changed.

Regards,
Christian.

Yes, TBH this was mostly to get discussion going how we'd best tackle this problem while being able to reuse the scheduler for long-running workloads.

I couldn't see any clear decision on your series, though, but one main difference I see is that this is intended for driver-internal use only. (I'm counting using the drm_scheduler as a helper for driver-private use). This is by no means a way to try tackle the indefinite fence problem.

Well this was just my latest try to tackle this, but essentially the problems are the same as with your approach: When we express such operations as dma_fence there is always the change that we leak that somewhere.

My approach of adding a flag noting that this operation is dangerous and can't be synced with something memory management depends on tried to contain this as much as possible, but Daniel still pretty clearly rejected it (for good reasons I think).


We could ofc invent a completely different data-type that abstracts the synchronization the scheduler needs in the long-running case, or each driver could hack something up, like sleeping in the prepare_job() or run_job() callback for throttling, but those waits should still be annotated in one way or annotated one way or another (and probably in a similar way across drivers) to make sure we don't do anything bad.

 So any suggestions as to what would be the better solution here would be appreciated.

Mhm, do we really the the GPU scheduler for that?

I mean in the 1 to 1 case  you basically just need a component which collects the dependencies as dma_fence and if all of them are fulfilled schedules a work item.

As long as the work item itself doesn't produce a dma_fence it can then still just wait for other none dma_fence dependencies.

Then the work function could submit the work and wait for the result.

The work item would then pretty much represent what you want, you can wait for it to finish and pass it along as long running dependency.

Maybe give it a funky name and wrap it up in a structure, but that's basically it.

This very much sounds like a i915_sw_fence for the dependency tracking and dma_fence_work for the actual work although it's completion fence is a dma_fence.

Although that goes against the whole idea of a condition for merging the xe driver would be that we implement some sort of minimal scaffolding for long-running workloads in the drm scheduler, and the thinking behind that is to avoid implementing intel-specific solutions like those...

Thanks,

Thomas



Regards,
Christian.


Thanks,

Thomas








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