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

Tue Apr 4 20:03:51 UTC 2023

On Tue, Apr 04, 2023 at 09:00:59PM +0200, Daniel Vetter wrote:
> On Tue, 4 Apr 2023 at 15:10, Christian König <christian.koenig at amd.com> 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 at linux.intel.com>
> > >>>
> > >>> 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).
> 
> Yeah I still don't like dma_fence that somehow have totally different
> semantics in that critical piece of "will it complete or will it
> deadlock?" :-)

Not going to touch LR dma-fences in this reply, I think we can continue
the LR fence discussion of the fork of this thread I just responded to.
Have a response the preempt fence discussion below.

> >
> > >
> > > 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.
> 
> Yeah that's the important thing, for long-running jobs dependencies as
> dma_fence should be totally fine. You're just not allowed to have any
> outgoing dma_fences at all (except the magic preemption fence).
> 
> > 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.
> 
> Like do we need this? If the kernel ever waits for a long-running
> compute job to finnish I'd call that a bug. Any functional
> dependencies between engines or whatever are userspace's problem only,
> which it needs to sort out using userspace memory fences.
> 
> The only things the kernel needs are some way to track dependencies as
> dma_fence (because memory management move the memory away and we need
> to move it back in, ideally pipelined). And it needs the special
> preempt fence (if we don't have pagefaults) so that you have a fence
> to attach to all the dma_resv for memory management purposes. Now the
> scheduler already has almost all the pieces (at least if we assume
> there's some magic fw which time-slices these contexts on its own),
> and we just need a few minimal changes:
> - allowing the scheduler to ignore the completion fence and just
> immediately push the next "job" in if its dependencies are ready
> - maybe minimal amounts of scaffolding to handle the preemption
> dma_fence because that's not entirely trivial. I think ideally we'd
> put that into drm_sched_entity since you can only ever have one active
> preempt dma_fence per gpu ctx/entity.
> 

Yep, preempt fence is per entity in Xe (xe_engine). We install these
into the VM and all external BOs mapped in the VM dma-resv slots.
Wondering if we can make all of this very generic between the DRM
scheduler + GPUVA...

Matt

> None of this needs a dma_fence_is_lr anywhere at all.
> 
> Of course there's the somewhat related issue of "how do we transport
> these userspace memory fences around from app to compositor", and
> that's a lot more gnarly. I still don't think dma_fence_is_lr is
> anywhere near what the solution should look like for that.
> -Daniel
> 
> 
> > Regards,
> > Christian.
> >
> > >
> > > Thanks,
> > >
> > > Thomas
> > >
> > >
> > >
> > >
> > >
> >
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch