Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations

Alex Deucher
2020-06-19T19:11:33Z

On Fri, Jun 19, 2020 at 2:09 PM Jerome Glisse <jglisse@redhat.com> wrote:
>
> On Fri, Jun 19, 2020 at 02:23:08PM -0300, Jason Gunthorpe wrote:
> > On Fri, Jun 19, 2020 at 06:19:41PM +0200, Daniel Vetter wrote:
> >
> > > The madness is only that device B's mmu notifier might need to wait
> > > for fence_B so that the dma operation finishes. Which in turn has to
> > > wait for device A to finish first.
> >
> > So, it sound, fundamentally you've got this graph of operations across
> > an unknown set of drivers and the kernel cannot insert itself in
> > dma_fence hand offs to re-validate any of the buffers involved?
> > Buffers which by definition cannot be touched by the hardware yet.
> >
> > That really is a pretty horrible place to end up..
> >
> > Pinning really is right answer for this kind of work flow. I think
> > converting pinning to notifers should not be done unless notifier
> > invalidation is relatively bounded.
> >
> > I know people like notifiers because they give a bit nicer performance
> > in some happy cases, but this cripples all the bad cases..
> >
> > If pinning doesn't work for some reason maybe we should address that?
>
> Note that the dma fence is only true for user ptr buffer which predate
> any HMM work and thus were using mmu notifier already. You need the
> mmu notifier there because of fork and other corner cases.
>
> For nouveau the notifier do not need to wait for anything it can update
> the GPU page table right away. Modulo needing to write to GPU memory
> using dma engine if the GPU page table is in GPU memory that is not
> accessible from the CPU but that's never the case for nouveau so far
> (but i expect it will be at one point).
>
>
> So i see this as 2 different cases, the user ptr case, which does pin
> pages by the way, where things are synchronous. Versus the HMM cases
> where everything is asynchronous.
>
>
> I probably need to warn AMD folks again that using HMM means that you
> must be able to update the GPU page table asynchronously without
> fence wait. The issue for AMD is that they already update their GPU
> page table using DMA engine. I believe this is still doable if they
> use a kernel only DMA engine context, where only kernel can queue up
> jobs so that you do not need to wait for unrelated things and you can
> prioritize GPU page table update which should translate in fast GPU
> page table update without DMA fence.

All devices which support recoverable page faults also have a
dedicated paging engine for the kernel driver which the driver already
makes use of. We can also update the GPU page tables with the CPU.

Alex

>
>
> > > Full disclosure: We are aware that we've designed ourselves into an
> > > impressive corner here, and there's lots of talks going on about
> > > untangling the dma synchronization from the memory management
> > > completely. But
> >
> > I think the documenting is really important: only GPU should be using
> > this stuff and driving notifiers this way. Complete NO for any
> > totally-not-a-GPU things in drivers/accel for sure.
>
> Yes for user that expect HMM they need to be asynchronous. But it is
> hard to revert user ptr has it was done a long time ago.
>
> Cheers,
> Jérôme
>
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