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

Jerome Glisse
2020-06-19T18:13:38Z

On Thu, Jun 11, 2020 at 07:35:35PM -0400, Felix Kuehling wrote:
> Am 2020-06-11 um 10:15 a.m. schrieb Jason Gunthorpe:
> > On Thu, Jun 11, 2020 at 10:34:30AM +0200, Daniel Vetter wrote:
> >>> I still have my doubts about allowing fence waiting from within shrinkers.
> >>> IMO ideally they should use a trywait approach, in order to allow memory
> >>> allocation during command submission for drivers that
> >>> publish fences before command submission. (Since early reservation object
> >>> release requires that).
> >> Yeah it is a bit annoying, e.g. for drm/scheduler I think we'll end up
> >> with a mempool to make sure it can handle it's allocations.
> >>
> >>> But since drivers are already waiting from within shrinkers and I take your
> >>> word for HMM requiring this,
> >> Yeah the big trouble is HMM and mmu notifiers. That's the really awkward
> >> one, the shrinker one is a lot less established.
> > I really question if HW that needs something like DMA fence should
> > even be using mmu notifiers - the best use is HW that can fence the
> > DMA directly without having to get involved with some command stream
> > processing.
> >
> > Or at the very least it should not be a generic DMA fence but a
> > narrowed completion tied only into the same GPU driver's command
> > completion processing which should be able to progress without
> > blocking.
> >
> > The intent of notifiers was never to endlessly block while vast
> > amounts of SW does work.
> >
> > Going around and switching everything in a GPU to GFP_ATOMIC seems
> > like bad idea.
> >
> >> I've pinged a bunch of armsoc gpu driver people and ask them how much this
> >> hurts, so that we have a clear answer. On x86 I don't think we have much
> >> of a choice on this, with userptr in amd and i915 and hmm work in nouveau
> >> (but nouveau I think doesn't use dma_fence in there).
>
> Soon nouveau will get company. We're working on a recoverable page fault
> implementation for HMM in amdgpu where we'll need to update page tables
> using the GPUs SDMA engine and wait for corresponding fences in MMU
> notifiers.

Note that HMM mandate, and i stressed that several time in the past,
that all GPU page table update are asynchronous and do not have to
wait on _anything_.

I understand that you use DMA engine for GPU page table update but
if you want to do so with HMM then you need a GPU page table update
only DMA context where all GPU page table update goes through and
where user space can not queue up job.

It can be for HMM only but if you want to mix HMM with non HMM then
everything need to be on that queue and other command queue will have
to depends on it.

Cheers,
Jérôme