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

Jason Gunthorpe
2020-06-11T14:15:23Z

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).

Right, nor will RDMA ODP.

Jason