Re: [PATCH 27/33] dma-direct: use node local allocations for coherent memory

[Robin Murphy]

On 10/01/18 15:30, Christoph Hellwig wrote:
> On Wed, Jan 10, 2018 at 12:06:22PM +0000, Robin Murphy wrote:
> > On 10/01/18 08:00, Christoph Hellwig wrote:
> > > To preserve the x86 behavior.
> >
> > And combined with patch 10/22 of the SWIOTLB refactoring, this means
> > SWIOTLB allocations will also end up NUMA-aware, right? Great, that's what
> > we want on arm64 too :)
>
> Well, only for swiotlb allocations that can be satisfied by
> dma_direct_alloc.  If we actually have to fall back to the swiotlb
> buffers there is not node affinity yet.

Yeah, when I looked into it I reached the conclusion that per-node 
bounce buffers probably weren't worth it - if you have to bounce you've 
already pretty much lost the performance game, and if the CPU doing the 
bouncing happens to be on a different node from the device you've 
certainly lost either way. Per-node CMA zones we definitely *would* 
like, but that's a future problem (it looks technically feasible without 
huge infrastructure changes, but fiddly).

Robin.
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