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

Robin Murphy
2018-01-10T16:49:58Z

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.