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

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Message-ID: <3f37ba8d-66fa-80e2-bbe1-77c2a4323d6b@arm.com>
Date:   Wed, 10 Jan 2018 16:49:58 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org, linux-mips@...ux-mips.org,
        linux-ia64@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, Guan Xuetao <gxt@...c.pku.edu.cn>,
        linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-c6x-dev@...ux-c6x.org, linux-hexagon@...r.kernel.org,
        x86@...nel.org, Konrad Rzeszutek Wilk <konrad@...nok.org>,
        linux-snps-arc@...ts.infradead.org,
        linux-m68k@...ts.linux-m68k.org, patches@...ups.riscv.org,
        linux-metag@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Michal Simek <monstr@...str.eu>, linux-parisc@...r.kernel.org,
        linux-cris-kernel@...s.com, linux-kernel@...r.kernel.org,
        linux-alpha@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 27/33] dma-direct: use node local allocations for coherent
 memory

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.