RE: [PATCH v7 0/3] make dma_alloc_coherent NUMA-aware by per-NUMA CMA

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Mike Kravetz [mailto:mike.kravetz@xxxxxxxxxx]
> Sent: Saturday, August 22, 2020 5:53 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; hch@xxxxxx;
> m.szyprowski@xxxxxxxxxxx; robin.murphy@xxxxxxx; will@xxxxxxxxxx;
> ganapatrao.kulkarni@xxxxxxxxxx; catalin.marinas@xxxxxxx;
> akpm@xxxxxxxxxxxxxxxxxxxx
> Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>;
> huangdaode <huangdaode@xxxxxxxxxx>; Linuxarm <linuxarm@xxxxxxxxxx>
> Subject: Re: [PATCH v7 0/3] make dma_alloc_coherent NUMA-aware by
> per-NUMA CMA
> 
> Hi Barry,
> Sorry for jumping in so late.
> 
> On 8/21/20 4:33 AM, Barry Song wrote:
> >
> > with per-numa CMA, smmu will get memory from local numa node to save
> command
> > queues and page tables. that means dma_unmap latency will be shrunk
> much.
> 
> Since per-node CMA areas for hugetlb was introduced, I have been thinking
> about the limited number of CMA areas.  In most configurations, I believe
> it is limited to 7.  And, IIRC it is not something that can be changed at
> runtime, you need to reconfig and rebuild to increase the number.  In contrast
> some configs have NODES_SHIFT set to 10.  I wasn't too worried because of
> the limited hugetlb use case.  However, this series is adding another user
> of per-node CMA areas.
> 
> With more users, should try to sync up number of CMA areas and number of
> nodes?  Or, perhaps I am worrying about nothing?

Hi Mike,
The current limitation is 8. If the server has 4 nodes and we enable both pernuma
CMA and hugetlb, the last node will fail to get one cma area as the default
global cma area will take 1 of 8. So users need to change menuconfig.
If the server has 8 nodes, we enable one of pernuma cma and hugetlb, one node
will fail to get cma.

We may set the default number of CMA areas as 8+MAX_NODES(if hugetlb enabled) +
MAX_NODES(if pernuma cma enabled) if we don't expect users to change config, but
right now hugetlb has not an option in Kconfig to enable or disable like pernuma cma
has DMA_PERNUMA_CMA.

> --
> Mike Kravetz

Thanks
Barry