RE: [PATCH v3 0/2] make dma_alloc_coherent NUMA-aware by per-NUMA CMA

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Song Bao Hua (Barry Song)
> Sent: Sunday, June 28, 2020 11:13 PM
> To: hch@lst.de; m.szyprowski@samsung.com; robin.murphy@arm.com;
> will@kernel.org; ganapatrao.kulkarni@cavium.com;
> catalin.marinas@arm.com
> Cc: iommu@lists.linux-foundation.org; Linuxarm <linuxarm@huawei.com>;
> linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; Song Bao
> Hua (Barry Song) <song.bao.hua@hisilicon.com>
> Subject: [PATCH v3 0/2] make dma_alloc_coherent NUMA-aware by
> per-NUMA CMA
> 
> Ganapatrao Kulkarni has put some effort on making arm-smmu-v3 use local
> memory to save command queues[1]. I also did similar job in patch
> "iommu/arm-smmu-v3: allocate the memory of queues in local numa node"
> [2] while not realizing Ganapatrao has done that before.
> 
> But it seems it is much better to make dma_alloc_coherent() to be inherently
> NUMA-aware on NUMA-capable systems.
> 
> Right now, smmu is using dma_alloc_coherent() to get memory to save queues
> and tables. Typically, on ARM64 server, there is a default CMA located at
> node0, which could be far away from node2, node3 etc.
> Saving queues and tables remotely will increase the latency of ARM SMMU
> significantly. For example, when SMMU is at node2 and the default global
> CMA is at node0, after sending a CMD_SYNC in an empty command queue, we
> have to wait more than 550ns for the completion of the command
> CMD_SYNC.
> However, if we save them locally, we only need to wait for 240ns.
> 
> 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.
> 
> Meanwhile, when iommu.passthrough is on, device drivers which call dma_
> alloc_coherent() will also get local memory and avoid the travel between
> numa nodes.
> 
> [1]
> https://lists.linuxfoundation.org/pipermail/iommu/2017-October/024455.htm
> l
> [2] https://www.spinics.net/lists/iommu/msg44767.html
> 
> -v3:
>   * move to use page_to_nid() while freeing cma with respect to Robin's
>   comment, but this will only work after applying my below patch:
>   "mm/cma.c: use exact_nid true to fix possible per-numa cma leak"
>   https://marc.info/?l=linux-mm&m=159333034726647&w=2
> 
>   * handle the case count <= 1 more properly according to Robin's
>   comment;
> 
>   * add pernuma_cma parameter to support dynamic setting of per-numa
>   cma size;
>   ideally we can leverage the CMA_SIZE_MBYTES, CMA_SIZE_PERCENTAGE and
>   "cma=" kernel parameter and avoid a new paramter separately for per-
>   numa cma. Practically, it is really too complicated considering the
>   below problems:
>   (1) if we leverage the size of default numa for per-numa, we have to
>   avoid creating two cma with same size in node0 since default cma is
>   probably on node0.
>   (2) default cma can consider the address limitation for old devices
>   while per-numa cma doesn't support GFP_DMA and GFP_DMA32. all
>   allocations with limitation flags will fallback to default one.
>   (3) hard to apply CMA_SIZE_PERCENTAGE to per-numa. it is hard to
>   decide if the percentage should apply to the whole memory size
>   or only apply to the memory size of a specific numa node.
>   (4) default cma size has CMA_SIZE_SEL_MIN and CMA_SIZE_SEL_MAX, it
>   makes things even more complicated to per-numa cma.
> 
>   I haven't figured out a good way to leverage the size of default cma
>   for per-numa cma. it seems a separate parameter for per-numa could
>   make life easier.
> 
>   * move dma_pernuma_cma_reserve() after hugetlb_cma_reserve() to
>   reuse the comment before hugetlb_cma_reserve() with respect to
>   Robin's comment
> 
> -v2:
>   * fix some issues reported by kernel test robot
>   * fallback to default cma while allocation fails in per-numa cma
>      free memory properly
> 
> Barry Song (2):
>   dma-direct: provide the ability to reserve per-numa CMA
>   arm64: mm: reserve per-numa CMA to localize coherent dma buffers
> 
>  .../admin-guide/kernel-parameters.txt         |  9 ++
>  arch/arm64/mm/init.c                          |  2 +
>  include/linux/dma-contiguous.h                |  4 +
>  kernel/dma/Kconfig                            | 10 ++
>  kernel/dma/contiguous.c                       | 98
> +++++++++++++++++--
>  5 files changed, 114 insertions(+), 9 deletions(-)

Gentle ping :-)

Thanks
Barry

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