[PATCH v4 0/2] make dma_alloc_coherent NUMA-aware by per-NUMA CMA

Barry Song
2020-07-23T13:16:23Z

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.html
[2] https://www.spinics.net/lists/iommu/msg44767.html

-v4:
* rebase on top of Christoph Hellwig's patch:
[PATCH v2] dma-contiguous: cleanup dma_alloc_contiguous
https://lore.kernel.org/linux-iommu/20200723120133.94105-1-hch@lst.de/
* cleanup according to Christoph's comment
* patch 2/2 rebased on top of linux-next to avoid arch/arm64 conflicts
* reserve cma by checking N_MEMORY rather than N_ONLINE

-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 | 94 +++++++++++++++++--
5 files changed, 109 insertions(+), 10 deletions(-)

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2.27.0