[bug report] iommu_dma_unmap_sg() is very slow then running IO from remote numa node

[Robin Murphy]

On 2021-07-09 09:38, Ming Lei wrote:
> Hello,
> 
> I observed that NVMe performance is very bad when running fio on one
> CPU(aarch64) in remote numa node compared with the nvme pci numa node.
> 
> Please see the test result[1] 327K vs. 34.9K.
> 
> Latency trace shows that one big difference is in iommu_dma_unmap_sg(),
> 1111 nsecs vs 25437 nsecs.

Are you able to dig down further into that? iommu_dma_unmap_sg() itself 
doesn't do anything particularly special, so whatever makes a difference 
is probably happening at a lower level, and I suspect there's probably 
an SMMU involved. If for instance it turns out to go all the way down to 
__arm_smmu_cmdq_poll_until_consumed() because polling MMIO from the 
wrong node is slow, there's unlikely to be much you can do about that 
other than the global "go faster" knobs (iommu.strict and 
iommu.passthrough) with their associated compromises.

Robin.

> [1] fio test & results
> 
> 1) fio test result:
> 
> - run fio on local CPU
> taskset -c 0 ~/git/tools/test/nvme/io_uring 10 1 /dev/nvme1n1 4k
> + fio --bs=4k --ioengine=io_uring --fixedbufs --registerfiles --hipri --iodepth=64 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 --filename=/dev/nvme1n1 --direct=1 --runtime=10 --numjobs=1 --rw=randread --name=test --group_reporting
> 
> IOPS: 327K
> avg latency of iommu_dma_unmap_sg(): 1111 nsecs
> 
> 
> - run fio on remote CPU
> taskset -c 80 ~/git/tools/test/nvme/io_uring 10 1 /dev/nvme1n1 4k
> + fio --bs=4k --ioengine=io_uring --fixedbufs --registerfiles --hipri --iodepth=64 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 --filename=/dev/nvme1n1 --direct=1 --runtime=10 --numjobs=1 --rw=randread --name=test --group_reporting
> 
> IOPS: 34.9K
> avg latency of iommu_dma_unmap_sg(): 25437 nsecs
> 
> 2) system info
> [root at ampere-mtjade-04 ~]# lscpu | grep NUMA
> NUMA node(s):                    2
> NUMA node0 CPU(s):               0-79
> NUMA node1 CPU(s):               80-159
> 
> lspci | grep NVMe
> 0003:01:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd NVMe SSD Controller SM981/PM981/PM983
> 
> [root at ampere-mtjade-04 ~]# cat /sys/block/nvme1n1/device/device/numa_node
> 0
> 
> 
> 
> Thanks,
> Ming
> 
> 
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