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[RFC,v2,0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency

Message ID 1592315993-164290-1-git-send-email-john.garry@huawei.com (mailing list archive)
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Series iommu/arm-smmu-v3: Improve cmdq lock efficiency | expand

Message

John Garry June 16, 2020, 1:59 p.m. UTC 
As mentioned in [0], the CPU may consume many cycles processing
arm_smmu_cmdq_issue_cmdlist(). One issue we find is the cmpxchg() loop to
get space on the queue takes approx 25% of the cycles for this function.

This series removes that cmpxchg().

For my NVMe test with 3x NVMe SSDs, I'm getting a ~24% throughput
increase:
Before: 1310 IOPs
After: 1630 IOPs

I also have a test harness to check the rate of DMA map+unmaps we can
achieve:

CPU count	32	64	128
Before:		63187	19418	10169
After:		93287	44789	15862

(unit is map+unmaps per CPU per second)

There's no specific problem that I know of with this series, as previous
issues should now be fixed - but I'm a bit nervous about how we deal with
the queue being full and wrapping.

And I want to test more.

Thanks

[0] https://lore.kernel.org/linux-iommu/B926444035E5E2439431908E3842AFD24B86DB@DGGEMI525-MBS.china.huawei.com/T/#ma02e301c38c3e94b7725e685757c27e39c7cbde3

John Garry (4):
  iommu/arm-smmu-v3: Fix trivial typo
  iommu/arm-smmu-v3: Calculate bits for prod and owner
  iommu/arm-smmu-v3: Always issue a CMD_SYNC per batch
  iommu/arm-smmu-v3: Remove cmpxchg() in arm_smmu_cmdq_issue_cmdlist()

 drivers/iommu/arm-smmu-v3.c | 210 ++++++++++++++++++++++--------------
 1 file changed, 131 insertions(+), 79 deletions(-)