[PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency

[John Garry]

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 a lot of time once we start getting many
CPUs contending - from experiment, for 64 CPUs contending the cmdq,
success rate is ~ 1 in 12, which is poor, but not totally awful.

This series removes that cmpxchg() and replaces with an atomic_add,
same as how the actual cmdq deals with maintaining the prod pointer.

For my NVMe test with 3x NVMe SSDs, I'm getting a ~24% throughput
increase:
Before: 1250K IOPs
After: 1550K IOPs

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

CPU count	8	16	32	64
Before:		282K	115K	36K	11K
After:		302K	193K	80K	30K

(unit is map+unmaps per CPU per second)

[0] https://lore.kernel.org/linux-iommu/B926444035E5E2439431908E3842AFD24B86DB@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#ma02e301c38c3e94b7725e685757c27e39c7cbde3

Differences to v1:
- Simplify by dropping patch to always issue a CMD_SYNC
- Use 64b atomic add, keeping prod in a separate 32b field

John Garry (2):
  iommu/arm-smmu-v3: Calculate max commands per batch
  iommu/arm-smmu-v3: Remove cmpxchg() in arm_smmu_cmdq_issue_cmdlist()

 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 166 ++++++++++++++------
 1 file changed, 114 insertions(+), 52 deletions(-)

-- 
2.26.2