-----Original Message-----
From: linux-kernel-owner@vger.kernel.org
[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of John Garry
Sent: Saturday, August 22, 2020 1:54 AM
To: will@kernel.org; robin.murphy@arm.com
Cc: joro@8bytes.org; linux-arm-kernel@lists.infradead.org;
iommu@lists.linux-foundation.org; maz@kernel.org; Linuxarm
[off-list ref]; linux-kernel@vger.kernel.org; John Garry
[off-list ref]
Subject: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency

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)

I have seen performance improvement on hns3 network by sending UDP with 1-32 threads:

Threads number        1        4           8         16       32
Before patch(TX Mbps)  7636.05  16444.36  21694.48  25746.40   25295.93
After  patch(TX Mbps)  7711.60  16478.98  26561.06  32628.75   33764.56

As you can see, for 8,16,32 threads, network TX throughput improve much. For 1 and 4 threads,
Tx throughput is almost seem before and after patch. This should be sensible as this patch
is mainly for decreasing the lock contention.

[0]
https://lore.kernel.org/linux-iommu/B926444035E5E2439431908E3842AFD2
4B86DB@DGGEMI525-MBS.china.huawei.com/T/#ma02e301c38c3e94b7725e
685757c27e39c7cbde3

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

Thanks
Barry


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