> -----Original Message-----
> From: linux-kernel-ow...@vger.kernel.org
> [mailto:linux-kernel-ow...@vger.kernel.org] On Behalf Of John Garry
> Sent: Saturday, August 22, 2020 1:54 AM
> To: w...@kernel.org; robin.mur...@arm.com
> Cc: j...@8bytes.org; linux-arm-ker...@lists.infradead.org;
> iommu@lists.linux-foundation.org; m...@kernel.org; Linuxarm
> <linux...@huawei.com>; linux-ker...@vger.kernel.org; John Garry
> <john.ga...@huawei.com>
> 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
> 4b8...@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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