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

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: linux-kernel-owner at vger.kernel.org
> [mailto:linux-kernel-owner at vger.kernel.org] On Behalf Of John Garry
> Sent: Saturday, August 22, 2020 1:54 AM
> To: will at kernel.org; robin.murphy at arm.com
> Cc: joro at 8bytes.org; linux-arm-kernel at lists.infradead.org;
> iommu at lists.linux-foundation.org; maz at kernel.org; Linuxarm
> <linuxarm at huawei.com>; linux-kernel at vger.kernel.org; John Garry
> <john.garry at 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
> 4B86DB at 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