linux-kernel - Re: [PATCH 0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency

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Date:   Wed, 8 Jul 2020 14:00:54 +0100
From:   John Garry <john.garry@...wei.com>
To:     <will@...nel.org>, <robin.murphy@....com>
CC:     <joro@...tes.org>, <trivial@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>, <maz@...nel.org>
Subject: Re: [PATCH 0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency

On 22/06/2020 18:28, John Garry wrote:

Hi, Can you guys let me know if this is on the radar at all?

I have been talking about this performance issue since Jan, and not 
getting anything really.

thanks

> 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)
> 
> [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 | 233 +++++++++++++++++++++++-------------
>   1 file changed, 151 insertions(+), 82 deletions(-)
>