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

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Message-ID: <20200921134324.GK2139@willie-the-truck>
Date:   Mon, 21 Sep 2020 14:43:25 +0100
From:   Will Deacon <will@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     robin.murphy@....com, joro@...tes.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, maz@...nel.org,
        linuxarm@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency

On Fri, Aug 21, 2020 at 09:54:20PM +0800, John Garry wrote:
> 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.

I'm still not a fan of this. Could you try to adapt the hacks I sent before,
please? I know they weren't quite right (I have no hardware to test on), but
the basic idea is to fall back to a spinlock if the cmpxchg() fails. The
queueing in the spinlock implementation should avoid the contention.

Thanks,

Will