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

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Date:   Thu, 16 Jul 2020 11:22:33 +0100
From:   Will Deacon <will@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     robin.murphy@....com, 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 Thu, Jul 16, 2020 at 11:19:41AM +0100, Will Deacon wrote:
> On Tue, Jun 23, 2020 at 01:28:36AM +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 approx 25% of the cycles for this function.
> > 
> > This series removes that cmpxchg().
> 
> How about something much simpler like the diff below?

Ah, scratch that, I don't drop the lock if we fail the cas with it held.
Let me hack it some more (I have no hardware so I can only build-test this).

Will