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

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Date:   Mon, 21 Sep 2020 14:58:10 +0100
From:   John Garry <john.garry@...wei.com>
To:     Will Deacon <will@...nel.org>
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 21/09/2020 14:43, Will Deacon wrote:
> 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.

OK, so if you're asking me to try this again, then I can do that, and 
see what it gives us.

John