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

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Date:   Thu, 16 Jul 2020 12:30:30 +0100
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>, Will Deacon <will@...nel.org>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "trivial@...nel.org" <trivial@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>, "maz@...nel.org" <maz@...nel.org>
Subject: Re: [PATCH 0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency

On 16/07/2020 12:22, Robin Murphy wrote:
> On 2020-07-16 11:56, John Garry wrote:
>> On 16/07/2020 11:28, Will Deacon wrote:
>>> On Thu, Jul 16, 2020 at 11:22:33AM +0100, Will Deacon wrote:
>>>> 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).
>>>
>>> Right, second attempt...
>>
>> I can try it, but if performance if not as good, then please check mine
>> further (patch 4/4 specifically) - performance is really good, IMHO.
> 
> Perhaps a silly question (I'm too engrossed in PMU world ATM to get
> properly back up to speed on this), but couldn't this be done without
> cmpxchg anyway? Instinctively it feels like instead of maintaining a
> literal software copy of the prod value, we could resolve the "claim my
> slot in the queue" part with atomic_fetch_add on a free-running 32-bit
> "pseudo-prod" index, then whoever updates the hardware deals with the
> truncation and wrap bit to convert it to an actual register value.
> 

That's what mine does. But I also need to take care of cmdq locking and 
how we unconditionally provide space.

Cheers,
John