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

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Date:   Fri, 13 Nov 2020 10:43:06 +0000
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 14:58, John Garry wrote:
> 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.
> 

JFYI, to prove that this is not a problem which affects only our HW, I 
managed to test an arm64 platform from another vendor. Generally I see 
the same issue, and this patchset actually helps that platform even more.

		CPUs	Before	After	% Increase
Huawei D06	8	282K	302K	7%
Other			379K	420K	11%

Huawei D06	16	115K	193K	68K
Other			102K	291K	185K

Huawei D06	32	36K	80K	122%
Other			41K	156K	280%

Huawei D06	64	11K	30K	172%
Other			6K	47K	683%

I tested with something like [1], so unit is map+unmaps per cpu per 
second - higher is better.

My D06 is memory poor, so would expect higher results otherwise (with 
more memory). Indeed, my D05 has memory on all nodes and performs better.

Anyway, I see that the implementation here is not perfect, and I could 
not get suggested approach to improve performance significantly. So back 
to the drawing board...

Thanks,
John

[1] 
https://lore.kernel.org/linux-iommu/20201102080646.2180-1-song.bao.hua@hisilicon.com/