linux-kernel - Re: [bug report] iommu_dma_unmap_sg() is very slow then running IO from remote numa node

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Message-ID: <ead87bf2-ddfa-eb67-db44-9619c6cdb714@huawei.com>
Date:   Wed, 28 Jul 2021 11:38:18 +0100
From:   John Garry <john.garry@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     Robin Murphy <robin.murphy@....com>,
        <linux-kernel@...r.kernel.org>, <linux-nvme@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>,
        "Will Deacon" <will@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [bug report] iommu_dma_unmap_sg() is very slow then running IO
 from remote numa node

On 28/07/2021 02:32, Ming Lei wrote:
> On Mon, Jul 26, 2021 at 3:51 PM John Garry<john.garry@...wei.com>  wrote:
>> On 23/07/2021 11:21, Ming Lei wrote:
>>>> Thanks, I was also going to suggest the latter, since it's what
>>>> arm_smmu_cmdq_issue_cmdlist() does with IRQs masked that should be most
>>>> indicative of where the slowness most likely stems from.
>>> The improvement from 'iommu.strict=0' is very small:
>>>
>> Have you tried turning off the IOMMU to ensure that this is really just
>> an IOMMU problem?
>>
>> You can try setting CONFIG_ARM_SMMU_V3=n in the defconfig or passing
>> cmdline param iommu.passthrough=1 to bypass the the SMMU (equivalent to
>> disabling for kernel drivers).
> Bypassing SMMU via iommu.passthrough=1 basically doesn't make a difference
> on this issue.

A ~90% throughput drop still seems to me to be too high to be a software 
issue. More so since I don't see similar on my system. And that 
throughput drop does not lead to a total CPU usage drop, from the fio log.

Do you know if anyone has run memory benchmark tests on this board to 
find out NUMA effect? I think lmbench or stream could be used for this.

Testing network performance in an equivalent fashion to storage could 
also be an idea.

Thanks,
John

> 
> And from fio log, submission latency is good, but completion latency
> is pretty bad,
> and maybe it is something that writing to PCI memory isn't committed to HW in
> time?