[bug report] iommu_dma_unmap_sg() is very slow then running IO from remote numa node

[John Garry]

On 28/07/2021 16:17, Ming Lei wrote:
>>>> 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.
> https://lore.kernel.org/lkml/YOhbc5C47IzC893B@T590/

Hi Ming,

Out of curiosity, did you investigate this topic any further?

And you also asked about my results earlier:

On 22/07/2021 16:54, Ming Lei wrote:
 >> [   52.035895] nvme 0000:81:00.0: Adding to iommu group 5
 >> [   52.047732] nvme nvme0: pci function 0000:81:00.0
 >> [   52.067216] nvme nvme0: 22/0/2 default/read/poll queues
 >> [   52.087318]  nvme0n1: p1
 >>
 >> So I get these results:
 >> cpu0 335K
 >> cpu32 346K
 >> cpu64 300K
 >> cpu96 300K
 >>
 >> So still not massive changes.
 > In your last email, the results are the following with irq mode io_uring:
 >
 >   cpu0  497K
 >   cpu4  307K
 >   cpu32 566K
 >   cpu64 488K
 >   cpu96 508K
 >
 > So looks you get much worse result with real io_polling?
 >

Would the expectation be that at least I get the same performance with 
io_polling here? Anything else to try which you can suggest to 
investigate this lower performance?

Thanks,
John