Re: performance regression noted in v5.11-rc after c062db039f40

[Lu Baolu <baolu.lu@linux.intel.com>]

On 2021/1/23 1:38, Robin Murphy wrote:
>>> I kind of believe it's due to the indirect calls. This is also reported
>>> on ARM.
>>>
>>> https://lore.kernel.org/linux-iommu/1610376862-927-1-git-send-email-isaacm@codeaurora.org/ 
>>>
>>>
>>> Maybe we can try changing indirect calls to static ones to verify this
>>> problem.
>>
>> I liked the idea of map_sg() enough to try my hand at building a PoC for
>> Intel, based on Isaac's patch series. It's just a cut-and-paste of the
>> generic iommu.c code with the indirect calls to ops->map() replaced.
>>
>> The indirect calls do not seem to be the problem. Calling intel_iommu_map
>> directly appears to be as costly as calling it indirectly.
>>
>> However, perhaps there are other ways map_sg() can be beneficial. In
>> v5.10, __domain_mapping and iommu_flush_write_buffer() appear to be
>> invoked just once for each large map operation, for example.
> 
> Oh, if the driver needs to do maintenance beyond just installing PTEs, 
> that should probably be devolved to iotlb_sync_map anyway. There's a 
> patch series here generalising that to be more useful, which is 
> hopefully just waiting to be merged now:
> 
> https://lore.kernel.org/linux-iommu/20210107122909.16317-1-yong.wu@mediatek.com/ 
> 

The iotlb_sync_map() could help here as far as I can see. I will post a
call-for-test patch set later.

> 
> Robin.

Best regards,
baolu
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