Re: [PATCH] kernel/resource: optimize find_next_iomem_res

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On Fri, May 31, 2024 at 1:57 AM Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
On Thu, May 30, 2024 at 10:36:57PM -0700, Chia-I Wu wrote:
> We can skip children resources when the parent resource does not cover
> the range.
>
> This should help vmf_insert_* users on x86, such as several DRM drivers.
> On my AMD Ryzen 5 7520C, when streaming data from cpu memory into amdgpu
> bo, the throughput goes from 5.1GB/s to 6.6GB/s.  perf report says
>
>   34.69%--__do_fault
>   34.60%--amdgpu_gem_fault
>   34.00%--ttm_bo_vm_fault_reserved
>   32.95%--vmf_insert_pfn_prot
>   25.89%--track_pfn_insert
>   24.35%--lookup_memtype
>   21.77%--pat_pagerange_is_ram
>   20.80%--walk_system_ram_range
>   17.42%--find_next_iomem_res
>
> before this change, and
>
>   26.67%--__do_fault
>   26.57%--amdgpu_gem_fault
>   25.83%--ttm_bo_vm_fault_reserved
>   24.40%--vmf_insert_pfn_prot
>   14.30%--track_pfn_insert
>   12.20%--lookup_memtype
>   9.34%--pat_pagerange_is_ram
>   8.22%--walk_system_ram_range
>   5.09%--find_next_iomem_res
>
> after.

Is there any documentation that explicitly says that the children resources
must not overlap parent's one? Do we have some test cases? (Either way they
needs to be added / expanded).
I think it's the opposite.  The assumption here is that a child is always a subset of its parent.  Thus, if the range to be checked is not covered by a parent, we can skip the children.

That's guaranteed by __request_resource.  I am less sure about __insert_resource but it appears to be the case too.  FWIW, resource_is_exclusive has the same assumption already.
 
It looks like I need to do some refactoring to add tests.


P.S> I'm not so sure about this change. It needs a thoroughly testing, esp.
in PCI case. Cc'ing to Ilpo.
What's special about PCI?

--
With Best Regards,
Andy Shevchenko



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