[PATCH] kernel/resource: optimize find_next_iomem_res

[Ilpo Järvinen]

On Fri, 31 May 2024, Chia-I Wu wrote:
> On Fri, May 31, 2024 at 1:57 AM Andy Shevchenko <andriy.shevchenko at linux.intel.com>
> 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.

Yes, the children resources are contained within the parent resource (at 
least in PCI but given the code, I'd expect that to be general state of 
affairs).

> 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?

-- 
 i.