Re: [PATCH] Reinstate "resource: avoid unnecessary lookups in find_next_iomem_res()"

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Mon, Nov 24, 2025 at 08:58:16AM -0800, Andrew Morton wrote:
> On Mon, 24 Nov 2025 16:53:49 +0000 Ilias Stamatis <ilstam@amazon.com> wrote:
> 
> > Commit 97523a4edb7b ("kernel/resource: remove first_lvl / siblings_only
> > logic") removed an optimization introduced by commit 756398750e11
> > ("resource: avoid unnecessary lookups in find_next_iomem_res()"). That
> > was not called out in the message of the first commit explicitly so it's
> > not entirely clear whether removing the optimization happened
> > inadvertently or not.
> > 
> > As the original commit message of the optimization explains there is no
> > point considering the children of a subtree in find_next_iomem_res() if
> > the top level range does not match. Reinstating the optimization results
> > in significant performance improvements in systems with very large iomem
> > maps when mmaping /dev/mem.
> 
> It would be great if we could quantify "significant performance
> improvements"?

+1. It also would be good to know which exact function(s) is a bottleneck.
The mentioned change updated a handful of them.

-- 
With Best Regards,
Andy Shevchenko