linux-kernel - Re: [PATCH] Reinstate "resource: avoid unnecessary lookups in find_next_iomem

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Message-ID: <aSSQXPz0f1A_Vb6Z@smile.fi.intel.com>
Date: Mon, 24 Nov 2025 19:05:32 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ilias Stamatis <ilstam@...zon.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, david@...nel.org, nadav.amit@...il.com,
	huang.ying.caritas@...il.com, bhe@...hat.com,
	nh-open-source@...zon.com
Subject: Re: [PATCH] Reinstate "resource: avoid unnecessary lookups in
 find_next_iomem_res()"

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@...zon.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