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

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Message-ID: <265028eb24f60ce45725d7887e93f76a3e86f897.camel@amazon.co.uk>
Date: Tue, 25 Nov 2025 14:23:38 +0000
From: "Stamatis, Ilias" <ilstam@...zon.co.uk>
To: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
CC: "nadav.amit@...il.com" <nadav.amit@...il.com>, "david@...nel.org"
	<david@...nel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"huang.ying.caritas@...il.com" <huang.ying.caritas@...il.com>,
	"bhe@...hat.com" <bhe@...hat.com>, "nh-open-source@...zon.com"
	<nh-open-source@...zon.com>
Subject: Re: [PATCH] Reinstate "resource: avoid unnecessary lookups in
 find_next_iomem_res()"

On Tue, 2025-11-25 at 12:23 +0200, andriy.shevchenko@...ux.intel.com wrote:
> Thanks for elaboration.
> 
> Please summarise this, add the performance test results and send a v2.
> Seems okay to me.

How does this look?

No code change, but happy to send a v2 if that makes things easier for merging.

Reinstate "resource: avoid unnecessary lookups in find_next_iomem_res()"

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 performance improvements in
systems where /proc/iomem is ~5k lines long. Calling mmap() on /dev/mem
in such platforms takes 700-1500μs without the optimisation and 10-50μs
with the optimisation.

Note that even though commit 97523a4edb7b removed the 'sibling_only'
parameter from next_resource(), newer kernels have basically reinstated
it under the name 'skip_children'.

Link: https://lore.kernel.org/all/20251124165349.3377826-1-ilstam@amazon.com/T/#u
Fixes: 97523a4edb7b ("kernel/resource: remove first_lvl / siblings_only logic")
Signed-off-by: Ilias Stamatis <ilstam@...zon.com>
Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>