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

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Message-ID: <aSSqIzJWKTe9hs2f@smile.fi.intel.com>
Date: Mon, 24 Nov 2025 20:55:31 +0200
From: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
To: "Stamatis, Ilias" <ilstam@...zon.co.uk>
Cc: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nadav.amit@...il.com" <nadav.amit@...il.com>,
	"david@...nel.org" <david@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.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 Mon, Nov 24, 2025 at 06:01:35PM +0000, Stamatis, Ilias wrote:
> On Mon, 2025-11-24 at 08:58 -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"?
> 
> Hi Andrew and Andy,
> 
> You are right to call that out and apologies for leaving it vague.
> 
> I've done my testing with older kernel versions in systems where `wc -l
> /proc/iomem` can return ~5k. In that environment I see mmaping parts of
> /dev/mem taking 700-1500μs without the optimisation and 10-50μs with the
> optimisation.
> 
> The real-world use case we care about is hypervisor live update where having to
> do lots of these mmaps() serially can significantly affect the guest downtime
> if the cost is 20-30x.

Thanks for providing this information.

> > It also would be good to know which exact function(s) is a bottleneck.
> 
> Perf tracing shows that ~95% of CPU time is spent in find_next_iomem_res(),

Have you investigated possibility to return that check directly into
the culprit?

> the full call stack being:
> 
>   find_next_iomem_res+0x3b ([kernel.kallsyms])
>   walk_system_ram_range+0x98 ([kernel.kallsyms])
>   pat_pagerange_is_ram+0x6e ([kernel.kallsyms])
>   reserve_pfn_range+0x47 ([kernel.kallsyms])
>   track_pfn_remap+0xb6 ([kernel.kallsyms])
>   remap_pfn_range+0x3b ([kernel.kallsyms])
>   mmap_mem+0x9e ([kernel.kallsyms])
>   mm_struct_mmap_region+0x1f3 ([kernel.kallsyms])
>   mmap_region+0xa3 ([kernel.kallsyms])

-- 
With Best Regards,
Andy Shevchenko