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

["andriy.shevchenko@linux.intel.com" <andriy.shevchenko@linux.intel.com>]

On Tue, Nov 25, 2025 at 09:56:36AM +0000, Stamatis, Ilias wrote:
> On Tue, 2025-11-25 at 08:50 +0200, andriy.shevchenko@linux.intel.com wrote:
> > On Mon, Nov 24, 2025 at 11:30:46PM +0000, Stamatis, Ilias wrote:
> > > On Mon, 2025-11-24 at 21:52 +0200, andriy.shevchenko@linux.intel.com wrote:
> > > > On Mon, Nov 24, 2025 at 07:35:31PM +0000, Stamatis, Ilias wrote:
> > > > > On Mon, 2025-11-24 at 20:55 +0200, andriy.shevchenko@linux.intel.com wrote:
> > > > > > 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@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"?
> > > > > > > 
> > > > > > > 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?
> > > > > 
> > > > > I'm sorry, I don't understand this. Could you please clarify what you mean?
> > > > > What do you consider to be the culprit and which check do you refer to?
> > > > 
> > > > The mentioned patch removed the check for siblings from next_resource().
> > > > The function that your test case complains about is find_next_iomem_res().
> > > > Hence, have you tried to reinstantiate the (removed) check from next_resource()
> > > > in find_next_iomem_res() and see if it helps?
> > > 
> > > next_resource() does accept a 'skip_children' parameter in the latest kernel
> > > today which is equivalent to the 'sibling_only' parameter in the older
> > > kernels.
> > 
> > It used to be
> > 
> >        if (sibling_only)
> >                return p->sibling;
> > 
> >        if (p->child)
> >                return p->child;
> >        ...
> 
> This returns p->sibling if sibling_only == true. 
> The return value might also be NULL.
> 
> > and become (in the latest kernels)
> > 
> >        if (!skip_children && p->child)
> >                return p->child;
> >        ...
> 
> 	if (!skip_children && p->child)
> 		return p->child;
> 	while (!p->sibling && p->parent) {
> 		p = p->parent;
> 		if (p == subtree_root)
> 			return NULL;
> 	}
> 	return p->sibling;
> 
> This is the full function on the latest kernel. If skip_children == true and
> there is a sibling, it also returns p->sibling.
> 
> If p->sibling is NULL, it'll try to get the parent. In the case of
> find_next_iomem_res() the parent will be iomem_resource, in which case the if
> (p == subtree_root) path is taken and we return NULL (same as the case of 
> p->sibling being NULL above). 

Thanks for elaboration.

Please summarise this, add the performance test results and send a v2.
Seems okay to me.

> > Can you elaborate how are they interoperable?
> > 
> > TL;DR: I don't think it's an equivalent.

So, it's not a literal equivalent, but it behaves in a very similar way.

-- 
With Best Regards,
Andy Shevchenko