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

["Stamatis, Ilias" <ilstam@amazon.co.uk>]

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"?

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.

> 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(), 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])
  do_mmap+0x3ea ([kernel.kallsyms])
  vm_mmap_pgoff+0xa2 ([kernel.kallsyms])
  ksys_mmap_pgoff+0xec ([kernel.kallsyms])
  do_syscall_64+0x29 ([kernel.kallsyms])
  entry_SYSCALL_64_after_hwframe+0x4b ([kernel.kallsyms])

Thanks,
Ilias