[linus:master] [maple_tree] 280b792cac: will-it-scale.per_process_ops 6.0% regression

[Liam R. Howlett]

On 26/05/25 03:23PM, Oliver Sang wrote:
> hi, Liam,
> 
> On Sat, May 23, 2026 at 11:12:24AM -0400, Liam R. Howlett wrote:
> 
> [...]
> 
> > > 
> > > I appled your patch upon below mainline tip commit when I checked.
> > > 6779b50faa562 ("Merge tag 'pci-v7.1-fixes-2' of git://git.kernel.org/pub/scm/linux/kernel/git/pci/pci")
> > > 
> > > then build kernels with attached config.
> > > 
> > > but found a big regression introduced by your patch.
> > > 
> > > =========================================================================================
> > > compiler/cpufreq_governor/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
> > >   gcc-14/performance/x86_64-rhel-9.4/process/100%/debian-13-x86_64-20250902.cgz/lkp-ivb-2ep2/mmap2/will-it-scale
> > > 
> > > commit:
> > >   6779b50faa562 ("Merge tag 'pci-v7.1-fixes-2' of git://git.kernel.org/pub/scm/linux/kernel/git/pci/pci")
> > >   942596e1c1037 ("maple_tree: Restore old tree layout using new scatter-gather node copy")
> > > 
> > > 6779b50faa562e6c 942596e1c1037f014638541bce4
> > > ---------------- ---------------------------
> > >          %stddev     %change         %stddev
> > >              \          |                \
> > >    7023559           -42.9%    4007886        will-it-scale.48.processes
> > >     146323           -42.9%      83497        will-it-scale.per_process_ops
> > >    7023559           -42.9%    4007886        will-it-scale.workload

...

> 
> > 
> > >       0.00           +14.8       14.81        perf-profile.calltrace.cycles-pp.mas_wr_split.mas_store_prealloc.__mmap_new_vma.__mmap_region.do_mmap
> > >       8.17            -8.2        0.00        perf-profile.children.cycles-pp.mas_wr_node_store
> > 
> > It looks like the slow path is taken 100% of the time in your run, for
> > all processes, always.
> > 
> > I understand why that is happening and I can work to avoid it (nudging
> > the write deeper into a split node), but I'd like clarification of my
> > questions above before I keep going here.
> 
> sorry that we cannot supply deep enough technical analysis such like why
> mainline tip (at least 6779b50faa562) has no regression already.
> 
> but in case you want to dig this deep, you could request us to test any
> fix/debug patch, whatever upon your last patch, current mainline tip, or
> 280b792cac. we just want to be able to supply any assistant to improve
> linux kernel code quality. thanks

Thanks!

Are you able to test this patch instead of the previous please?  I had
already started this when you responded so I'd like to see if I'm
correct in what is going on, at least.

Regards,
Liam
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