Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per_sec 63.0% regression

[David Hildenbrand]

On 28.01.25 12:31, Peter Zijlstra wrote:
> On Tue, Jan 28, 2025 at 11:05:17AM +0100, David Hildenbrand wrote:
> > On 28.01.25 10:57, kernel test robot wrote:
> > > hi, Qi Zheng,
> > >
> > > this is more a FYI report than a regression report.
> > >
> > > by 4817f70c25, parent/4817f70c25 configs have below diff,
> > >
> > > --- /pkg/linux/x86_64-rhel-9.4/gcc-12/718b13861d2256ac95d65b892953282a63faf240/.config  2025-01-27 16:20:43.419181382 +0800
> > > +++ /pkg/linux/x86_64-rhel-9.4/gcc-12/4817f70c25b63ee5e6fd42d376700c058ae16a96/.config  2025-01-26 09:27:16.848625105 +0800
> > > @@ -1236,6 +1236,8 @@ CONFIG_IOMMU_MM_DATA=y
> > >    CONFIG_EXECMEM=y
> > >    CONFIG_NUMA_MEMBLKS=y
> > >    CONFIG_NUMA_EMU=y
> > > +CONFIG_ARCH_SUPPORTS_PT_RECLAIM=y
> > > +CONFIG_PT_RECLAIM=y
> > >
> > >    #
> > >    # Data Access Monitoring
> > >
> > >
> > > this report seems show the impact of PT_CLAIM feature for this stress-ng case.
> > >
> > > To us, this is not a code logic regression, but is kind of 'regression' from a
> > > new feature. anyway, below full report just FYI.
> >
> > mmapaddr test case seems to mostly do mmap+munmap. No obvious sign of
> > MADV_DONTNEED, unless buried somewhere :)
> >
> > So either
> >
> > (1) The series is reclaiming page tables outside of MADV_DONTNEED, which
> >      it shouldn't -- in particular not during munmap() where that happens
> >      already using the "ordinary" page table removal code for removed
> >      VMAs.
> >
> > (2) This is the effect of MMU_GATHER_RCU_TABLE_FREE that gets selected?
> >
> >
> > I recall a recent series to select MMU_GATHER_RCU_TABLE_FREE on x86
> > unconditionally (@Peter, @Rik).
>
> Those changes should not have made it to Linus yet.
>
> /me updates git and checks...
>
> nope, nothing changed there ... yet

Sorry, I wasn't quite clear. CONFIG_PT_RECLAIM made it upstream, which 
has "select MMU_GATHER_RCU_TABLE_FREE" in kconfig.

So I'm wondering if the degradation we see in this report is due to 
MMU_GATHER_RCU_TABLE_FREE being selected by CONFIG_PT_RECLAIM, and we'd 
get the same result (degradation) when unconditionally enabling 
MMU_GATHER_RCU_TABLE_FREE.

--
Cheers,

David / dhildenb