Re: [linus:master] [mm] f822a9a81a: stress-ng.bigheap.realloc_calls_per_sec 37.3% regression

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Aug 07, 2025 at 09:36:38PM +0530, Dev Jain wrote:

> > > > commit:
> > > >    94dab12d86 ("mm: call pointers to ptes as ptep")
> > > >    f822a9a81a ("mm: optimize mremap() by PTE batching")
> > > >
> > > > 94dab12d86cf77ff f822a9a81a31311d67f260aea96
> > > > ---------------- ---------------------------
> > > >           %stddev     %change         %stddev
> > > >               \          |                \
> > > >       13777 ± 37%     +45.0%      19979 ± 27%
> > > > numa-vmstat.node1.nr_slab_reclaimable
> > > >      367205            +2.3%     375703 vmstat.system.in
> > > >       55106 ± 37%     +45.1%      79971 ± 27%
> > > > numa-meminfo.node1.KReclaimable
> > > >       55106 ± 37%     +45.1%      79971 ± 27%
> > > > numa-meminfo.node1.SReclaimable
> > > >      559381           -37.3%     350757
> > > > stress-ng.bigheap.realloc_calls_per_sec
> > > >       11468            +1.2%      11603 stress-ng.time.system_time
> > > >      296.25            +4.5%     309.70 stress-ng.time.user_time
> > > >        0.81 ±187%    -100.0%       0.00 perf-sched.sch_delay.avg.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0
> > > >        9.36 ±165%    -100.0%       0.00 perf-sched.sch_delay.max.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0
> > > >        0.81 ±187%    -100.0%       0.00 perf-sched.wait_time.avg.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0
> > > >        9.36 ±165%    -100.0%       0.00 perf-sched.wait_time.max.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0

Hm is lack of zap some kind of clue here?

> > > >        5.50 ± 17%    +390.9%      27.00 ± 56% perf-c2c.DRAM.local
> > > >      388.50 ± 10%    +114.7%     834.17 ± 33% perf-c2c.DRAM.remote
> > > >        1214 ± 13%    +107.3%       2517 ± 31% perf-c2c.HITM.local
> > > >      135.00 ± 19%    +130.9%     311.67 ± 32% perf-c2c.HITM.remote
> > > >        1349 ± 13%    +109.6%       2829 ± 31% perf-c2c.HITM.total
> > >
> > > Yeah this also looks pretty consistent too...
> >
> > It almost looks like some kind of NUMA effects?
> >
> > I would have expected that it's the overhead of the vm_normal_folio(),
> > but not sure how that corresponds to the SLAB + local vs. remote stats.
> > Maybe they are just noise?
> Is there any way of making the robot test again? As you said, the only
> suspect is vm_normal_folio(), nothing seems to pop up...
>

Not sure there's much point in that, these tests are run repeatedly and
statistical analysis taken from them so what would another run accomplish unless
there's something very consistently wrong with the box that happens only to
trigger at your commit?

Cheers, Lorenzo