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

[Jann Horn <jannh@google.com>]

On Thu, Aug 7, 2025 at 10:28 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
> On Thu, Aug 07, 2025 at 04:17:09PM +0800, kernel test robot wrote:
> > 94dab12d86cf77ff f822a9a81a31311d67f260aea96
> > ---------------- ---------------------------
> >          %stddev     %change         %stddev
> >              \          |                \
> >      13777 ą 37%     +45.0%      19979 ą 27%  numa-vmstat.node1.nr_slab_reclaimable
> >     367205            +2.3%     375703        vmstat.system.in
> >      55106 ą 37%     +45.1%      79971 ą 27%  numa-meminfo.node1.KReclaimable
> >      55106 ą 37%     +45.1%      79971 ą 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 ą187%    -100.0%       0.00        perf-sched.sch_delay.avg.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0
> >       9.36 ą165%    -100.0%       0.00        perf-sched.sch_delay.max.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0
> >       0.81 ą187%    -100.0%       0.00        perf-sched.wait_time.avg.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0
> >       9.36 ą165%    -100.0%       0.00        perf-sched.wait_time.max.ms.__cond_resched.zap_pte_range.zap_pmd_range.isra.0
> >       5.50 ą 17%    +390.9%      27.00 ą 56%  perf-c2c.DRAM.local
> >     388.50 ą 10%    +114.7%     834.17 ą 33%  perf-c2c.DRAM.remote
> >       1214 ą 13%    +107.3%       2517 ą 31%  perf-c2c.HITM.local
> >     135.00 ą 19%    +130.9%     311.67 ą 32%  perf-c2c.HITM.remote
> >       1349 ą 13%    +109.6%       2829 ą 31%  perf-c2c.HITM.total
>
> Yeah this also looks pretty consistent too...

FWIW, HITM hat different meanings depending on exactly which
microarchitecture that test happened on; the message says it is from
Sapphire Rapids, which is a successor of Ice Lake, so HITM is less
meaningful than if it came from a pre-IceLake system (see
https://lore.kernel.org/all/CAG48ez3RmV6SsVw9oyTXxQXHp3rqtKDk2qwJWo9TGvXCq7Xr-w@mail.gmail.com/).

To me those numbers mainly look like you're accessing a lot more
cache-cold data. (On pre-IceLake they would indicate cacheline
bouncing, but I guess here they probably don't.) And that makes sense,
since before the patch, this path was just moving PTEs around without
looking at the associated pages/folios; basically more or less like a
memcpy() on x86-64. But after the patch, for every 8 bytes that you
copy, you have to load a cacheline from the vmemmap to get the page.