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

[Dev Jain <dev.jain@arm.com>]

On 07/08/25 10:37 pm, Lorenzo Stoakes wrote:
> On Thu, Aug 07, 2025 at 10:34:43PM +0530, Dev Jain wrote:
>> On 07/08/25 9:46 pm, Lorenzo Stoakes wrote:
>>> On Thu, Aug 07, 2025 at 05:10:17PM +0100, Lorenzo Stoakes wrote:
>>>> 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
>>> Let me play around on my test box roughly and see if I can repro
>> So I tested with
>> ./stress-ng --timeout 1 --times --verify --metrics --no-rand-seed --oom-avoid --bigheap 20
>> extracted the number out of the line containing the output "realloc calls per sec", did an
>> avg and standard deviation over 20 runs. Before the patch:
>>
>> Average realloc calls/sec: 196907.380000
>> Standard deviation        : 12685.721021
>>
>> After the patch:
>>
>> Average realloc calls/sec: 187894.300500
>> Standard deviation        : 12494.153533
>>
>> which is 5% approx.
>>
> Are you testing that on x86-64 bare metal?

Qemu VM on x86-64.

>
> Anyway this is _not_ what I get.
>
> I am testing on my test box, and seeing a _very significant_ regression as reported.
>
> I am narrowing down the exact cause and will report back. Non-NUMA box, recent
> uArch, dedicated machine.

Oops. Thanks for testing. Lemme stare at my patch for some more time :)

>
> Cheers, Lorenzo