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 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.