Re: [PATCH v2 1/2] mm: use aligned address in clear_gigantic_page()

Kefeng Wang
2024-10-30T05:05:44Z

On 2024/10/30 11:21, Huang, Ying wrote:
> Kefeng Wang <wangkefeng.wang@huawei.com> writes:
>
>> On 2024/10/30 9:04, Huang, Ying wrote:
>>> David Hildenbrand <david@redhat.com> writes:
>>>
>>>> On 29.10.24 14:04, Kefeng Wang wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> That should all be cleaned up ... process_huge_page() likely
>>>>>>>>>>>> shouldn't
>>>>>>>>>>>
>>>>>>>>>>> Yes, let's fix the bug firstly,
>>>>>>>>>>>
>>>>>>>>>>>> be even consuming "nr_pages".
>>>>>>>>>>>
>>>>>>>>>>> No sure about this part, it uses nr_pages as the end and calculate
>>>>>>>>>>> the
>>>>>>>>>>> 'base'.
>>>>>>>>>>
>>>>>>>>>> It should be using folio_nr_pages().
>>>>>>>>>
>>>>>>>>> But process_huge_page() without an explicit folio argument, I'd like to
>>>>>>>>> move the aligned address calculate into the folio_zero_user and
>>>>>>>>> copy_user_large_folio(will rename it to folio_copy_user()) in the
>>>>>>>>> following cleanup patches, or do it in the fix patches?
>>>>>>>>
>>>>>>>> First, why does folio_zero_user() call process_huge_page() for *a small
>>>>>>>> folio*? Because we like or code to be extra complicated to understand?
>>>>>>>> Or am I missing something important?
>>>>>>>
>>>>>>> The folio_zero_user() used for PMD-sized THP and HugeTLB before, and
>>>>>>> after anon mTHP supported, it is used for order-2~order-PMD-order THP
>>>>>>> and HugeTLB, so it won't process a small folio if I understand correctly.
>>>>>>
>>>>>> And unfortunately neither the documentation nor the function name
>>>>>> expresses that :(
>>>>>>
>>>>>> I'm happy to review any patches that improve the situation here :)
>>>>>>
>>>>> Actually, could we drop the process_huge_page() totally, from my
>>>>> testcase[1], process_huge_page() is not better than clear/copy page
>>>>> from start to last, and sequential clearing/copying maybe more
>>>>> beneficial to the hardware prefetching, and is there a way to let lkp
>>>>> to test to check the performance, since the process_huge_page()
>>>>> was submitted by Ying, what's your opinion?
>>> I don't think that it's a good idea to revert the commit without
>>> studying and root causing the issues. I can work together with you on
>>> that. If we have solid and well explained data to prove
>>> process_huge_page() isn't benefitial, we can revert the commit.
>>
>>
>> Take 'fallocate 20G' as an example， before
>>
>> Performance counter stats for 'taskset -c 10 fallocate -l 20G
>> /mnt/hugetlbfs/test':
>
> IIUC, fallocate will zero pages, but will not touch them at all, right?
> If so, no cache benefit from clearing referenced page last.

Yes, for this case, only clear page.
>
>> 3,118.94 msec task-clock # 0.999 CPUs
>> utilized
>> 30 context-switches # 0.010 K/sec
>> 1 cpu-migrations # 0.000 K/sec
>> 136 page-faults # 0.044 K/sec
>> 8,092,075,873 cycles #
>> 2.594 GHz (92.82%)
>> 1,624,587,663 instructions # 0.20 insn per
>> cycle (92.83%)
>> 395,341,850 branches # 126.755 M/sec
>> (92.82%)
>> 3,872,302 branch-misses # 0.98% of all
>> branches (92.83%)
>> 1,398,066,701 L1-dcache-loads # 448.251 M/sec
>> (92.82%)
>> 58,124,626 L1-dcache-load-misses # 4.16% of all
>> L1-dcache accesses (92.82%)
>> 1,032,527 LLC-loads # 0.331 M/sec
>> (92.82%)
>> 498,684 LLC-load-misses # 48.30% of all
>> LL-cache accesses (92.84%)
>> 473,689,004 L1-icache-loads # 151.875 M/sec
>> (92.82%)
>> 356,721 L1-icache-load-misses # 0.08% of all
>> L1-icache accesses (92.85%)
>> 1,947,644,987 dTLB-loads # 624.458 M/sec
>> (92.95%)
>> 10,185 dTLB-load-misses # 0.00% of all
>> dTLB cache accesses (92.96%)
>> 474,622,896 iTLB-loads # 152.174 M/sec
>> (92.95%)
>> 94 iTLB-load-misses # 0.00% of all
>> iTLB cache accesses (85.69%)
>>
>> 3.122844830 seconds time elapsed
>>
>> 0.000000000 seconds user
>> 3.107259000 seconds sys
>>
>> and after（clear from start to end）
>>
>> Performance counter stats for 'taskset -c 10 fallocate -l 20G
>> /mnt/hugetlbfs/test':
>>
>> 1,135.53 msec task-clock # 0.999 CPUs
>> utilized
>> 10 context-switches # 0.009 K/sec
>> 1 cpu-migrations # 0.001 K/sec
>> 137 page-faults # 0.121 K/sec
>> 2,946,673,587 cycles #
>> 2.595 GHz (92.67%)
>> 1,620,704,205 instructions # 0.55 insn per
>> cycle (92.61%)
>> 394,595,772 branches # 347.499 M/sec
>> (92.60%)
>> 130,756 branch-misses # 0.03% of all
>> branches (92.84%)
>> 1,396,726,689 L1-dcache-loads # 1230.022 M/sec
>> (92.96%)
>> 338,344 L1-dcache-load-misses # 0.02% of all
>> L1-dcache accesses (92.95%)
>> 111,737 LLC-loads # 0.098 M/sec
>> (92.96%)
>> 67,486 LLC-load-misses # 60.40% of all
>> LL-cache accesses (92.96%)
>> 418,198,663 L1-icache-loads # 368.285 M/sec
>> (92.96%)
>> 173,764 L1-icache-load-misses # 0.04% of all
>> L1-icache accesses (92.96%)
>> 2,203,364,632 dTLB-loads # 1940.385 M/sec
>> (92.96%)
>> 17,195 dTLB-load-misses # 0.00% of all
>> dTLB cache accesses (92.95%)
>> 418,198,365 iTLB-loads # 368.285 M/sec
>> (92.96%)
>> 79 iTLB-load-misses # 0.00% of all
>> iTLB cache accesses (85.34%)
>>
>> 1.137015760 seconds time elapsed
>>
>> 0.000000000 seconds user
>> 1.131266000 seconds sys
>>
>> The IPC improved a lot，less LLC-loads and more L1-dcache-loads， but
>> this depends on the implementation of the microarchitecture.
>
> Anyway, we need to avoid (or reduce at least) the pure memory clearing
> performance. Have you double checked whether process_huge_page() is
> inlined? Perf-profile result can be used to check this too.
>

Yes, I'm sure the process_huge_page() is inlined.

> When you say from start to end, you mean to use clear_gigantic_page()
> directly, or change process_huge_page() to clear page from start to end?
>

Using clear_gigantic_page() and changing process_huge_page() to clear
page from start to end are both good for performance when sequential
clearing, but no random test so far.

>> 1) Will test some rand test to check the different of performance as
>> David suggested.
>>
>> 2) Hope the LKP to run more tests since it is very useful(more test
>> set and different machines)
>
> I'm starting to use LKP to test.

Greet.