Re: [PATCH] mm: shmem: convert to use folio_zero_range()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/10/25 10:59, Huang, Ying wrote:
> Hi, Kefeng,
> 
> Kefeng Wang <wangkefeng.wang@huawei.com> writes:
> 
>> +CC Huang Ying,
>>
>> On 2024/10/23 6:56, Barry Song wrote:
>>> On Wed, Oct 23, 2024 at 4:10 AM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>>>>
>>>>
>> ...
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On 2024/10/17 23:09, Matthew Wilcox wrote:
>>>>>>>>>>>>>>>>>>> On Thu, Oct 17, 2024 at 10:25:04PM +0800, Kefeng Wang wrote:
>>>>>>>>>>>>>>>>>>>> Directly use folio_zero_range() to cleanup code.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Are you sure there's no performance regression introduced by this?
>>>>>>>>>>>>>>>>>>> clear_highpage() is often optimised in ways that we can't optimise for
>>>>>>>>>>>>>>>>>>> a plain memset().  On the other hand, if the folio is large, maybe a
>>>>>>>>>>>>>>>>>>> modern CPU will be able to do better than clear-one-page-at-a-time.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Right, I missing this, clear_page might be better than memset, I change
>>>>>>>>>>>>>>>>>> this one when look at the shmem_writepage(), which already convert to
>>>>>>>>>>>>>>>>>> use folio_zero_range() from clear_highpage(), also I grep
>>>>>>>>>>>>>>>>>> folio_zero_range(), there are some other to use folio_zero_range().
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> fs/bcachefs/fs-io-buffered.c:           folio_zero_range(folio, 0,
>>>>>>>>>>>>>>>>>> folio_size(folio));
>>>>>>>>>>>>>>>>>> fs/bcachefs/fs-io-buffered.c:                   folio_zero_range(f,
>>>>>>>>>>>>>>>>>> 0, folio_size(f));
>>>>>>>>>>>>>>>>>> fs/bcachefs/fs-io-buffered.c:                   folio_zero_range(f,
>>>>>>>>>>>>>>>>>> 0, folio_size(f));
>>>>>>>>>>>>>>>>>> fs/libfs.c:     folio_zero_range(folio, 0, folio_size(folio));
>>>>>>>>>>>>>>>>>> fs/ntfs3/frecord.c:             folio_zero_range(folio, 0,
>>>>>>>>>>>>>>>>>> folio_size(folio));
>>>>>>>>>>>>>>>>>> mm/page_io.c:   folio_zero_range(folio, 0, folio_size(folio));
>>>>>>>>>>>>>>>>>> mm/shmem.c:             folio_zero_range(folio, 0, folio_size(folio));
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> IOW, what performance testing have you done with this patch?
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> No performance test before, but I write a testcase,
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> 1) allocate N large folios (folio_alloc(PMD_ORDER))
>>>>>>>>>>>>>>>>>> 2) then calculate the diff(us) when clear all N folios
>>>>>>>>>>>>>>>>>>             clear_highpage/folio_zero_range/folio_zero_user
>>>>>>>>>>>>>>>>>> 3) release N folios
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> the result(run 5 times) shown below on my machine,
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> N=1,
>>>>>>>>>>>>>>>>>>                 clear_highpage  folio_zero_range    folio_zero_user
>>>>>>>>>>>>>>>>>>            1      69                   74                 177
>>>>>>>>>>>>>>>>>>            2      57                   62                 168
>>>>>>>>>>>>>>>>>>            3      54                   58                 234
>>>>>>>>>>>>>>>>>>            4      54                   58                 157
>>>>>>>>>>>>>>>>>>            5      56                   62                 148
>>>>>>>>>>>>>>>>>> avg       58                   62.8               176.8
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> N=100
>>>>>>>>>>>>>>>>>>                 clear_highpage  folio_zero_range    folio_zero_user
>>>>>>>>>>>>>>>>>>            1    11015                 11309               32833
>>>>>>>>>>>>>>>>>>            2    10385                 11110               49751
>>>>>>>>>>>>>>>>>>            3    10369                 11056               33095
>>>>>>>>>>>>>>>>>>            4    10332                 11017               33106
>>>>>>>>>>>>>>>>>>            5    10483                 11000               49032
>>>>>>>>>>>>>>>>>> avg     10516.8               11098.4             39563.4
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> N=512
>>>>>>>>>>>>>>>>>>                 clear_highpage  folio_zero_range   folio_zero_user
>>>>>>>>>>>>>>>>>>            1    55560                 60055              156876
>>>>>>>>>>>>>>>>>>            2    55485                 60024              157132
>>>>>>>>>>>>>>>>>>            3    55474                 60129              156658
>>>>>>>>>>>>>>>>>>            4    55555                 59867              157259
>>>>>>>>>>>>>>>>>>            5    55528                 59932              157108
>>>>>>>>>>>>>>>>>> avg     55520.4               60001.4            157006.6
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> folio_zero_user with many cond_resched(), so time fluctuates a lot,
>>>>>>>>>>>>>>>>>> clear_highpage is better folio_zero_range as you said.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Maybe add a new helper to convert all folio_zero_range(folio, 0,
>>>>>>>>>>>>>>>>>> folio_size(folio))
>>>>>>>>>>>>>>>>>> to use clear_highpage + flush_dcache_folio?
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> If this also improves performance for other existing callers of
>>>>>>>>>>>>>>>>> folio_zero_range(), then that's a positive outcome.
>>>>>>>>>>>>>>>>
>>>>>>>>>>> ...
>>>>>>>>>>>
>>>>>>>>>>>>>> hi Kefeng,
>>>>>>>>>>>>>> what's your point? providing a helper like clear_highfolio() or similar?
>>>>>>>>>>>>>
>>>>>>>>>>>>> Yes, from above test, using clear_highpage/flush_dcache_folio is better
>>>>>>>>>>>>> than using folio_zero_range() for folio zero(especially for large
>>>>>>>>>>>>> folio), so I'd like to add a new helper, maybe name it folio_zero()
>>>>>>>>>>>>> since it zero the whole folio.
>>>>>>>>>>>>
>>>>>>>>>>>> we already have a helper like folio_zero_user()?
>>>>>>>>>>>> it is not good enough?
>>>>>>>>>>>
>>>>>>>>>>> Since it is with many cond_resched(), the performance is worst...
>>>>>>>>>>
>>>>>>>>>> Not exactly? It should have zero cost for a preemptible kernel.
>>>>>>>>>> For a non-preemptible kernel, it helps avoid clearing the folio
>>>>>>>>>> from occupying the CPU and starving other processes, right?
>>>>>>>>>
>>>>>>>>> --- a/mm/shmem.c
>>>>>>>>> +++ b/mm/shmem.c
>>>>>>>>>
>>>>>>>>> @@ -2393,10 +2393,7 @@ static int shmem_get_folio_gfp(struct inode
>>>>>>>>> *inode, pgoff_t index,
>>>>>>>>>               * it now, lest undo on failure cancel our earlier guarantee.
>>>>>>>>>               */
>>>>>>>>>
>>>>>>>>>              if (sgp != SGP_WRITE && !folio_test_uptodate(folio)) {
>>>>>>>>> -               long i, n = folio_nr_pages(folio);
>>>>>>>>> -
>>>>>>>>> -               for (i = 0; i < n; i++)
>>>>>>>>> -                       clear_highpage(folio_page(folio, i));
>>>>>>>>> +               folio_zero_user(folio, vmf->address);
>>>>>>>>>                      flush_dcache_folio(folio);
>>>>>>>>>                      folio_mark_uptodate(folio);
>>>>>>>>>              }
>>>>>>>>>
>>>>>>>>> Do we perform better or worse with the following?
>>>>>>>>
>>>>>>>> Here is for SGP_FALLOC, vmf = NULL, we could use folio_zero_user(folio,
>>>>>>>> 0), I think the performance is worse, will retest once I can access
>>>>>>>> hardware.
>>>>>>>
>>>>>>> Perhaps, since the current code uses clear_hugepage(). Does using
>>>>>>> index << PAGE_SHIFT as the addr_hint offer any benefit?
>>>>>>>
>>>>>>
>>>>>> when use folio_zero_user(), the performance is vary bad with above
>>>>>> fallocate test(mount huge=always),
>>>>>>
>>>>>>           folio_zero_range   clear_highpage         folio_zero_user
>>>>>> real    0m1.214s             0m1.111s              0m3.159s
>>>>>> user    0m0.000s             0m0.000s              0m0.000s
>>>>>> sys     0m1.210s             0m1.109s              0m3.152s
>>>>>>
>>>>>> I tried with addr_hint = 0/index << PAGE_SHIFT, no obvious different.
>>>>>
>>>>> Interesting. Does your kernel have preemption disabled or
>>>>> preemption_debug enabled?
>>>>
>>>> ARM64 server, CONFIG_PREEMPT_NONE=y
>>> this explains why the performance is much worse.
>>>
>>>>
>>>>>
>>>>> If not, it makes me wonder whether folio_zero_user() in
>>>>> alloc_anon_folio() is actually improving performance as expected,
>>>>> compared to the simpler folio_zero() you plan to implement. :-)
>>>>
>>>> Yes, maybe, the folio_zero_user(was clear_huge_page) is from
>>>> 47ad8475c000 ("thp: clear_copy_huge_page"), so original clear_huge_page
>>>> is used in HugeTLB, clear PUD size maybe spend many time, but for PMD or
>>>> other size of large folio, cond_resched is not necessary since we
>>>> already have some folio_zero_range() to clear large folio, and no issue
>>>> was reported.
>>> probably worth an optimization. calling cond_resched() for each page
>>> seems too aggressive and useless.
>>
>> After some test, I think the cond_resched() is not the root cause,
>> no performance gained with batched cond_resched(), even I kill
>> cond_resched() from process_huge_page, no improvement.
>>
>> But when I unconditionally use clear_gigantic_page() in
>> folio_zero_user(patched), there is big improvement with above
>> fallocate on tmpfs(mount huge=always), also I test some other testcase,
>>
>>
>> 1) case-anon-w-seq-mt: (2M PMD THP)
>>
>> base:
>> real    0m2.490s    0m2.254s    0m2.272s
>> user    1m59.980s   2m23.431s   2m18.739s
>> sys     1m3.675s    1m15.462s   1m15.030s	
>>
>> patched:
>> real    0m2.234s    0m2.225s    0m2.159s
>> user    2m56.105s   2m57.117s   3m0.489s
>> sys     0m17.064s   0m17.564s   0m16.150s
>>
>> Patched kernel win on sys and bad in user, but real is almost same,
>> maybe a little better than base.
> 
> We can find user time difference.  That means the original cache hot
> behavior still applies on your system.
> 
> However, it appears that the performance to clear page from end to begin
> is really bad on your system.
> 
> So, I suggest to revise the current implementation to use sequential
> clearing as much as possible.
> 

I test case-anon-cow-seq-hugetlb for copy_user_large_folio()

base:
real    0m6.259s    0m6.197s    0m6.316s
user    1m31.176s   1m27.195s   1m29.594s
sys     7m44.199s   7m51.490s   8m21.149s

patched(use copy_user_gigantic_page for 2M hugetlb too)
real    0m3.182s    0m3.002s    0m2.963s
user    1m19.456s   1m3.107s    1m6.447s
sys     2m59.222s   3m10.899s   3m1.027s

and sequential copy is better than the current implementation,
so I will use sequential clear and copy.



>> 2) case-anon-w-seq-hugetlb:(2M PMD HugeTLB)
>>
>> base:
>> real    0m5.175s    0m5.117s    0m4.856s
>> user    5m15.943s   5m7.567s    4m29.273s
>> sys     2m38.503s   2m21.949s   2m21.252s
>>
>> patched:
>> real    0m4.966s    0m4.841s    0m4.561s
>> user    6m30.123s   6m9.516s    5m49.733s
>> sys     0m58.503s   0m47.847s   0m46.785s
>>
>>
>> This case is similar to the case1.
>>
>> 3) fallocate hugetlb 20G (2M PMD HugeTLB)
>>
>> base:
>> real    0m3.016s    0m3.019s    0m3.018s
>> user    0m0.000s    0m0.000s    0m0.000s
>> sys     0m3.009s    0m3.012s    0m3.010s
>>
>> patched:
>>
>> real    0m1.136s    0m1.136s    0m1.136s
>> user    0m0.000s    0m0.000s    0m0.004s
>> sys     0m1.133s    0m1.133s    0m1.129s
>>
>>
>> There is big win on patched kernel, and it is similar to above tmpfs
>> test, so maybe we could revert the commit c79b57e462b5 ("mm: hugetlb:
>> clear target sub-page last when clearing huge page").
> 
> --
> Best Regards,
> Huang, Ying
>