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

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

On 2024/10/25 15:47, Huang, Ying wrote:
> Kefeng Wang <wangkefeng.wang@huawei.com> writes:
> 
>> 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.
> 
> Sorry, it appears that you misunderstanding my suggestion.  I suggest to
> revise process_huge_page() to use more sequential memory clearing and
> copying to improve its performance on your platform.
> 
> --
> Best Regards,
> Huang, Ying
> 
>>>> 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").

I tried the following changes,
diff --git a/mm/memory.c b/mm/memory.c
index 66cf855dee3f..e5cc75adfa10 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6777,7 +6777,7 @@ static inline int process_huge_page(
                 base = 0;
                 l = n;
                 /* Process subpages at the end of huge page */
-               for (i = nr_pages - 1; i >= 2 * n; i--) {
+               for (i = 2 * n; i < nr_pages; i++) {
                         cond_resched();
                         ret = process_subpage(addr + i * PAGE_SIZE, i, 
arg);
                         if (ret)

Since n = 0, so the copying is from start to end now, but not
improvement for case-anon-cow-seq-hugetlb,

and if use copy_user_gigantic_pager, the time reduced from 6s to 3s

diff --git a/mm/memory.c b/mm/memory.c
index fe21bd3beff5..2c6532d21d84 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6876,10 +6876,7 @@ int copy_user_large_folio(struct folio *dst, 
struct folio *src,
                 .vma = vma,
         };

-       if (unlikely(nr_pages > MAX_ORDER_NR_PAGES))
-               return copy_user_gigantic_page(dst, src, addr_hint, vma, 
nr_pages);
-
-       return process_huge_page(addr_hint, nr_pages, copy_subpage, &arg);
+       return copy_user_gigantic_page(dst, src, addr_hint, vma, nr_pages);
  }






> 
> --
> Best Regards,
> Huang, Ying
>