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

["Huang, Ying" <ying.huang@intel.com>]

Kefeng Wang <wangkefeng.wang@huawei.com> writes:

> 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);
>  }

It appears that we have code generation issue here.  Can you check it?
Whether code is inlined in the same way?

Maybe we can start with

modified   mm/memory.c
@@ -6714,7 +6714,7 @@ EXPORT_SYMBOL(__might_fault);
  * operation.  The target subpage will be processed last to keep its
  * cache lines hot.
  */
-static inline int process_huge_page(
+static __always_inline int process_huge_page(
 	unsigned long addr_hint, unsigned int nr_pages,
 	int (*process_subpage)(unsigned long addr, int idx, void *arg),
 	void *arg)

--
Best Regards,
Huang, Ying