Message-ID: <225fd9dd-2b97-4ec6-a9a6-fe148c4b901e@arm.com>
Date: Thu, 5 Jun 2025 08:15:56 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Xavier Xia <xavier.qyxia@...il.com>
Cc: Xavier Xia <xavier_qy@....com>, 21cnbao@...il.com, dev.jain@....com,
 ioworker0@...il.com, akpm@...ux-foundation.org, catalin.marinas@....com,
 david@...hat.com, gshan@...hat.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, will@...nel.org, willy@...radead.org,
 ziy@...dia.com, Barry Song <baohua@...nel.org>, linux-mm@...ck.org
Subject: Re: [PATCH v6] arm64/mm: Optimize loop to reduce redundant operations
 of contpte_ptep_get

On 05/06/2025 06:54, Xavier Xia wrote:
> Hi Ryan,
> 
> Thank you for your review, and for reproducing and verifying the test cases.
> I am using a Gmail email to reply to your message, hoping you can receive it.
> Please check the details below.

Ahh yes, this arrived in my inbox without issue!

Thanks,
Ryan


> 
> 
> 
> On Thu, Jun 5, 2025 at 11:20 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> On 10/05/2025 13:59, Xavier Xia wrote:
>>> This commit optimizes the contpte_ptep_get and contpte_ptep_get_lockless
>>> function by adding early termination logic. It checks if the dirty and
>>> young bits of orig_pte are already set and skips redundant bit-setting
>>> operations during the loop. This reduces unnecessary iterations and
>>> improves performance.
>>>
>>> In order to verify the optimization performance, a test function has been
>>> designed. The function's execution time and instruction statistics have
>>> been traced using perf, and the following are the operation results on a
>>> certain Qualcomm mobile phone chip:
>>>
>>> Test Code:
>>
>> nit: It would have been good to include the source for the whole program,
>> including #includes and the main() function to make it quicker for others to get
>> up and running.
> 
> OK, I will pay attention to it in the future. This test case is quite
> simple, so I didn't add it.
> 
>>
>>>
>>>       #define PAGE_SIZE 4096
>>>       #define CONT_PTES 16
>>>       #define TEST_SIZE (4096* CONT_PTES * PAGE_SIZE)
>>>       #define YOUNG_BIT 8
>>>       void rwdata(char *buf)
>>>       {
>>>               for (size_t i = 0; i < TEST_SIZE; i += PAGE_SIZE) {
>>>                       buf[i] = 'a';
>>>                       volatile char c = buf[i];
>>>               }
>>>       }
>>>       void clear_young_dirty(char *buf)
>>>       {
>>>               if (madvise(buf, TEST_SIZE, MADV_FREE) == -1) {
>>>                       perror("madvise free failed");
>>>                       free(buf);
>>>                       exit(EXIT_FAILURE);
>>>               }
>>>               if (madvise(buf, TEST_SIZE, MADV_COLD) == -1) {
>>>                       perror("madvise free failed");
>>>                       free(buf);
>>>                       exit(EXIT_FAILURE);
>>>               }
>>
>> nit: MADV_FREE clears both young and dirty so I don't think MADV_COLD is
>> required? (MADV_COLD only clears young I think?)
> 
> You're right, MADV_COLD here can probably be removed.
> 
>>
>>>       }
>>>       void set_one_young(char *buf)
>>>       {
>>>               for (size_t i = 0; i < TEST_SIZE; i += CONT_PTES * PAGE_SIZE) {
>>>                       volatile char c = buf[i + YOUNG_BIT * PAGE_SIZE];
>>>               }
>>>       }
>>>
>>>       void test_contpte_perf() {
>>>               char *buf;
>>>               int ret = posix_memalign((void **)&buf, CONT_PTES * PAGE_SIZE,
>>>                               TEST_SIZE);
>>>               if ((ret != 0) || ((unsigned long)buf % CONT_PTES * PAGE_SIZE)) {
>>>                       perror("posix_memalign failed");
>>>                       exit(EXIT_FAILURE);
>>>               }
>>>
>>>               rwdata(buf);
>>>       #if TEST_CASE2 || TEST_CASE3
>>>               clear_young_dirty(buf);
>>>       #endif
>>>       #if TEST_CASE2
>>>               set_one_young(buf);
>>>       #endif
>>>
>>>               for (int j = 0; j < 500; j++) {
>>>                       mlock(buf, TEST_SIZE);
>>>
>>>                       munlock(buf, TEST_SIZE);
>>>               }
>>>               free(buf);
>>>       }
>>>
>>>       Descriptions of three test scenarios
>>>
>>> Scenario 1
>>>       The data of all 16 PTEs are both dirty and young.
>>>       #define TEST_CASE2 0
>>>       #define TEST_CASE3 0
>>>
>>> Scenario 2
>>>       Among the 16 PTEs, only the 8th one is young, and there are no dirty ones.
>>>       #define TEST_CASE2 1
>>>       #define TEST_CASE3 0
>>>
>>> Scenario 3
>>>       Among the 16 PTEs, there are neither young nor dirty ones.
>>>       #define TEST_CASE2 0
>>>       #define TEST_CASE3 1
>>>
>>> Test results
>>>
>>> |Scenario 1         |       Original|       Optimized|
>>> |-------------------|---------------|----------------|
>>> |instructions       |    37912436160|     18731580031|
>>> |test time          |         4.2797|          2.2949|
>>> |overhead of        |               |                |
>>> |contpte_ptep_get() |         21.31%|           4.80%|
>>>
>>> |Scenario 2         |       Original|       Optimized|
>>> |-------------------|---------------|----------------|
>>> |instructions       |    36701270862|     36115790086|
>>> |test time          |         3.2335|          3.0874|
>>> |Overhead of        |               |                |
>>> |contpte_ptep_get() |         32.26%|          33.57%|
>>>
>>> |Scenario 3         |       Original|       Optimized|
>>> |-------------------|---------------|----------------|
>>> |instructions       |    36706279735|     36750881878|
>>> |test time          |         3.2008|          3.1249|
>>> |Overhead of        |               |                |
>>> |contpte_ptep_get() |         31.94%|          34.59%|
>>>
>>> For Scenario 1, optimized code can achieve an instruction benefit of 50.59%
>>> and a time benefit of 46.38%.
>>> For Scenario 2, optimized code can achieve an instruction count benefit of
>>> 1.6% and a time benefit of 4.5%.
>>> For Scenario 3, since all the PTEs have neither the young nor the dirty
>>> flag, the branches taken by optimized code should be the same as those of
>>> the original code. In fact, the test results of optimized code seem to be
>>> closer to those of the original code.
>>
>> I re-ran these tests on Apple M2 with 4K base pages + 64K mTHP.
>>
>> Scenario 1: reduced to 56% of baseline execution time
>> Scenario 2: reduced to 89% of baseline execution time
>> Scenario 3: reduced to 91% of baseline execution time
>>
>> I'm pretty amazed that scenario 3 got faster given it is doing the same number
>> of loops.
> 
> It seems that the data you obtained is similar to my test data. For
> scenario 3, it's
> faster even when running the same code, which I can't quite figure out either.
> 
>>>
>>> It can be proven through test function that the optimization for
>>> contpte_ptep_get is effective. Since the logic of contpte_ptep_get_lockless
>>> is similar to that of contpte_ptep_get, the same optimization scheme is
>>> also adopted for it.
>>>
>>> Reviewed-by: Barry Song <baohua@...nel.org>
>>> Signed-off-by: Xavier Xia <xavier_qy@....com>
>>
>> I don't love the extra complexity, but this version is much tidier. While the
>> micro-benchmark is clearly contrived, it shows that there will be cases where it
>> will be faster and there are no cases where it is slower. This will probably be
>> more valuable for 16K kernels because the number of PTEs in a contpte block is
>> 128 there:
> 
> Okay, this version has been revised multiple times based on your
> previous feedback
> and Barry's comments, and it seems much less complicated to understand now. :)
> 
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>> Tested-by: Ryan Roberts <ryan.roberts@....com>
>>
>>> ---
>>> Changes in v6:
>>> - Move prot = pte_pgprot(pte_mkold(pte_mkclean(pte))) into the contpte_is_consistent(),
>>>   as suggested by Barry.
>>> - Link to v5: https://lore.kernel.org/all/20250509122728.2379466-1-xavier_qy@163.com/
>>>
>>> Changes in v5:
>>> - Replace macro CHECK_CONTPTE_CONSISTENCY with inline function contpte_is_consistent
>>>   for improved readability and clarity, as suggested by Barry.
>>> - Link to v4: https://lore.kernel.org/all/20250508070353.2370826-1-xavier_qy@163.com/
>>>
>>> Changes in v4:
>>> - Convert macro CHECK_CONTPTE_FLAG to an internal loop for better readability.
>>> - Refactor contpte_ptep_get_lockless using the same optimization logic, as suggested by Ryan.
>>> - Link to v3: https://lore.kernel.org/all/3d338f91.8c71.1965cd8b1b8.Coremail.xavier_qy@163.com/
>>> ---
>>>  arch/arm64/mm/contpte.c | 74 +++++++++++++++++++++++++++++++++++------
>>>  1 file changed, 64 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>>> index bcac4f55f9c1..71efe7dff0ad 100644
>>> --- a/arch/arm64/mm/contpte.c
>>> +++ b/arch/arm64/mm/contpte.c
>>> @@ -169,17 +169,46 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
>>>       for (i = 0; i < CONT_PTES; i++, ptep++) {
>>>               pte = __ptep_get(ptep);
>>>
>>> -             if (pte_dirty(pte))
>>> +             if (pte_dirty(pte)) {
>>>                       orig_pte = pte_mkdirty(orig_pte);
>>> -
>>> -             if (pte_young(pte))
>>> +                     for (; i < CONT_PTES; i++, ptep++) {
>>> +                             pte = __ptep_get(ptep);
>>> +                             if (pte_young(pte)) {
>>> +                                     orig_pte = pte_mkyoung(orig_pte);
>>> +                                     break;
>>> +                             }
>>> +                     }
>>> +                     break;
>>> +             }
>>> +
>>> +             if (pte_young(pte)) {
>>>                       orig_pte = pte_mkyoung(orig_pte);
>>> +                     i++;
>>> +                     ptep++;
>>> +                     for (; i < CONT_PTES; i++, ptep++) {
>>> +                             pte = __ptep_get(ptep);
>>> +                             if (pte_dirty(pte)) {
>>> +                                     orig_pte = pte_mkdirty(orig_pte);
>>> +                                     break;
>>> +                             }
>>> +                     }
>>> +                     break;
>>> +             }
>>>       }
>>>
>>>       return orig_pte;
>>>  }
>>>  EXPORT_SYMBOL_GPL(contpte_ptep_get);
>>>
>>> +static inline bool contpte_is_consistent(pte_t pte, unsigned long pfn,
>>> +                                     pgprot_t orig_prot)
>>> +{
>>> +     pgprot_t prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
>>> +
>>> +     return pte_valid_cont(pte) && pte_pfn(pte) == pfn &&
>>> +                     pgprot_val(prot) == pgprot_val(orig_prot);
>>> +}
>>> +
>>>  pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
>>>  {
>>>       /*
>>> @@ -202,7 +231,6 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
>>>       pgprot_t orig_prot;
>>>       unsigned long pfn;
>>>       pte_t orig_pte;
>>> -     pgprot_t prot;
>>>       pte_t *ptep;
>>>       pte_t pte;
>>>       int i;
>>> @@ -219,18 +247,44 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
>>>
>>>       for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
>>>               pte = __ptep_get(ptep);
>>> -             prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
>>>
>>> -             if (!pte_valid_cont(pte) ||
>>> -                pte_pfn(pte) != pfn ||
>>> -                pgprot_val(prot) != pgprot_val(orig_prot))
>>> +             if (!contpte_is_consistent(pte, pfn, orig_prot))
>>>                       goto retry;
>>>
>>> -             if (pte_dirty(pte))
>>> +             if (pte_dirty(pte)) {
>>>                       orig_pte = pte_mkdirty(orig_pte);
>>> +                     for (; i < CONT_PTES; i++, ptep++, pfn++) {
>>> +                             pte = __ptep_get(ptep);
>>> +
>>> +                             if (!contpte_is_consistent(pte, pfn, orig_prot))
>>> +                                     goto retry;
>>> +
>>> +                             if (pte_young(pte)) {
>>> +                                     orig_pte = pte_mkyoung(orig_pte);
>>> +                                     break;
>>> +                             }
>>> +                     }
>>> +                     break;
>>
>> I considered for a while whether it is safe for contpte_ptep_get_lockless() to
>> exit early having not seen every PTE in the contpte block and confirmed that
>> they are all consistent. I eventually concluded that it is, as long as all the
>> PTEs that it does check are consistent I believe this is fine.
> 
> So, it looks like my changes here will be okay.
> 
>>
>>> +             }
>>>
>>> -             if (pte_young(pte))
>>> +             if (pte_young(pte)) {
>>>                       orig_pte = pte_mkyoung(orig_pte);
>>> +                     i++;
>>> +                     ptep++;
>>> +                     pfn++;
>>> +                     for (; i < CONT_PTES; i++, ptep++, pfn++) {
>>> +                             pte = __ptep_get(ptep);
>>> +
>>> +                             if (!contpte_is_consistent(pte, pfn, orig_prot))
>>> +                                     goto retry;
>>> +
>>> +                             if (pte_dirty(pte)) {
>>> +                                     orig_pte = pte_mkdirty(orig_pte);
>>> +                                     break;
>>> +                             }
>>> +                     }
>>> +                     break;
>>> +             }
>>>       }
>>>
>>>       return orig_pte;
>>

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