Message-ID: <CAEmg6AUBf1wVjXSoqBseWffLbixUV7U-nY52ScKCeNXwrkBcqg@mail.gmail.com>
Date: Thu, 5 Jun 2025 13:54:38 +0800
From: Xavier Xia <xavier.qyxia@...il.com>
To: Ryan Roberts <ryan.roberts@....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

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.



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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