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Message-ID: <CAGsJ_4wq0HD=Q-URO766zz=M8yyUxauhRoF9CTDkAgE5Favg-A@mail.gmail.com>
Date: Sat, 10 May 2025 15:20:48 +0800
From: Barry Song <21cnbao@...il.com>
To: Xavier Xia <xavier_qy@....com>
Cc: ryan.roberts@....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
Subject: Re: [PATCH v5] arm64/mm: Optimize loop to reduce redundant operations
of contpte_ptep_get
On Fri, May 9, 2025 at 8:29 PM Xavier Xia <xavier_qy@....com> 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:
>
> #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);
> }
> }
> 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.
>
> 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.
>
> Signed-off-by: Xavier Xia <xavier_qy@....com>
You can probably consolidate the logic below into contpte_is_consistent(), which
would allow you to
1. Eliminate the repeated pattern: prot =
pte_pgprot(pte_mkold(pte_mkclean(pte)))
2. Remove the argument "pgprot_t prot" for contpte_is_consistent().
+ prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
+
+ if (!contpte_is_consistent(pte, pfn,
prot, orig_prot))
+ goto retry;
If performance takes precedence over code simplicity, and you're planning to
send a new version with the above refinements, feel free to include:
Reviewed-by: Barry Song <baohua@...nel.org>
> ---
> 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, 65 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> index bcac4f55f9c1..1037450b68e9 100644
> --- a/arch/arm64/mm/contpte.c
> +++ b/arch/arm64/mm/contpte.c
> @@ -169,17 +169,44 @@ 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 prot, pgprot_t orig_prot)
> +{
> + 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)
> {
> /*
> @@ -221,16 +248,45 @@ pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> 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, prot, orig_prot))
> goto retry;
>
> - if (pte_dirty(pte))
> + if (pte_dirty(pte)) {
> orig_pte = pte_mkdirty(orig_pte);
> -
> - if (pte_young(pte))
> + for (; i < CONT_PTES; i++, ptep++, pfn++) {
> + pte = __ptep_get(ptep);
> + prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
> +
> + if (!contpte_is_consistent(pte, pfn, prot, orig_prot))
> + goto retry;
> +
> + if (pte_young(pte)) {
> + orig_pte = pte_mkyoung(orig_pte);
> + break;
> + }
> + }
> + break;
> + }
> +
> + if (pte_young(pte)) {
> orig_pte = pte_mkyoung(orig_pte);
> + i++;
> + ptep++;
> + pfn++;
> + for (; i < CONT_PTES; i++, ptep++, pfn++) {
> + pte = __ptep_get(ptep);
> + prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
> +
> + if (!contpte_is_consistent(pte, pfn, prot, orig_prot))
> + goto retry;
> +
> + if (pte_dirty(pte)) {
> + orig_pte = pte_mkdirty(orig_pte);
> + break;
> + }
> + }
> + break;
> + }
> }
>
> return orig_pte;
> --
> 2.34.1
>
Thanks
Barry
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