Re: [PATCH v4] arm64/mm: Optimize loop to reduce redundant operations of contpte_ptep_get

Barry Song
2025-05-09T02:11:55Z

On Thu, May 8, 2025 at 7:04 PM Xavier Xia <xavier_qy@163.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@163.com>
> ---
> arch/arm64/mm/contpte.c | 71 +++++++++++++++++++++++++++++++++++------
> 1 file changed, 62 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> index bcac4f55f9c1..e9882ec782fc 100644
> --- a/arch/arm64/mm/contpte.c
> +++ b/arch/arm64/mm/contpte.c
> @@ -169,17 +169,41 @@ 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);
>
> +#define CHECK_CONTPTE_CONSISTENCY(pte, pfn, prot, orig_prot) \
> + (!pte_valid_cont(pte) || pte_pfn(pte) != pfn || \
> + pgprot_val(prot) != pgprot_val(orig_prot))

maybe make it a static inline function to improve readability. Also,
the name appears to
be not good: CHECK_CONTPTE_CONSISTENCY is actually checking for inconsistency,
not consistency.

it might be:

static inline bool contpte_is_consistent(...)
{
return pte_valid_cont(pte) && pte_pfn(pte) == pfn &&
pgprot_val(prot) == pgprot_val(orig_prot);
}

or another better name.

> +
> pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
> {
> /*
> @@ -221,16 +245,45 @@ 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 (CHECK_CONTPTE_CONSISTENCY(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 (CHECK_CONTPTE_CONSISTENCY(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 (CHECK_CONTPTE_CONSISTENCY(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