From 093ae7a033cfde536db997e7dc4e829ce65fb38a Mon Sep 17 00:00:00 2001 From: Xavier Xia Date: Tue, 24 Jun 2025 23:25:49 +0800 Subject: [PATCH] arm64/mm: Optimize loop to reduce redundant operations of contpte_ptep_get 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: #include #include #include #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); } int main(void) { test_contpte_perf(); return 0; } 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. Ryan 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 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: Ryan Roberts Tested-by: Ryan Roberts Reviewed-by: Barry Song Signed-off-by: Xavier Xia Link: https://lore.kernel.org/r/20250624152549.2647828-1-xavier.qyxia@gmail.com Signed-off-by: Catalin Marinas --- 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 bcac4f55f9c14..71efe7dff0ad7 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 @@ retry: 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; + } - 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; -- 2.51.0