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

Xavier
2025-05-09T10:18:58Z

At 2025-05-09 10:09:21, "Barry Song" <21cnbao@gmail.com> wrote:
>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.
>

You're right. What's being checked here is the inconsistency. I will make the modification
in the next version. Thank you for your suggestion.

--

Thanks,
Xavier