Re: [mm/contpte v3 1/1] mm/contpte: Optimize loop to reduce redundant operations

Barry Song
2025-05-01T21:35:17Z

On Fri, May 2, 2025 at 9:19 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Fri, May 2, 2025 at 12:41 AM Xavier <xavier_qy@163.com> wrote:
> >
> >
> >
> > Hi Barry,
> >
> >
> > At 2025-05-01 07:17:36, "Barry Song" <21cnbao@gmail.com> wrote:
> > >On Tue, Apr 22, 2025 at 9:34 PM Xavier <xavier_qy@163.com> wrote:
> > >>
> > >>
> > >> Hi all,
> > >>
> > >>
> > >> At 2025-04-16 20:54:47, "Ryan Roberts" <ryan.roberts@arm.com> wrote:
> > >> >On 15/04/2025 09:22, Xavier wrote:
> > >> >> This commit optimizes the contpte_ptep_get 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.
> > >> >>
> > >> >> Signed-off-by: Xavier <xavier_qy@163.com>
> > >> >> ---
> > >> >> arch/arm64/mm/contpte.c | 20 ++++++++++++++++++--
> > >> >> 1 file changed, 18 insertions(+), 2 deletions(-)
> > >> >>
> > >> >> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > >> >> index bcac4f55f9c1..0acfee604947 100644
> > >> >> --- a/arch/arm64/mm/contpte.c
> > >> >> +++ b/arch/arm64/mm/contpte.c
> > >> >> @@ -152,6 +152,16 @@ void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
> > >> >> }
> > >> >> EXPORT_SYMBOL_GPL(__contpte_try_unfold);
> > >> >>
> > >> >> +/* Note: in order to improve efficiency, using this macro will modify the
> > >> >> + * passed-in parameters.*/
> > >> >> +#define CHECK_CONTPTE_FLAG(start, ptep, orig_pte, flag) \
> > >> >> + for (; (start) < CONT_PTES; (start)++, (ptep)++) { \
> > >> >> + if (pte_##flag(__ptep_get(ptep))) { \
> > >> >> + orig_pte = pte_mk##flag(orig_pte); \
> > >> >> + break; \
> > >> >> + } \
> > >> >> + }
> > >> >
> > >> >I'm really not a fan of this macro, it just obfuscates what is going on. I'd
> > >> >personally prefer to see the 2 extra loops open coded below.
> > >> >
> > >> >Or even better, could you provide results comparing this 3 loop version to the
> > >> >simpler approach I suggested previously? If the performance is similar (which I
> > >> >expect it will be, especially given Barry's point that your test always ensures
> > >> >the first PTE is both young and dirty) then I'd prefer to go with the simpler code.
> > >> >
> > >>
> > >> Based on the discussions in the previous email, two modifications were adopted
> > >> and tested, and the results are as follows:
> > >>
> > >> Modification 1
> > >>
> > >> pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> > >> {
> > >> pte_t pte;
> > >> int i;
> > >>
> > >> ptep = contpte_align_down(ptep);
> > >>
> > >> for (i = 0; i < CONT_PTES; i++, ptep++) {
> > >> pte = __ptep_get(ptep);
> > >>
> > >> if (pte_dirty(pte)) {
> > >> orig_pte = pte_mkdirty(orig_pte);
> > >> if (pte_young(orig_pte))
> > >> break;
> > >> }
> > >>
> > >> if (pte_young(pte)) {
> > >> orig_pte = pte_mkyoung(orig_pte);
> > >> if (pte_dirty(orig_pte))
> > >> break;
> > >> }
> > >> }
> > >>
> > >> return orig_pte;
> > >> }
> > >>
> > >> Modification 2
> > >>
> > >> pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> > >> {
> > >> pte_t pte;
> > >> int i;
> > >>
> > >> ptep = contpte_align_down(ptep);
> > >>
> > >> for (i = 0; i < CONT_PTES; i++, ptep++) {
> > >> pte = __ptep_get(ptep);
> > >>
> > >> if (pte_dirty(pte)) {
> > >> orig_pte = pte_mkdirty(orig_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;
> > >> }
> > >>
> > >> 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| Modification 1| Modification 2|
> > >> |-------------------|---------------|----------------|----------------|
> > >> |instructions | 37912436160| 18303833386| 18731580031|
> > >> |test time | 4.2797| 2.2687| 2.2949|
> > >> |overhead of | | | |
> > >> |contpte_ptep_get() | 21.31%| 4.72%| 4.80%|
> > >>
> > >> |Scenario 2 | Original| Modification 1| Modification 2|
> > >> |-------------------|---------------|----------------|----------------|
> > >> |instructions | 36701270862| 38729716276| 36115790086|
> > >> |test time | 3.2335| 3.5732| 3.0874|
> > >> |Overhead of | | | |
> > >> |contpte_ptep_get() | 32.26%| 41.35%| 33.57%|
> > >>
> > >> |Scenario 3 | Original| Modification 1| Modification 2|
> > >> |-------------------|---------------|----------------|----------------|
> > >> |instructions | 36706279735| 38305241759| 36750881878|
> > >> |test time | 3.2008| 3.5389| 3.1249|
> > >> |Overhead of | | | |
> > >> |contpte_ptep_get() | 31.94%| 41.30%| 34.59%|
> > >>
> > >>
> > >> For Scenario 1, Modification 1 can achieve an instruction count benefit of
> > >> 51.72% and a time benefit of 46.99%. Modification 2 can achieve an instruction
> > >> benefit of 50.59% and a time benefit of 46.38%.
> > >>
> > >> For Scenarios 2, Modification 2 can achieve an instruction count benefit of
> > >> 1.6% and a time benefit of 4.5%. while Modification 1 significantly increases
> > >> the instructions and time due to additional conditional checks.
> > >>
> > >> For Scenario 3, since all the PTEs have neither the young nor the dirty flag,
> > >> the branches taken by Modification 1 and Modification 2 should be the same as
> > >> those of the original code. In fact, the test results of Modification 2 seem
> > >> to be closer to those of the original code. I don't know why there is a
> > >> performance regression in Modification 1.
> > >>
> > >> Therefore, I believe modifying the code according to Modification 2 can bring
> > >> maximum benefits. Everyone can discuss whether this approach is acceptable,
> > >> and if so, I will send Patch V4 to proceed with submitting this modification.
> > >>
> > >
> > >modification 2 is not correct. if pte0~pte14 are all young and no one
> > >is dirty, we are
> > >having lots of useless "for (; i < CONT_PTES; i++, ptep++)"
> > >
> > > 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;
> > > }
> > >
> >
> > I didn't understand which part you referred to when you said there were a lot of
> > useless loops. According to the scenario you mentioned, "if pte0~pte14 are all
> > young and no one is dirty", Modification 2 will enter the following branch when
> > judging pte0:
> >
> > if (pte_young(pte)) {
> > orig_pte = pte_mkyoung(orig_pte);
> > // The dirty status of pte0 has already been checked, skip it.
> > i++;
> > ptep++;
> > // Then we only need to check whether pte1~pte15 are dirty.
> > for (; i < CONT_PTES; i++, ptep++) {
> > pte = __ptep_get(ptep);
> > if (pte_dirty(pte)) {
> > // Exit as soon as a dirty entry is found.
> > orig_pte = pte_mkdirty(orig_pte);
> > break;
> > }
> > }
> > // Exit directly here without going through the outer loop again.
> > break;
> > }
> >
> > In this scenario, the total number of judgments in Modification 2 is nearly half less
> > than that of the original code. I should have understood it correctly, right?
>
> You're right—I missed the part where you're also taking a break, even though
> no one is dirty. Based on your data, modification 2 seems to be good.
>
> But I don't quite understand why you are doing
> i++;
> ptep++;
> before for (; i < CONT_PTES; i++, ptep++).
>
> it seems to be wrong. if i==15, you will get i=16. you are skipping
> the pte_dirty
> check for i==15. it is also true for any value between 0 and 15. My
> point is that
> you should drop it and re-test.

Sorry, I missed that you already checked pte_dirty(pte) before calling
pte_young(). So please ignore my comment. The code's a bit hard to
follow now. :-)

>
> >
> >
> > --
> >
> > Thanks,
> > Xavier
>

Thanks
barry