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Message-ID: <CAGsJ_4zKFiCEUnFjb3qThoM2FG4XCsmeU=JeTLDXjMU2KONu-Q@mail.gmail.com>
Date: Fri, 2 May 2025 09:19:09 +1200
From: Barry Song <21cnbao@...il.com>
To: Xavier <xavier_qy@....com>
Cc: Ryan Roberts <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: [mm/contpte v3 1/1] mm/contpte: Optimize loop to reduce redundant operations
On Fri, May 2, 2025 at 12:41 AM Xavier <xavier_qy@....com> wrote:
>
>
>
> Hi Barry,
>
>
> At 2025-05-01 07:17:36, "Barry Song" <21cnbao@...il.com> wrote:
> >On Tue, Apr 22, 2025 at 9:34 PM Xavier <xavier_qy@....com> wrote:
> >>
> >>
> >> Hi all,
> >>
> >>
> >> At 2025-04-16 20:54:47, "Ryan Roberts" <ryan.roberts@....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@....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.
>
>
> --
>
> Thanks,
> Xavier
Thanks
barry
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