Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant operations

Xavier
2025-04-12T05:35:05Z

Hi,

At 2025-04-12 13:05:13, "Lance Yang" <ioworker0@gmail.com> wrote:
>On Sat, Apr 12, 2025 at 1:30 AM Dev Jain <dev.jain@arm.com> wrote:
>>
>> +others
>>
>> On 11/04/25 2:55 am, Barry Song wrote:
>> > On Mon, Apr 7, 2025 at 9:23 PM Xavier <xavier_qy@163.com> 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 | 13 +++++++++++--
>> >> 1 file changed, 11 insertions(+), 2 deletions(-)
>> >>
>> >> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>> >> index bcac4f55f9c1..ca15d8f52d14 100644
>> >> --- a/arch/arm64/mm/contpte.c
>> >> +++ b/arch/arm64/mm/contpte.c
>> >> @@ -163,17 +163,26 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
>> >>
>> >> pte_t pte;
>> >> int i;
>> >> + bool dirty = false;
>> >> + bool young = false;
>> >>
>> >> ptep = contpte_align_down(ptep);
>> >>
>> >> for (i = 0; i < CONT_PTES; i++, ptep++) {
>> >> pte = __ptep_get(ptep);
>> >>
>> >> - if (pte_dirty(pte))
>> >> + if (!dirty && pte_dirty(pte)) {
>> >> + dirty = true;
>> >> orig_pte = pte_mkdirty(orig_pte);
>> >> + }
>> >>
>> >> - if (pte_young(pte))
>> >> + if (!young && pte_young(pte)) {
>> >> + young = true;
>> >> orig_pte = pte_mkyoung(orig_pte);
>> >> + }
>> >> +
>> >> + if (dirty && young)
>> >> + break;
>> >
>> > This kind of optimization is always tricky. Dev previously tried a similar
>> > approach to reduce the loop count, but it ended up causing performance
>> > degradation:
>> > https://lore.kernel.org/linux-mm/20240913091902.1160520-1-dev.jain@arm.com/
>> >
>> > So we may need actual data to validate this idea.

You're absolutely right. Optimization must be proven by test data. I will
conduct some tests and validations next.

>>
>> The original v2 patch does not work, I changed it to the following:
>>
>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>> index bcac4f55f9c1..db0ad38601db 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);
>>
>> +#define CHECK_CONTPTE_FLAG(start, ptep, orig_pte, flag) \
>> + int _start; \
>> + pte_t *_ptep = ptep; \
>> + for (_start = start; _start < CONT_PTES; _start++, ptep++) { \
>> + if (pte_##flag(__ptep_get(_ptep))) { \
>> + orig_pte = pte_mk##flag(orig_pte); \
>> + break; \
>> + } \
>> + }
>> +
>> pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
>> {
>> /*
>> @@ -169,11 +179,17 @@ 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);
>> + CHECK_CONTPTE_FLAG(i, ptep, orig_pte, young);
>> + break;
>> + }
>>
>> - if (pte_young(pte))
>> + if (pte_young(pte)) {
>> orig_pte = pte_mkyoung(orig_pte);
>> + CHECK_CONTPTE_FLAG(i, ptep, orig_pte, dirty);
>> + break;
>> + }
>> }
>>
>> return orig_pte;
>>
>> Some rudimentary testing with micromm reveals that this may be
>> *slightly* faster. I cannot say for sure yet.

Thank you for your suggestions on the modification. I will write some
test cases for verification.

>
>Yep, this change works as expected, IIUC.
>
>However, I'm still wondering if the added complexity is worth it for
>such a slight/negligible performance gain. That said, if we have
>solid numbers/data to back it up, all doubts would disappear ;)
>

Indeed, unless there is a noticeable improvement in efficiency, increasing
complexity may not be worth the cost. I will summarize some test data
to determine whether this optimization is worthwhile.

------
Thanks,
Xavier