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

[Dev Jain <dev.jain@arm.com>]

+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.

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.

> 
>>          }
>>
>>          return orig_pte;
>> --
>> 2.34.1
>>
> 
> Thanks
> Barry
>