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

Ryan Roberts
2025-04-14T08:08:18Z

On 12/04/2025 06:05, Lance Yang 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.
>>
>> 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.
>
> 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 ;)

I agree with Barry; we need clear performance improvement numbers to consider
this type of optimization. I doubt there will be measurable improvement for 4K
and 64K base pages (because the the number of PTEs in a contpte block are 16 and
32 respectively). But 16K base pages may benefit given there are 128 PTEs in a
contpte block for that case.

Also FWIW, I'm struggling to understand CHECK_CONTPTE_FLAG(); Perhaps something
like this would suffice?

----8<----
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index 55107d27d3f8..7787b116b339 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -169,11 +169,17 @@ 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);
+ if (pte_young(orig_pte))
+ break;
+ }

- if (pte_young(pte))
+ if (pte_young(pte)) {
orig_pte = pte_mkyoung(orig_pte);
+ if (pte_dirty(orig_pte))
+ break;
+ }
}

return orig_pte;
----8<----

Thanks,
Ryan

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