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

Dev Jain
2025-04-14T09:18:56Z

On 14/04/25 1:36 pm, Ryan Roberts wrote:
> 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?

The idea of CHECK_CONTPTE_FLAG() is to check only for the other flag
until it's found; the diff below will still keep checking for both until
both are found.

>
> ----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
>>>>
>>>
>