Re: [mm/contpte v3 1/1] mm/contpte: Optimize loop to reduce redundant operations

David Laight
2025-04-16T10:31:43Z

On Tue, 15 Apr 2025 16:22:05 +0800
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.

Benchmarks?

As has been pointed out before CONT_PTES is small and IIRC dirty+young
is unusual.

>
> Signed-off-by: Xavier <xavier_qy@163.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.*/

... this macro modifies ...

But you can make it obvious my passing by reference.
The compiler will generate the same code.

David

> +#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; \
> + } \
> + }
> +
> 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;