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

Ryan Roberts
2025-04-16T12:58:13Z

On 15/04/2025 09:22, Xavier 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 | 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.*/
> +#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; \
> + } \
> + }

I'm really not a fan of this macro, it just obfuscates what is going on. I'd
personally prefer to see the 2 extra loops open coded below.

Or even better, could you provide results comparing this 3 loop version to the
simpler approach I suggested previously? If the performance is similar (which I
expect it will be, especially given Barry's point that your test always ensures
the first PTE is both young and dirty) then I'd prefer to go with the simpler code.

> +
> 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;

If we decide this is all worth the trouble, then I think we can (and *should*,
in order to be consistent) pull a similar stunt in contpte_ptep_get_lockless().

Thanks,
Ryan