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

Barry Song
2025-04-10T21:27:56Z

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.

> }
>
> return orig_pte;
> --
> 2.34.1
>

Thanks
Barry