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

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Message-ID: <CAGsJ_4wsW+urp6gbp+yDG40fZw9Bszny2iJVsV_AzzGqgQy+4g@mail.gmail.com>
Date: Fri, 11 Apr 2025 09:25:39 +1200
From: Barry Song <21cnbao@...il.com>
To: Xavier <xavier_qy@....com>
Cc: catalin.marinas@....com, will@...nel.org, akpm@...ux-foundation.org, 
	ryan.roberts@....com, ioworker0@...il.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant operations

On Mon, Apr 7, 2025 at 9:23 PM Xavier <xavier_qy@....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@....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