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

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Message-ID: <20250416095706.47a8afa2@pumpkin>
Date: Wed, 16 Apr 2025 09:57:06 +0100
From: David Laight <david.laight.linux@...il.com>
To: Xavier <xavier_qy@....com>
Cc: ryan.roberts@....com, dev.jain@....com, ioworker0@...il.com,
 21cnbao@...il.com, akpm@...ux-foundation.org, catalin.marinas@....com,
 david@...hat.com, gshan@...hat.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, will@...nel.org, willy@...radead.org,
 ziy@...dia.com
Subject: Re: [mm/contpte v3 1/1] mm/contpte: Optimize loop to reduce
 redundant operations

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

Benchmarks?

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

> 
> Signed-off-by: Xavier <xavier_qy@....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;