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

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Message-ID: <eba7a8fa-0b85-4b30-ab3e-9c0a65b7dc80@arm.com>
Date: Wed, 16 Apr 2025 13:54:47 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Xavier <xavier_qy@....com>, dev.jain@....com, ioworker0@...il.com,
 21cnbao@...il.com
Cc: 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 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@....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