linux-kernel - Re: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of the compiler

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Date:   Fri, 5 Feb 2021 10:09:28 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        mhocko@...e.com, song.bao.hua@...ilicon.com, david@...hat.com,
        naoya.horiguchi@....com, duanxiongchun@...edance.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of
 the compiler

On Thu, Feb 04, 2021 at 11:50:43AM +0800, Muchun Song wrote:
> We cannot optimize if a "struct page" crosses page boundaries. If
> it is true, we can optimize the code with the help of a compiler.
> When free_vmemmap_pages_per_hpage() returns zero, most functions are
> optimized by the compiler.

"When the "struct page size" crosses page boundaries we cannot
 make use of this feature.
 Let free_vmemmap_pages_per_hpage() return zero if that is the case,
 most of the functions can be optimized away."

I think the above is more clear, but just a suggestion.
 

> Signed-off-by: Muchun Song <songmuchun@...edance.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

> ---
>  include/linux/hugetlb.h |  3 ++-
>  mm/hugetlb_vmemmap.c    | 13 +++++++++++++
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 822ab2f5542a..7bfb06e16298 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -878,7 +878,8 @@ extern bool hugetlb_free_vmemmap_enabled;
>  
>  static inline bool is_hugetlb_free_vmemmap_enabled(void)
>  {
> -	return hugetlb_free_vmemmap_enabled;
> +	return hugetlb_free_vmemmap_enabled &&
> +	       is_power_of_2(sizeof(struct page));
>  }
>  #else
>  static inline bool is_hugetlb_free_vmemmap_enabled(void)
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 8efad9978821..068d0e0cebc8 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -211,6 +211,12 @@ early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
>   */
>  static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
>  {
> +	/*
> +	 * This check aims to let the compiler help us optimize the code as
> +	 * much as possible.
> +	 */
> +	if (!is_power_of_2(sizeof(struct page)))
> +		return 0;
>  	return h->nr_free_vmemmap_pages;
>  }
>  
> @@ -280,6 +286,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>  	BUILD_BUG_ON(NR_USED_SUBPAGE >=
>  		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
>  
> +	/*
> +	 * The compiler can help us to optimize this function to null
> +	 * when the size of the struct page is not power of 2.
> +	 */
> +	if (!is_power_of_2(sizeof(struct page)))
> +		return;
> +
>  	if (!hugetlb_free_vmemmap_enabled)
>  		return;
>  
> -- 
> 2.11.0
> 
> 

-- 
Oscar Salvador
SUSE L3