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

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Date:   Thu, 4 Feb 2021 14:33:42 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     <duanxiongchun@...edance.com>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-fsdevel@...r.kernel.org>, <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>, <osalvador@...e.de>, <mhocko@...e.com>,
        <song.bao.hua@...ilicon.com>, <david@...hat.com>,
        <naoya.horiguchi@....com>
Subject: Re: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of
 the compiler

On 2021/2/4 11:50, 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.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

I like it. Thanks.
Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>

> ---
>  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;
>  
>