linux-kernel - Re: [PATCH v18 9/9] mm: hugetlb: optimize the code with the help of the compiler

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Date:   Wed, 10 Mar 2021 16:41:23 +0100
From:   Michal Hocko <mhocko@...e.com>
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,
        osalvador@...e.de, song.bao.hua@...ilicon.com, david@...hat.com,
        naoya.horiguchi@....com, joao.m.martins@...cle.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Miaohe Lin <linmiaohe@...wei.com>,
        Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [PATCH v18 9/9] mm: hugetlb: optimize the code with the help of
 the compiler

On Mon 08-03-21 18:28:07, Muchun Song wrote:
> 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 am confused. Don't you check for this in early_hugetlb_free_vmemmap_param already?
Why do we need any runtime checks?

> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Tested-by: Chen Huang <chenhuang5@...wei.com>
> Tested-by: Bodeddula Balasubramaniam <bodeddub@...zon.com>
> ---
>  include/linux/hugetlb.h | 3 ++-
>  mm/hugetlb_vmemmap.c    | 7 +++++++
>  mm/hugetlb_vmemmap.h    | 6 ++++++
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index c70421e26189..333dd0479fc2 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -880,7 +880,8 @@ 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 33e42678abe3..1ba1ef45c48c 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -265,6 +265,13 @@ 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;
>  
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index cb2bef8f9e73..29aaaf7b741e 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -21,6 +21,12 @@ void hugetlb_vmemmap_init(struct hstate *h);
>   */
>  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;
>  }
>  #else
> -- 
> 2.11.0
> 

-- 
Michal Hocko
SUSE Labs