linux-kernel - Re: [PATCH v8 12/12] mm/hugetlb: Optimize the code with the help of the compiler

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Message-ID: <375d6bad6bb37e3626f71bfabc20b384@suse.de>
Date:   Thu, 10 Dec 2020 11:25:02 +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,
        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 v8 12/12] mm/hugetlb: Optimize the code with the help of
 the compiler

On 2020-12-10 04:55, 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.

As I said earlier, I would squash this patch with patch#10 and
remove the !is_power_of_2 check in hugetlb_vmemmap_init and leave
only the check for the boot parameter.
That should be enough.

>  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));

Why? hugetlb_free_vmemmap_enabled can only become true
if the is_power_of_2 check succeeds in early_hugetlb_free_vmemmap_param.
The "is_power_of_2" check here can go.

> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index 0a1c0d33a316..5f5e90c81cd2 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -21,7 +21,7 @@ void free_huge_page_vmemmap(struct hstate *h, struct
> page *head);
>   */
>  static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate 
> *h)
>  {
> -	return h->nr_free_vmemmap_pages;
> +	return h->nr_free_vmemmap_pages && is_power_of_2(sizeof(struct 
> page));

If hugetlb_free_vmemmap_enabled is false, hugetlb_vmemmap_init() leaves
h->nr_free_vmemmap_pages unset to 0, so no need for the is_power_of_2 
check here.


-- 
Oscar Salvador
SUSE L3