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

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Date:   Thu, 11 Mar 2021 15:33:20 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oscar Salvador <osalvador@...e.de>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [External] Re: [PATCH v18 9/9] mm: hugetlb: optimize the code
 with the help of the compiler

On Wed, Mar 10, 2021 at 11:41 PM Michal Hocko <mhocko@...e.com> wrote:
>
> 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?

Right.

> Why do we need any runtime checks?

If the size of the struct page is not power of 2, compiler can think
is_hugetlb_free_vmemmap_enabled() always return false. So
the code snippet of this user can be optimized away.

E.g.

if (is_hugetlb_free_vmemmap_enabled())
        /* do something */

The compiler can drop "/* do something */" directly, because
it knows is_hugetlb_free_vmemmap_enabled() always returns
false.

Thanks.

>
> > 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