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

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Message-ID: <CAMZfGtW5uHYiA_1an3W-jEmemsoN3Org7JwieeE2V271wh9X-A@mail.gmail.com>
Date:   Thu, 11 Mar 2021 17:08:34 +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 Thu, Mar 11, 2021 at 4:55 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Thu 11-03-21 15:33:20, Muchun Song wrote:
> > 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.
>
> OK, so this is a micro-optimization to generate a better code?

Right.

> Is this measurable to warrant more code?

I have disassembled the code to confirm this behavior.
I know this is not the hot path. But it actually can decrease
the code size.

Thanks.

> --
> Michal Hocko
> SUSE Labs