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

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Date:   Thu, 10 Dec 2020 21:29:51 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>, 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>, 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>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.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>
Subject: Re: [External] Re: [PATCH v8 12/12] mm/hugetlb: Optimize the code
 with the help of the compiler

On Thu, Dec 10, 2020 at 9:16 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Thu, Dec 10, 2020 at 08:14:18PM +0800, Muchun Song wrote:
> > Yeah, you are right. But if we do this check can make the code simple.
> >
> > For example, here is a code snippet.
> >
> > void func(void)
> > {
> >         if (free_vmemmap_pages_per_hpage())
> >                 return;
> >         /* Do something */
> > }
> >
> > With this patch, the func will be optimized to null when is_power_of_2
> > returns false.
> >
> > void func(void)
> > {
> > }
> >
> > Without this patch, the compiler cannot do this optimization.
>
> Ok, I misread the changelog.
>
> So, then is_hugetlb_free_vmemmap_enabled, free_huge_page_vmemmap,
> free_vmemmap_pages_per_hpage and hugetlb_vmemmap_init are optimized
> out, right?

Yes, that's right. I have disassembled to make sure of this. Thanks.

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun