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 20:14:18 +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 7:39 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> 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.

Yeah, you are right. I just want the compiler to do optimization.

>
> >  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.

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.

Thanks.

>
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun