Re: [External] Re: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of the compiler

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Feb 5, 2021 at 5:09 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Feb 04, 2021 at 11:50:43AM +0800, 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.
>
> "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 think the above is more clear, but just a suggestion.

OK. I will use it. Thanks for your suggestions.

>
>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>

Thanks.

>
> > ---
> >  include/linux/hugetlb.h |  3 ++-
> >  mm/hugetlb_vmemmap.c    | 13 +++++++++++++
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 822ab2f5542a..7bfb06e16298 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -878,7 +878,8 @@ 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 8efad9978821..068d0e0cebc8 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -211,6 +211,12 @@ early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
> >   */
> >  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;
> >  }
> >
> > @@ -280,6 +286,13 @@ 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;
> >
> > --
> > 2.11.0
> >
> >
>
> --
> Oscar Salvador
> SUSE L3