Re: [PATCH RESEND v2 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Barry Song <21cnbao@gmail.com>]

On Sat, Sep 18, 2021 at 10:31 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Sat, Sep 18, 2021 at 12:55 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > On Sat, Sep 18, 2021 at 12:08 AM Muchun Song <songmuchun@bytedance.com> wrote:
> > >
> > > The page_head_if_fake() is used throughout memory management and the
> > > conditional check requires checking a global variable, although the
> > > overhead of this check may be small, it increases when the memory
> > > cache comes under pressure. Also, the global variable will not be
> > > modified after system boot, so it is very appropriate to use static
> > > key machanism.
> > >
> > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > ---
> > >  include/linux/hugetlb.h    |  6 +++++-
> > >  include/linux/page-flags.h |  6 ++++--
> > >  mm/hugetlb_vmemmap.c       | 10 +++++-----
> > >  3 files changed, 14 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > > index f7ca1a3870ea..ee3ddf3d12cf 100644
> > > --- a/include/linux/hugetlb.h
> > > +++ b/include/linux/hugetlb.h
> > > @@ -1057,7 +1057,11 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
> > >  #endif /* CONFIG_HUGETLB_PAGE */
> > >
> > >  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > > -extern bool hugetlb_free_vmemmap_enabled;
> > > +DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> > > +                        hugetlb_free_vmemmap_enabled_key);
> > > +#define hugetlb_free_vmemmap_enabled                                    \
> > > +       static_key_enabled(&hugetlb_free_vmemmap_enabled_key)
> > > +
> > >  #else
> > >  #define hugetlb_free_vmemmap_enabled   false
> > >  #endif
> > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > > index 7b1a918ebd43..d68d2cf30d76 100644
> > > --- a/include/linux/page-flags.h
> > > +++ b/include/linux/page-flags.h
> > > @@ -185,7 +185,8 @@ enum pageflags {
> > >  #ifndef __GENERATING_BOUNDS_H
> > >
> > >  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > > -extern bool hugetlb_free_vmemmap_enabled;
> > > +DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> > > +                        hugetlb_free_vmemmap_enabled_key);
> > >
> > >  /*
> > >   * If the feature of freeing some vmemmap pages associated with each HugeTLB
> > > @@ -204,7 +205,8 @@ extern bool hugetlb_free_vmemmap_enabled;
> > >   */
> > >  static __always_inline const struct page *page_head_if_fake(const struct page *page)
> > >  {
> > > -       if (!hugetlb_free_vmemmap_enabled)
> > > +       if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> > > +                                &hugetlb_free_vmemmap_enabled_key))
> >
> > A question bothering me is that we still have hugetlb_free_vmemmap_enabled
> > defined as static_key_enabled(&hugetlb_free_vmemmap_enabled_key).
> > but here you are using static_branch_maybe() with the CONFIG and refer the key
> > directly.
> > Do we only need one of them? Or something is wrong?
> >
>
> Yeah, we only need one. But my consideration is that we
> use static_branch_maybe() for performance sensitive places.
> So I do not change hugetlb_free_vmemmap_enabled
> to static_branch_maybe(), this can reduce some codes
> that need to be updated when the static key is enabled.
> Actually, the user of hugetlb_free_vmemmap_enabled
> is not performance sensitive.

not quite sure if an unified inline API will be better, e.g.

#ifdef CONFIG_SCHED_SMT
extern struct static_key_false sched_smt_present;

static __always_inline bool sched_smt_active(void)
{
        return static_branch_likely(&sched_smt_present);
}
#else
static inline bool sched_smt_active(void) { return false; }
#endif

but in your case, CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
is always true in your page_head_if_fake(). Why do we check it
again?

>
> Thanks.

Thanks
barry