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Sat, 18 Sep 2021 04:48:16 -0700 (PDT) MIME-Version: 1.0 References: <20210917034815.80264-1-songmuchun@bytedance.com> <20210917034815.80264-3-songmuchun@bytedance.com> In-Reply-To: From: Muchun Song Date: Sat, 18 Sep 2021 19:47:38 +0800 Message-ID: Subject: Re: [PATCH RESEND v2 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key To: Barry Song <21cnbao@gmail.com> Cc: Mike Kravetz , Andrew Morton , Oscar Salvador , Michal Hocko , Barry Song , David Hildenbrand , Chen Huang , "Bodeddula, Balasubramaniam" , Jonathan Corbet , Matthew Wilcox , Xiongchun duan , fam.zheng@bytedance.com, Muchun Song , Qi Zheng , linux-doc@vger.kernel.org, LKML , Linux-MM Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E430D5068A45 X-Stat-Signature: a9seim5bii7r19ohoqfd7akcfzs17kr5 Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=bytedance-com.20210112.gappssmtp.com header.s=20210112 header.b=47B5COgW; spf=pass (imf01.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.214.174 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-HE-Tag: 1631965698-634682 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Sep 18, 2021 at 7:15 PM Barry Song <21cnbao@gmail.com> wrote: > > On Sat, Sep 18, 2021 at 10:31 PM Muchun Song 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 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 > > > > --- > > > > 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 Alright, I can change hugetlb_free_vmemmap_enabled to an inline function. > > but in your case, CONFIG_HUGETLB_PAGE_FREE_VMEMMAP > is always true in your page_head_if_fake(). Why do we check it > again? That is CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON not CONFIG_HUGETLB_PAGE_FREE_VMEMMAP. Thanks