On Sat, Sep 18, 2021 at 7:15 PM Barry Song [off-list ref] wrote:

On Sat, Sep 18, 2021 at 10:31 PM Muchun Song [off-list ref] wrote:

quoted

On Sat, Sep 18, 2021 at 12:55 PM Barry Song [off-list ref] wrote:

quoted

On Sat, Sep 18, 2021 at 12:08 AM Muchun Song [off-list ref] wrote:

quoted

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 <redacted>
---
 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