[PATCH v12 13/13] mm/hugetlb: Optimize the code with the help of the compiler

Muchun Song
2021-01-06T14:23:16Z

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.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
include/linux/hugetlb.h | 3 ++-
mm/hugetlb_vmemmap.c | 7 +++++++
mm/hugetlb_vmemmap.h | 5 +++--
3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 05fd2db09b78..b685bc4d79d5 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -792,7 +792,8 @@ 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 6b8f7bb2273e..5ea12c7507a6 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -250,6 +250,13 @@ 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;

diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 8fd9ae113dbd..e8de41295d4d 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -17,11 +17,12 @@ void hugetlb_vmemmap_init(struct hstate *h);

/*
* How many vmemmap pages associated with a HugeTLB page that can be freed
- * to the buddy allocator.
+ * to the buddy allocator. The checking of the is_power_of_2() aims to let
+ * the compiler help us optimize the code as much as possible.
*/
static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
{
- return h->nr_free_vmemmap_pages;
+ return is_power_of_2(sizeof(struct page)) ? h->nr_free_vmemmap_pages : 0;
}
#else
static inline void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
--
2.11.0