Re: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of the compiler
From: Oscar Salvador <osalvador@suse.de>
Date: 2021-02-05 09:13:33
Also in:
linux-fsdevel, linux-mm, lkml
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.
Signed-off-by: Muchun Song <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
quoted hunk ↗ jump to hunk
--- 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