[PATCH] mm/sparse: Optimize section number calculations using bit shifts

[Zhen Ni]

Add SECTIONS_PER_ROOT_SHIFT = ilog2(SECTIONS_PER_ROOT) with correctness
guaranteed by BUILD_BUG_ON in sparse_init(). Convert SECTION_NR_TO_ROOT
to use right shift instead of division for better performance. Add
SECTION_NR_IN_ROOT() macro to improve code readability.

This improves code efficiency in hot paths where __nr_to_section() is
frequently called, such as sparse_init() and memory section management
operations.

Performance verification in sparse_init() on ARM (8GB RAM, 4 NUMA nodes):

    sparse_init()
    |
    +----> memblocks_present()
    |
    +----> section initialization (sparse_init_nid loop)

Time measurement points:

    [T1] sparse_init start
         |
         v
    [T2] memblocks_present() complete
         |
         v
    [T3] sparse_init_nid() loop complete / sparse_init end

Measurement values:
    memblocks_present_cycles = T2 - T1
    section_initialization_cycles = T3 - T2
    total_cycles = T3 - T1

Before (division):
[    0.000000] sparse_init: total 7538 cycles
[    0.000000]   memblocks_present: 4232 cycles
[    0.000000]   section initialization: 3261 cycles

After (bit shift):
[    0.000000] sparse_init: total 5641 cycles
[    0.000000]   memblocks_present: 3562 cycles
[    0.000000]   section initialization: 2057 cycles

Performance improvement:
  Total: (7538-5641)/7538 = 25.2% faster
  memblocks_present: (4232-3562)/4232 = 15.8% faster
  section initialization: (3261-2057)/3261 = 36.9% faster

Signed-off-by: Zhen Ni <zhen.ni@xxxxxxxxxxxx>
---
 include/linux/mmzone.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 9adb2ad21da5..5daf471f6823 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -2035,11 +2035,14 @@ struct mem_section {
 
 #ifdef CONFIG_SPARSEMEM_EXTREME
 #define SECTIONS_PER_ROOT       (PAGE_SIZE / sizeof (struct mem_section))
+#define SECTIONS_PER_ROOT_SHIFT ilog2(SECTIONS_PER_ROOT)
 #else
 #define SECTIONS_PER_ROOT	1
+#define SECTIONS_PER_ROOT_SHIFT 0
 #endif
 
-#define SECTION_NR_TO_ROOT(sec)	((sec) / SECTIONS_PER_ROOT)
+#define SECTION_NR_TO_ROOT(sec)	((sec) >> SECTIONS_PER_ROOT_SHIFT)
+#define SECTION_NR_IN_ROOT(sec)	((sec) & SECTION_ROOT_MASK)
 #define NR_SECTION_ROOTS	DIV_ROUND_UP(NR_MEM_SECTIONS, SECTIONS_PER_ROOT)
 #define SECTION_ROOT_MASK	(SECTIONS_PER_ROOT - 1)
 
@@ -2065,7 +2068,7 @@ static inline struct mem_section *__nr_to_section(unsigned long nr)
 	if (!mem_section || !mem_section[root])
 		return NULL;
 #endif
-	return &mem_section[root][nr & SECTION_ROOT_MASK];
+	return &mem_section[root][SECTION_NR_IN_ROOT(nr)];
 }
 extern size_t mem_section_usage_size(void);
 
-- 
2.20.1