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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/16/26 04:59, Zhen Ni wrote:
> 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@easystack.cn>
> ---
>  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)
>  

From a compiler POV, "/ SECTIONS_PER_ROOT" is exactly the same as >>
ilog2(SECTIONS_PER_ROOT) *as long as* the variable we are processing is
"unsigned long".

The compiler should be smart enough to figure out that out with
SECTIONS_PER_ROOT being known at compiletime.

Can you compare the generated code __nr_to_section() to see if and why the
compiler fails to optimize that?

-- 
Cheers,

David