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

["zhen.ni" <zhen.ni@easystack.cn>]

在 2026/6/16 14:32, Mike Rapoport 写道:
> Hi,
> 
> On Tue, Jun 16, 2026 at 10:59:42AM +0800, 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
> 
> This is a nice improvement, but it's not the hot path. I believe you can
> derive improvement to __nr_to_section() from these measurements.

sparse_init() is not a hot path, but it invokes __nr_to_section() in a
tight loop, making it a good measurement point to demonstrate the
performance improvement.

>   
>> 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)
>>   
>> @@ -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)];
> 
> The explicit masking is clearer IMO.
> 
>>   }
>>   extern size_t mem_section_usage_size(void);
> 
> Hmm, I don't see BUILD_BUG_ON() you mention in the changelog.
>   
>> -- 
>> 2.20.1
>>
> 

Regarding the BUILD_BUG_ON, it is in sparse_init() at line 419:

void __init sparse_init(void)
{
     ...
     /* see include/linux/mmzone.h 'struct mem_section' definition */
     BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
     ...
}

This guarantees that sizeof(struct mem_section) is a power of 2, and 
since SECTIONS_PER_ROOT = PAGE_SIZE / sizeof(struct mem_section) and 
PAGE_SIZE is always a power of 2, SECTIONS_PER_ROOT is guaranteed to be 
a power of 2 as well, validating the use of bit shifts.


Thanks,
Zhen