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

[Mike Rapoport]

On Tue, Jun 16, 2026 at 03:29:38PM +0800, zhen.ni wrote:
> 在 2026/6/16 14:32, Mike Rapoport 写道:
> > On Tue, Jun 16, 2026 at 10:59:42AM +0800, Zhen Ni wrote:
> > > 
> > > 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.
 
Right, and explanation along these lines should be in the changelog.

> > > 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)];
> > 
> > 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.

This was not clear from reading the changelog. 
 
> Thanks,
> Zhen

-- 
Sincerely yours,
Mike.