Re: [External] Re: [PATCH v18 9/9] mm: hugetlb: optimize the code with the help of the compiler

[Michal Hocko]

On Thu 11-03-21 15:33:20, Muchun Song wrote:
> On Wed, Mar 10, 2021 at 11:41 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Mon 08-03-21 18:28:07, Muchun Song wrote:
> > > 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 am confused. Don't you check for this in early_hugetlb_free_vmemmap_param already?
> 
> Right.
> 
> > Why do we need any runtime checks?
> 
> If the size of the struct page is not power of 2, compiler can think
> is_hugetlb_free_vmemmap_enabled() always return false. So
> the code snippet of this user can be optimized away.
> 
> E.g.
> 
> if (is_hugetlb_free_vmemmap_enabled())
>         /* do something */
> 
> The compiler can drop "/* do something */" directly, because
> it knows is_hugetlb_free_vmemmap_enabled() always returns
> false.

OK, so this is a micro-optimization to generate a better code?
Is this measurable to warrant more code?
-- 
Michal Hocko
SUSE Labs