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

On Thu 11-03-21 15:33:20, Muchun Song wrote:

On Wed, Mar 10, 2021 at 11:41 PM Michal Hocko [off-list ref] wrote:

quoted

On Mon 08-03-21 18:28:07, Muchun Song wrote:

quoted

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.

quoted

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