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

[Muchun Song]

On Thu, Mar 11, 2021 at 4:55 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> 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?

Right.

> Is this measurable to warrant more code?

I have disassembled the code to confirm this behavior.
I know this is not the hot path. But it actually can decrease
the code size.

Thanks.

> --
> Michal Hocko
> SUSE Labs