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

[Michal Hocko <mhocko@suse.com>]

On Thu 11-03-21 17:08:34, Muchun Song wrote:
> On Thu, Mar 11, 2021 at 4:55 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 11-03-21 15:33:20, Muchun Song wrote:
> > > On Wed, Mar 10, 2021 at 11:41 PM Michal Hocko <mhocko@suse.com> 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.

struct page which is not power of 2 is not a common case. Are you sure
it makes sense to micro optimize for an outliar. If you really want to
microptimize then do that for a common case - the feature being
disabled - via static key.
-- 
Michal Hocko
SUSE Labs