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 5:39 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 11-03-21 17:08:34, Muchun Song wrote:
> > 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.
>
> struct page which is not power of 2 is not a common case.

I know this is not a common case. But the check of
is_power_of_2(sizeof(struct page)) does not bring extra
runtime overhead. It just tells the compiler to optimize code
as much as possible.

> 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.

We cannot optimize the code size (vmlinux) even if we use a static
key when the size is not power of 2.

Sorry. I am confused why you disagree with this change.
It does not bring any disadvantages.

> --
> Michal Hocko
> SUSE Labs