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. 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
