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
