On Wed 07-08-19 08:31:09, Wei Yang wrote: > On Tue, Aug 06, 2019 at 11:29:52AM +0200, Vlastimil Babka wrote: > >On 8/6/19 10:11 AM, Wei Yang wrote: > >> When addr is out of the range of the whole rb_tree, pprev will points to > >> the biggest node. find_vma_prev gets is by going through the right most > > > >s/biggest/last/ ? or right-most? > > > >> node of the tree. > >> > >> Since only the last node is the one it is looking for, it is not > >> necessary to assign pprev to those middle stage nodes. By assigning > >> pprev to the last node directly, it tries to improve the function > >> locality a little. > > > >In the end, it will always write to the cacheline of pprev. The caller has most > >likely have it on stack, so it's already hot, and there's no other CPU stealing > >it. So I don't understand where the improved locality comes from. The compiler > >can also optimize the patched code so the assembly is identical to the previous > >code, or vice versa. Did you check for differences? > > Vlastimil > > Thanks for your comment. > > I believe you get a point. I may not use the word locality. This patch tries > to reduce some unnecessary assignment of pprev. > > Original code would assign the value on each node during iteration, this is > what I want to reduce. Is there any measurable difference (on micro benchmarks or regular workloads)? -- Michal Hocko SUSE Labs
