On Tue, 10 Nov 2020 11:57:53 -0800 Roman Gushchin <guro@xxxxxx> wrote: > In general it's unknown in advance if a slab page will contain > accounted objects or not. In order to avoid memory waste, an > obj_cgroup vector is allocated dynamically when a need to account > of a new object arises. Such approach is memory efficient, but > requires an expensive cmpxchg() to set up the memcg/objcgs pointer, > because an allocation can race with a different allocation on another > cpu. > > But in some common cases it's known for sure that a slab page will > contain accounted objects: if the page belongs to a slab cache with a > SLAB_ACCOUNT flag set. It includes such popular objects like > vm_area_struct, anon_vma, task_struct, etc. > > In such cases we can pre-allocate the objcgs vector and simple assign > it to the page without any atomic operations, because at this early > stage the page is not visible to anyone else. Was there any measurable performance change from this?
