Re: [PATCH 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab caches with SLAB_ACCOUNT

[Andrew Morton]

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?