Re: [PATCH 1/2] mm: memcontrol: remove mem_cgroup_kmem_disabled

[Kairui Song <ryncsn@gmail.com>]

Michal Hocko <mhocko@suse.com> 于2022年8月30日周二 14:45写道：
>
> On Tue 30-08-22 13:59:48, Kairui Song wrote:
> > From: Kairui Song <kasong@tencent.com>
> >
> > There are currently two helpers for checking if cgroup kmem
> > accounting is enabled:
> >
> > - mem_cgroup_kmem_disabled
> > - memcg_kmem_enabled
>
> Yes, this is a bit confusing indeed!
>
> > mem_cgroup_kmem_disabled is a simple helper that returns true if
> > cgroup.memory=nokmem is specified, otherwise returns false.
> >
> > memcg_kmem_enabled is a bit different, it returns true if
> > cgroup.memory=nokmem is not specified and there is at least one
> > non-root cgroup ever created. And once there is any non-root memcg
> > created, it won't go back to return false again.
> >
> > This may help improve performance for some corner use cases where
> > the user enables memory cgroup and kmem accounting globally but never
> > create any cgroup.
> >
> > Considering that corner case is rare, especially nowadays cgroup is
> > widely used as a standard way to organize services.
>
> Is it really that rare? Most configurations would use a default setup, so
> both MEMCG enabled and without nokmem on cmd line yet the memory
> controller is not enabled in their setups.

Actually I don't have too much confidence saying that as well... but
AFAIK, almost all distros will create a few sub cgroup on boot by the
init (eg. openrc, finit, systemd).
Maybe it's not that rare indeed.

>
> > And the "once
> > enabled never disable" behavior is kind of strange. This commit simplifies
> > the behavior of memcg_kmem_enabled, making it simply the opposite of
> > mem_cgroup_kmem_disabled, always true if cgroup.memory=nokmem is
> > not specified. So mem_cgroup_kmem_disabled can be dropped.
> >
> > This simplifies the code, and besides, memcg_kmem_enabled makes use
> > of static key so it has a lower overhead.
>
> I agree that this is slightly confusing and undocumented. The first step
> would be finding out why we need both outside of the memcg proper.
>
> E.g. it doesn't make much sense to me that count_objcg_event uses the
> command line variant when it should be using the dynamic (and more
> optimized no branch) variant.
>
> On the other hand pcpu_alloc_chunk seems to be different because it can
> be called before the controller is enabled but maybe we do not need to
> waste memory before that? Similarly new_kmalloc_cache. I suspect these
> are mostly to simplify the code and reduce special casing.

Yes, that's very insightful, let me tidy up the code and logic behind
and send a V2 later.