linux-kernel - Re: [PATCH 1/2] mm: memcontrol: remove mem_cgroup_kmem

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Date:   Tue, 30 Aug 2022 15:45:01 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     cgroups@...r.kernel.org, linux-mm@...ck.org,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: memcontrol: remove mem_cgroup_kmem_disabled

Michal Hocko <mhocko@...e.com> 于2022年8月30日周二 15:12写道：
>
> On Tue 30-08-22 15:06:57, Kairui Song wrote:
> > Michal Hocko <mhocko@...e.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@...cent.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).
>
> Yeah, but do they enable the memory controller as well? Unless I am
> missing something this will require at least one memcg enabled cgroup to
> be created.

Systemd enable memory controller by default since ver 238 from 2018,
but I'm not sure about the others.
Now I think I was wrong about the assumption, will be sure to do more
homework next time.

And thanks for the review!