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

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Date:   Tue, 30 Aug 2022 09:12:48 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Kairui Song <ryncsn@...il.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

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.

-- 
Michal Hocko
SUSE Labs