Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 30 Aug 2022 09:12:48 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Kairui Song <ryncsn@gmail.com>
Cc:     cgroups@vger.kernel.org, linux-mm@kvack.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm: memcontrol: remove mem_cgroup_kmem_disabled
Message-ID: <Yw24cFGTtOZF7Cgw@dhcp22.suse.cz>
References: <20220830055949.12640-1-ryncsn@gmail.com>
 <20220830055949.12640-2-ryncsn@gmail.com>
 <Yw2x6T3xchjpzX7j@dhcp22.suse.cz>
 <CAMgjq7A+Ju247rC7+fo9jcZpK8rkd0Ou6e=7qzgL4rV3JWBX5w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAMgjq7A+Ju247rC7+fo9jcZpK8rkd0Ou6e=7qzgL4rV3JWBX5w@mail.gmail.com>
Precedence: bulk

On Tue 30-08-22 15:06:57, Kairui Song wrote:
> 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).

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