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;
MIME-Version: 1.0
References: <20220830055949.12640-1-ryncsn@gmail.com> <20220830055949.12640-2-ryncsn@gmail.com>
 <Yw2x6T3xchjpzX7j@dhcp22.suse.cz>
In-Reply-To: <Yw2x6T3xchjpzX7j@dhcp22.suse.cz>
From:   Kairui Song <ryncsn@gmail.com>
Date:   Tue, 30 Aug 2022 15:06:57 +0800
Message-ID: <CAMgjq7A+Ju247rC7+fo9jcZpK8rkd0Ou6e=7qzgL4rV3JWBX5w@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm: memcontrol: remove mem_cgroup_kmem_disabled
To:     Michal Hocko <mhocko@suse.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
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

Michal Hocko <mhocko@suse.com> =E4=BA=8E2022=E5=B9=B48=E6=9C=8830=E6=97=A5=
=E5=91=A8=E4=BA=8C 14:45=E5=86=99=E9=81=93=EF=BC=9A
>
> 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=3Dnokmem is specified, otherwise returns false.
> >
> > memcg_kmem_enabled is a bit different, it returns true if
> > cgroup.memory=3Dnokmem 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 simplif=
ies
> > the behavior of memcg_kmem_enabled, making it simply the opposite of
> > mem_cgroup_kmem_disabled, always true if cgroup.memory=3Dnokmem 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.