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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id k7-20020a056830168700b006705cddb73fsm1205278otr.31.2022.12.06.17.58.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Dec 2022 17:58:16 -0800 (PST) Date: Tue, 6 Dec 2022 17:58:14 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: Johannes Weiner cc: Andrew Morton , Linus Torvalds , Hugh Dickins , Shakeel Butt , Michal Hocko , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] mm: memcontrol: deprecate charge moving In-Reply-To: <20221206171340.139790-4-hannes@cmpxchg.org> Message-ID: <02b9663-4377-bd67-8da2-aad72240da@google.com> References: <20221206171340.139790-1-hannes@cmpxchg.org> <20221206171340.139790-4-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 6 Dec 2022, Johannes Weiner wrote: > Charge moving mode in cgroup1 allows memory to follow tasks as they > migrate between cgroups. This is, and always has been, a questionable > thing to do - for several reasons. > > First, it's expensive. Pages need to be identified, locked and > isolated from various MM operations, and reassigned, one by one. > > Second, it's unreliable. Once pages are charged to a cgroup, there > isn't always a clear owner task anymore. Cache isn't moved at all, for > example. Mapped memory is moved - but if trylocking or isolating a > page fails, it's arbitrarily left behind. Frequent moving between > domains may leave a task's memory scattered all over the place. > > Third, it isn't really needed. Launcher tasks can kick off workload > tasks directly in their target cgroup. Using dedicated per-workload > groups allows fine-grained policy adjustments - no need to move tasks > and their physical pages between control domains. The feature was > never forward-ported to cgroup2, and it hasn't been missed. > > Despite it being a niche usecase, the maintenance overhead of > supporting it is enormous. Because pages are moved while they are live > and subject to various MM operations, the synchronization rules are > complicated. There are lock_page_memcg() in MM and FS code, which > non-cgroup people don't understand. In some cases we've been able to > shift code and cgroup API calls around such that we can rely on native > locking as much as possible. But that's fragile, and sometimes we need > to hold MM locks for longer than we otherwise would (pte lock e.g.). > > Mark the feature deprecated. Hopefully we can remove it soon. > > Signed-off-by: Johannes Weiner Acked-by: Hugh Dickins but I wonder if it would be helpful to mention move_charge_at_immigrate in the deprecation message: maybe the first line should be "Cgroup memory moving (move_charge_at_immigrate) is deprecated.\n" > --- > Documentation/admin-guide/cgroup-v1/memory.rst | 11 ++++++++++- > mm/memcontrol.c | 4 ++++ > 2 files changed, 14 insertions(+), 1 deletion(-) > > diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst > index 60370f2c67b9..87d7877b98ec 100644 > --- a/Documentation/admin-guide/cgroup-v1/memory.rst > +++ b/Documentation/admin-guide/cgroup-v1/memory.rst > @@ -86,6 +86,8 @@ Brief summary of control files. > memory.swappiness set/show swappiness parameter of vmscan > (See sysctl's vm.swappiness) > memory.move_charge_at_immigrate set/show controls of moving charges > + This knob is deprecated and shouldn't be > + used. > memory.oom_control set/show oom controls. > memory.numa_stat show the number of memory usage per numa > node > @@ -717,9 +719,16 @@ Soft limits can be setup by using the following commands (in this example we > It is recommended to set the soft limit always below the hard limit, > otherwise the hard limit will take precedence. > > -8. Move charges at task migration > +8. Move charges at task migration (DEPRECATED!) > ================================= > > +THIS IS DEPRECATED! > + > +It's expensive and unreliable! It's better practice to launch workload > +tasks directly from inside their target cgroup. Use dedicated workload > +cgroups to allow fine-grained policy adjustments without having to > +move physical pages between control domains. > + > Users can move charges associated with a task along with task migration, that > is, uncharge task's pages from the old cgroup and charge them to the new cgroup. > This feature is not supported in !CONFIG_MMU environments because of lack of > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index b696354c1b21..e650a38d9a90 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3919,6 +3919,10 @@ static int mem_cgroup_move_charge_write(struct cgroup_subsys_state *css, > { > struct mem_cgroup *memcg = mem_cgroup_from_css(css); > > + pr_warn_once("Cgroup memory moving is deprecated. " > + "Please report your usecase to linux-mm@kvack.org if you " > + "depend on this functionality.\n"); > + > if (val & ~MOVE_MASK) > return -EINVAL; > > -- > 2.38.1 > >