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From:   Chunxin Zang <zangchunxin@bytedance.com>
Date:   Tue, 22 Sep 2020 17:43:52 +0800
Message-ID: <CAKRVAeN5U6S78jF1n8nCs5ioAdqvVn5f6GGTAnA93g_J0daOLw@mail.gmail.com>
Subject: Re: [External] Re: [PATCH] mm/memcontrol: Add the drop_cache
 interface for cgroup v2
To:     Michal Hocko <mhocko@suse.com>
Cc:     Yafang Shao <laoar.shao@gmail.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, lizefan@huawei.com,
        Jonathan Corbet <corbet@lwn.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, kafai@fb.com,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        andriin@fb.com, john.fastabend@gmail.com, kpsingh@chromium.org,
        Cgroups <cgroups@vger.kernel.org>, linux-doc@vger.kernel.org,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>, bpf@vger.kernel.org
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On Mon, Sep 21, 2020 at 7:05 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 21-09-20 18:55:40, Yafang Shao wrote:
> > On Mon, Sep 21, 2020 at 4:12 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Mon 21-09-20 16:02:55, zangchunxin@bytedance.com wrote:
> > > > From: Chunxin Zang <zangchunxin@bytedance.com>
> > > >
> > > > In the cgroup v1, we have 'force_mepty' interface. This is very
> > > > useful for userspace to actively release memory. But the cgroup
> > > > v2 does not.
> > > >
> > > > This patch reuse cgroup v1's function, but have a new name for
> > > > the interface. Because I think 'drop_cache' may be is easier to
> > > > understand :)
> > >
> > > This should really explain a usecase. Global drop_caches is a terribl=
e
> > > interface and it has caused many problems in the past. People have
> > > learned to use it as a remedy to any problem they might see and cause
> > > other problems without realizing that. This is the reason why we even
> > > log each attempt to drop caches.
> > >
> > > I would rather not repeat the same mistake on the memcg level unless
> > > there is a very strong reason for it.
> > >
> >
> > I think we'd better add these comments above the function
> > mem_cgroup_force_empty() to explain why we don't want to expose this
> > interface in cgroup2, otherwise people will continue to send this
> > proposal without any strong reason.
>
> I do not mind people sending this proposal.  "V1 used to have an
> interface, we need it in v2 as well" is not really viable without
> providing more reasoning on the specific usecase.
>
> _Any_ patch should have a proper justification. This is nothing really
> new to the process and I am wondering why this is coming as a surprise.
>

I'm so sorry for that.
My usecase is that there are two types of services in one server. They
have difference
priorities. Type_A has the highest priority, we need to ensure it's
schedule latency=E3=80=81I/O
latency=E3=80=81memory enough. Type_B has the lowest priority, we expect it
will not affect
Type_A when executed.
So Type_A could use memory without any limit. Type_B could use memory
only when the
memory is absolutely sufficient. But we cannot estimate how much
memory Type_B should
use. Because everything is dynamic. So we can't set Type_B's memory.high.

So we want to release the memory of Type_B when global memory is
insufficient in order
to ensure the quality of service of Type_A . In the past, we used the
'force_empty' interface
of cgroup v1.

> --
> Michal Hocko
> SUSE Labs

Best wishes
Chunxin