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Date:   Mon, 21 Sep 2020 13:36:46 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     zangchunxin@...edance.com, Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, lizefan@...wei.com,
        Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, kafai@...com,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        andriin@...com, john.fastabend@...il.com, kpsingh@...omium.org,
        Cgroups <cgroups@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org
Subject: Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2

On Mon 21-09-20 19:23:01, Yafang Shao wrote:
> On Mon, Sep 21, 2020 at 7:05 PM Michal Hocko <mhocko@...e.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@...e.com> wrote:
> > > >
> > > > On Mon 21-09-20 16:02:55, zangchunxin@...edance.com wrote:
> > > > > From: Chunxin Zang <zangchunxin@...edance.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 terrible
> > > > 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.
> >
> 
> Container users always want to drop cache in a specific container,
> because they used to use drop_caches to fix memory pressure issues.

This is exactly the kind of problems we have seen in the past. There
should be zero reason to addre potential reclaim problems by dropping
page cache on the floor. There is a huge cargo cult about this
procedure and I have seen numerous reports when people complained about
performance afterwards just to learn that the dropped page cache was one
of the resons for that.

> Although drop_caches can cause some unexpected issues, it could also
> fix some issues.

"Some issues" is way too general. We really want to learn about those
issues and address them properly.

> So container users want to use it in containers as
> well.
> If this feature is not implemented in cgroup, they will ask you why
> but there is no explanation in the kernel.

There is no usecase that would really require it so far.

> Regarding the memory.high, it is not perfect as well, because you have
> to set it to 0 to drop_caches, and the processes in the containers
> have to reclaim pages as well because they reach the memory.high, but
> memory.force_empty won't make other processes to reclaim.

Since 536d3bf261a2 ("mm: memcontrol: avoid workload stalls when lowering
memory.high") the limit is set after the reclaim so the race window when
somebody would be pushed to high limit reclaim is reduced. But I do
agree this is just a workaround.

> That doesn't mean I agree to add this interface, while I really mean
> that if we discard one feature we'd better explain why.

We need to understand why somebody wants an interface because once it is
added it will have to be maintained for ever.
-- 
Michal Hocko
SUSE Labs

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