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Date:   Thu, 3 Jan 2019 19:53:33 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Yang Shi <yang.shi@linux.alibaba.com>
Cc:     hannes@cmpxchg.org, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty
Message-ID: <20190103185333.GX31793@dhcp22.suse.cz>
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On Thu 03-01-19 10:40:54, Yang Shi wrote:
> 
> 
> On 1/3/19 10:13 AM, Michal Hocko wrote:
> > On Thu 03-01-19 09:33:14, Yang Shi wrote:
> > > 
> > > On 1/3/19 2:12 AM, Michal Hocko wrote:
> > > > On Thu 03-01-19 04:05:30, Yang Shi wrote:
> > > > > Currently, force empty reclaims memory synchronously when writing to
> > > > > memory.force_empty.  It may take some time to return and the afterwards
> > > > > operations are blocked by it.  Although it can be interrupted by signal,
> > > > > it still seems suboptimal.
> > > > Why it is suboptimal? We are doing that operation on behalf of the
> > > > process requesting it. What should anybody else pay for it? In other
> > > > words why should we hide the overhead?
> > > Please see the below explanation.
> > > 
> > > > > Now css offline is handled by worker, and the typical usecase of force
> > > > > empty is before memcg offline.  So, handling force empty in css offline
> > > > > sounds reasonable.
> > > > Hmm, so I guess you are talking about
> > > > echo 1 > $MEMCG/force_empty
> > > > rmdir $MEMCG
> > > > 
> > > > and you are complaining that the operation takes too long. Right? Why do
> > > > you care actually?
> > > We have some usecases which create and remove memcgs very frequently, and
> > > the tasks in the memcg may just access the files which are unlikely accessed
> > > by anyone else. So, we prefer force_empty the memcg before rmdir'ing it to
> > > reclaim the page cache so that they don't get accumulated to incur
> > > unnecessary memory pressure. Since the memory pressure may incur direct
> > > reclaim to harm some latency sensitive applications.
> > Yes, this makes sense to me.
> > 
> > > And, the create/remove might be run in a script sequentially (there might be
> > > a lot scripts or applications are run in parallel to do this), i.e.
> > > mkdir cg1
> > > do something
> > > echo 0 > cg1/memory.force_empty
> > > rmdir cg1
> > > 
> > > mkdir cg2
> > > ...
> > > 
> > > The creation of the afterwards memcg might be blocked by the force_empty for
> > > long time if there are a lot page caches, so the overall throughput of the
> > > system may get hurt.
> > Is there any reason for your scripts to be strictly sequential here? In
> > other words why cannot you offload those expensive operations to a
> > detached context in _userspace_?
> 
> I would say it has not to be strictly sequential. The above script is just
> an example to illustrate the pattern. But, sometimes it may hit such pattern
> due to the complicated cluster scheduling and container scheduling in the
> production environment, for example the creation process might be scheduled
> to the same CPU which is doing force_empty. I have to say I don't know too
> much about the internals of the container scheduling.

In that case I do not see a strong reason to implement the offloding
into the kernel. It is an additional code and semantic to maintain.

I think it is more important to discuss whether we want to introduce
force_empty in cgroup v2.
-- 
Michal Hocko
SUSE Labs