linux-kernel - Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty

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Message-ID: <20190103181329.GW31793@dhcp22.suse.cz>
Date:   Thu, 3 Jan 2019 19:13:29 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     hannes@...xchg.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty

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_?
-- 
Michal Hocko
SUSE Labs