linux-kernel - Re: [patch -mm 3/4] mm, memcg: replace memory.oom

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Message-ID: <20180123151300.GP1526@dhcp22.suse.cz>
Date:   Tue, 23 Jan 2018 16:13:00 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        kernel-team@...com, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch -mm 3/4] mm, memcg: replace memory.oom_group with policy
 tunable

On Wed 17-01-18 14:18:33, David Rientjes wrote:
> On Wed, 17 Jan 2018, Michal Hocko wrote:
> 
> > Absolutely agreed! And moreover, there are not all that many ways what
> > to do as an action. You just kill a logical entity - be it a process or
> > a logical group of processes. But you have way too many policies how
> > to select that entity. Do you want to chose the youngest process/group
> > because all the older ones have been computing real stuff and you would
> > lose days of your cpu time? Or should those who pay more should be
> > protected (aka give them static priorities), or you name it...
> > 
> 
> That's an argument for making the interface extensible, yes.

And there is no interface to control the selection yet so we can develop
one on top.
 
> > I am sorry, I still didn't grasp the full semantic of the proposed
> > soluton but the mere fact it is starting by conflating selection and the
> > action is a no go and a wrong API. This is why I've said that what you
> > (David) outlined yesterday is probably going to suffer from a much
> > longer discussion and most likely to be not acceptable. Your patchset
> > proves me correct...
> 
> I'm very happy to change the API if there are better suggestions.  That 
> may end up just being an memory.oom_policy file, as this implements, and 
> separating out a new memory.oom_action that isn't a boolean value to 
> either do a full group kill or only a single process.  Or it could be what 
> I suggested in my mail to Tejun, such as "hierarchy killall" written to
> memory.oom_policy, which would specify a single policy and then an 
> optional mechanism.  With my proposed patchset, there would then be three 
> policies: "none", "cgroup", and "tree" and one possible optional 
> mechanism: "killall".

You haven't convinced me at all. This all sounds more like "what if"
than a really thought through interface. I've tried to point out that
having a real policy driven victim selection is a _hard_ thing to do
_right_.

On the other hand oom_group makes semantic sense. It controls the
killable entity and there are usecases which want to consider the full
memcg as a single killable entity. No matter what selection policy we
chose on top. It is just a natural API.

Now you keep arguing about the victim selection and different strategies
to implement it. We will not move forward as long as you keep conflating
the two things, I am afraid.
-- 
Michal Hocko
SUSE Labs