linux-kernel - Re: [PATCH v10 2/9] cpuset: Add new v2 cpuset.sched.domain

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Message-ID: <20180702163225.GH533219@devbig577.frc2.facebook.com>
Date:   Mon, 2 Jul 2018 09:32:25 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH v10 2/9] cpuset: Add new v2 cpuset.sched.domain_root flag

Hello,

On Fri, Jun 22, 2018 at 11:00:03AM +0800, Waiman Long wrote:
> On 06/21/2018 05:20 PM, Peter Zijlstra wrote:
> >> As for the inconsistency between the real root and the container root,
> >> this is true for almost all the controllers. So it is a generic problem.
> >> One possible solution is to create a kind a pseudo root cgroup for the
> >> container that looks and feels like a real root. But is there really a
> >> need to do that?
> > I don't really know. I thought the idea was to make containers
> > indistinguishable from a real system. Now I know we're really rather far
> > away from that in reality, and I really have no clue how important all
> > that is.
> 
> That will certainly be the ideal.

Sure, ideal in theoretical sense; however, the practical cost-benefit
ratio of trying to make containers indistinguishible from system
doesn't seem enough to justify the effort.  Not yet anyway.  It'd be
nice to not paint ourselves into a corner where we can't get the
equivalence without major interface changes later but I think that's
about the extent we should go for now.

> > It all depends on how exactly this works; is it like I assumed, that
> > this file is owned by the parent instead of the current directory? And
> > that if you namespace this, you have an effective read-only file?
> 
> Yes, that is right.
> 
> > Then fixing the inconsistency is trivial; simply provide a read-only
> > file for the actual root cgroup too.
> >
> > And if the solution is trivial, I don't see a good reason not to do it.
> 
> Do you mean providing a flag like READONLY_AT_ROOT so that it will be
> read-only at the real root? That is an cgroup architectural decision
> that needs input from Tejun. Anyway, this issue is not specific to this
> patchset and I would like to break it out as a separate discussion
> independent of this patchset.

Yeah, it's a larger problem than cpuset and different controllers
trying it in different ways isn't a good idea anyway.  Let's shelve
this topic for now.

Thanks.

-- 
tejun