linux-kernel - Re: [PATCH v4] cpuset: Enable cpuset controller in default hierarchy

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Date:   Tue, 20 Mar 2018 05:25:41 +0100
From:   Mike Galbraith <efault@....de>
To:     Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>
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,
        torvalds@...ux-foundation.org, Roman Gushchin <guro@...com>
Subject: Re: [PATCH v4] cpuset: Enable cpuset controller in default hierarchy

On Mon, 2018-03-19 at 17:41 -0400, Waiman Long wrote:
> On 03/19/2018 04:49 PM, Mike Galbraith wrote:
> > On Mon, 2018-03-19 at 08:34 -0700, Tejun Heo wrote:
> >> Hello, Mike.
> >>
> >> On Thu, Mar 15, 2018 at 03:49:01AM +0100, Mike Galbraith wrote:
> >>> Under the hood v2 details are entirely up to you.  My input ends at
> >>> please don't leave dynamic partitioning standing at the dock when v2
> >>> sails.
> >> So, this isn't about implementation details but about what the
> >> interface achieves - ie, what's the actual function?  The only thing I
> >> can see is blocking the entity which is configuring the hierarchy from
> >> making certain configs.  While that might be useful in some specific
> >> use cases, it seems to miss the bar for becoming its own kernel
> >> feature.  After all, nothing prevents the same entity from clearing
> >> the exlusive bit and making the said changes.
> > Yes, privileged contexts can maliciously or stupidly step all over one
> > other no matter what you do (finite resource), but oxymoron creation
> > (CPUs simultaneously balanced and isolated) should be handled.  If one
> > context can allocate a set overlapping a set another context intends to
> > or already has detached from scheduler domains, both are screwed.
> >
> > 	-Mike
> 
> The allocations of CPUs to child cgroups should be controlled by the
> parent cgroup. It is the parent's fault if some CPUs are in both
> balanced and isolated cgroups.
> 
> How about we don't allow turning off scheduling if the CPUs aren't
> exclusive from the parent's perspective? So you can't create an isolated
> cgroup if the CPUs aren't exclusive. Will this be a good enough compromise?

Sure.  The kernel need only ensure its own sanity.  Userspace conflicts
are more or less a non-issue.  In practice, all players but one will
have been constrained or eliminated prior to any partitioning.

	-Mike