From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] cpuset: Enforce that a child's cpus must be a subset of
 the parent
Date: Thu, 31 May 2018 09:19:42 -0700
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Waiman Long <longman@redhat.com>, Zefan Li <lizefan@huawei.com>, Johannes Weiner <hannes@cmpxchg.org>, Ingo Molnar <mingo@redhat.com>, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@fb.com, pjt@google.com, luto@amacapital.net, Mike Galbraith <efault@gmx.de>, torvalds@linux-foundation.org, Roman Gushchin <guro@fb.com>, Juri Lelli <juri.lelli@redhat.com>, Patrick Bellasi <patrick.bellasi@arm.com>

Hello,

On Thu, May 31, 2018 at 06:16:45PM +0200, Peter Zijlstra wrote:
> > So, let's please stay away from it even if that means a bit of
> > overhead in terms of interface.
> 
> Urgh, that again :/

Yeah, well, it's pretty important.

> I'm still not convinced by your arguments though. The root container can
> access all the sub-groups anyway and can grub around in them to take
> away resources if it really wants to.

That's really messy and if you delegated away a subtree, you can't
walk the subtree in a race free way, not easily anyway.

> For cpuset in particular randomly restricting on the ancestor level can
> create an unrecoverable trainwreck inside a container. Affinities are
> not recoverable. Once a runnable task ends up with an empty set, its
> affinities are reset and the smaller (empty) set is lost.

Yeah, for cpuset, it's messier, but it isn't different from hotunplug
scenario, right?  I think the best we can do there is putting ancestor
operation on an equal footing as hotplug ops.

Thanks.

-- 
tejun