linux-kernel - Re: [PATCH] cpuset: Enforce that a child's cpus must be a subset of the parent

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Message-ID: <20180531161942.GW1351649@devbig577.frc2.facebook.com>
Date:   Thu, 31 May 2018 09:19:42 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>, Zefan Li <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] cpuset: Enforce that a child's cpus must be a subset of
 the parent

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