linux-kernel - Re: [PATCH v7 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

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Date:   Thu, 26 Aug 2021 18:00:47 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <llong@...hat.com>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Koutný <mkoutny@...e.com>
Subject: Re: [PATCH v7 5/6] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

Hello,

On Thu, Aug 26, 2021 at 11:01:30PM -0400, Waiman Long wrote:
> What I am doing here is setting a high bar for transitioning from member to
> either "root" or "isolated". Once it becomes a partition, there are multiple
> ways that can make it invalid. I am fine with that. However, I am not sure
> it is a good idea to allow users to echo "root" to cpuset.cpus.partition
> anywhere in the cgroup hierarchy and require them to read it back to see if
> it succeed.

The problem is that the "high" bar is rather arbitrary. It might feel like a
good idea to some but not to others. There are no clear technical reasons or
principles for rules to be set this particular way.

> All the checking are done with cpuset_rwsem held. So there shouldn't be any
> racing. Of course, a hotplug can immediately follow and make the partition
> invalid.

Imagine a system which dynamically on/offlines its cpus based on load or
whatever and also configures partitions for cases where the needed cpus are
online. If the partitions are set up while the cpus are online, it'd work as
expected - partitions are in effect when the system can support them and
ignored otherwise. However, if the partition configuration is attempted
while the cpus happen to be offline, the configuration will fail, and there
is no guaranteed way to make that configuration stick short of disabling
hotplug operations. This is a pretty jarring brekage happening exactly
because the behavior is an inconsistent amalgam.

It's usually not a good sign if interface restrictions can be added or
removed because how one feels without clear functional reasons and often
indicates that there's something broken, which seems to be the case here
too.

Thanks.

-- 
tejun