linux-kernel - Re: [PATCH v8 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

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Message-ID: <0bdfb7f2-61ee-7b2a-fdb2-3c41f6d6ade0@redhat.com>
Date:   Wed, 1 Dec 2021 09:26:43 -0500
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Michal Koutný <mkoutny@...e.com>,
        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>
Subject: Re: [PATCH v8 5/6] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

On 11/30/21 22:56, Waiman Long wrote:
> On 11/30/21 12:11, Tejun Heo wrote:
>
>
>>>      Once becoming a partition root, the following two rules restrict
>>>      what changes can be made to "cpuset.cpus".
>>>
>>>      1) The value must be exclusive.
>>>      2) If child cpusets exist, the value must be a superset of what
>>>         are defined in the child cpusets.
>>>
>>>      The second rule applies even for "member". Other changes to
>>>      "cpuset.cpus" that do not violate the above rules are always
>>>      allowed.
>> While it isn't necessarily tied to this series, it's a big no-no to 
>> restrict
>> what a parent can do depending on what its descendants are doing. A 
>> cgroup
>> higher up in the hierarchy should be able to change configuration 
>> however it
>> sees fit as deligation breaks down otherwise.
>>
>> Maybe you can argue that cpuset is special and shouldn't be subject 
>> to such
>> convention but I can't see strong enough justifications especially given
>> that most of these restrictions can be broken by hotplug operations 
>> anyway
>> and thus need code to handle those situations.
>
> These are all pre-existing restrictions before the introduction of 
> partition. These are checks done in validate_change(). I am just 
> saying out loud the existing behavior. If you think that needs to be 
> changed, I am fine with that. However, it will be a separate patch as 
> it is not a behavior that is introduced by this series.

Of the 2 restrictions listed above, the exclusivity rule is due to the 
use of CS_CPU_EXCLUSIVE flag. I think it is reasonable as it affects 
only siblings, not the parent.

The second restriction was found during my testing. It is caused by the 
following code in validate_change():

         /* Each of our child cpusets must be a subset of us */
         ret = -EBUSY;
         cpuset_for_each_child(c, css, cur)
                 if (!is_cpuset_subset(c, trial))
                         goto out;

It seems that this code was there since v2.6.12 (the beginning of the 
git era). Later in function, we have

         /* On legacy hierarchy, we must be a subset of our parent 
cpuset. */
         ret = -EACCES;
         if (!is_in_v2_mode() && !is_cpuset_subset(trial, par))
                 goto out;

This is actually a duplicate in the case of legacy hierarchy.

I can add a patch to take out the first code block above which I think 
is where most of your objections are. Then I can remove the 2nd 
restriction in my documentation. I would like to emphasize that this is 
a pre-existing behavior which I just happen to document.

Cheers,
Longman