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

[Waiman Long]

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