Messages in this thread | | | Date | Wed, 1 Dec 2021 09:26:43 -0500 | Subject | Re: [PATCH v8 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst | From | Waiman Long <> |
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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
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