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
