> cgroup users often need a way to determine when a cgroup's > subhierarchy becomes empty so that it can be cleaned up. cgroup > currently provides release_agent for it; unfortunately, this mechanism > is riddled with issues. > > * It delivers events by forking and execing a userland binary > specified as the release_agent. This is a long deprecated method of > notification delivery. It's extremely heavy, slow and cumbersome to > integrate with larger infrastructure. > > * There is single monitoring point at the root. There's no way to > delegate management of a subtree. > > * The event isn't recursive. It triggers when a cgroup doesn't have > any tasks or child cgroups. Events for internal nodes trigger only > after all children are removed. This again makes it impossible to > delegate management of a subtree. > > * Events are filtered from the kernel side. "notify_on_release" file > is used to subscribe to or suppress release event. This is > unnecessarily complicated and probably done this way because event > delivery itself was expensive. > > This patch implements interface file "cgroup.populated" which can be > used to monitor whether the cgroup's subhierarchy has tasks in it or > not. Its value is 0 if there is no task in the cgroup and its > descendants; otherwise, 1, and kernfs_notify() notificaiton is > triggers when the value changes, which can be monitored through poll > and [di]notify. > > This is a lot ligther and simpler and trivially allows delegating > management of subhierarchy - subhierarchy monitoring can block further > propgation simply by putting itself or another process in the root of > the subhierarchy and monitor events that it's interested in from there > without interfering with monitoring higher in the tree. > > v2: Patch description updated as per Serge. > > v3: "cgroup.subtree_populated" renamed to "cgroup.populated". The > subtree_ prefix was a bit confusing because > "cgroup.subtree_control" uses it to denote the tree rooted at the > cgroup sans the cgroup itself while the populated state includes > the cgroup itself. > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxx> > Cc: Lennart Poettering <lennart@xxxxxxxxxxxxxx> Acked-by: Li Zefan <lizefan@xxxxxxxxxx> _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
