cgroup: implement cgroup_subsys->implicit_on_dfl
Some controllers, perf_event for now and possibly freezer in the
future, don't really make sense to control explicitly through
"cgroup.subtree_control". For example, the primary role of perf_event
is identifying the cgroups of tasks; however, because the controller
also keeps a small amount of state per cgroup, it can't be replaced
with simple cgroup membership tests.
This patch implements cgroup_subsys->implicit_on_dfl flag. When set,
the controller is implicitly enabled on all cgroups on the v2
hierarchy so that utility type controllers such as perf_event can be
enabled and function transparently.
An implicit controller doesn't show up in "cgroup.controllers" or
"cgroup.subtree_control", is exempt from no internal process rule and
can be stolen from the default hierarchy even if there are non-root
csses.
v2: Reimplemented on top of the recent updates to css handling and
subsystem rebinding. Rebinding implicit subsystems is now a
simple matter of exempting it from the busy subsystem check.
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 590291d..34b42f0 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -451,6 +451,19 @@
bool early_init:1;
/*
+ * If %true, the controller, on the default hierarchy, doesn't show
+ * up in "cgroup.controllers" or "cgroup.subtree_control", is
+ * implicitly enabled on all cgroups on the default hierarchy, and
+ * bypasses the "no internal process" constraint. This is for
+ * utility type controllers which is transparent to userland.
+ *
+ * An implicit controller can be stolen from the default hierarchy
+ * anytime and thus must be okay with offline csses from previous
+ * hierarchies coexisting with csses for the current one.
+ */
+ bool implicit_on_dfl:1;
+
+ /*
* If %false, this subsystem is properly hierarchical -
* configuration, resource accounting and restriction on a parent
* cgroup cover those of its children. If %true, hierarchy support