cgroup: add cgroup_subsys->css_e_css_changed()

Add a new cgroup_subsys operatoin ->css_e_css_changed().  This is
invoked if any of the effective csses seen from the css's cgroup may
have changed.  This will be used to implement cgroup writeback
support.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

include/linux/cgroup.h

@@ -615,6 +615,7 @@ struct cgroup_subsys {
 	void (*css_released)(struct cgroup_subsys_state *css);
 	void (*css_free)(struct cgroup_subsys_state *css);
 	void (*css_reset)(struct cgroup_subsys_state *css);
+	void (*css_e_css_changed)(struct cgroup_subsys_state *css);
 
 	int (*can_attach)(struct cgroup_subsys_state *css,
 			  struct cgroup_taskset *tset);

kernel/cgroup.c

@@ -2836,6 +2836,24 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 		}
 	}
 
+	/*
+	 * The effective csses of all the descendants (excluding @cgrp) may
+	 * have changed.  Subsystems can optionally subscribe to this event
+	 * by implementing ->css_e_css_changed() which is invoked if any of
+	 * the effective csses seen from the css's cgroup may have changed.
+	 */
+	for_each_subsys(ss, ssid) {
+		struct cgroup_subsys_state *this_css = cgroup_css(cgrp, ss);
+		struct cgroup_subsys_state *css;
+
+		if (!ss->css_e_css_changed || !this_css)
+			continue;
+
+		css_for_each_descendant_pre(css, this_css)
+			if (css != this_css)
+				ss->css_e_css_changed(css);
+	}
+
 	kernfs_activate(cgrp->kn);
 	ret = 0;
 out_unlock: