cgroup: implement cgroup_subsys->css_reset()

cgroup is implementing support for subsystem dependency which would
require a way to enable a subsystem even when it's not directly
configured through "cgroup.subtree_control".

The previous patches added support for explicitly and implicitly
enabled subsystems and showing/hiding their interface files.  An
explicitly enabled subsystem may become implicitly enabled if it's
turned off through "cgroup.subtree_control" but there are subsystems
depending on it.  In such cases, the subsystem, as it's turned off
when seen from userland, shouldn't enforce any resource control.
Also, the subsystem may be explicitly turned on later again and its
interface files should be as close to the intial state as possible.

This patch adds cgroup_subsys->css_reset() which is invoked when a css
is hidden.  The callback should disable resource control and reset the
state to the vanilla state.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 331fa296..3a6b77d 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2740,17 +2740,25 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	/*
 	 * All tasks are migrated out of disabled csses.  Kill or hide
 	 * them.  A css is hidden when the userland requests it to be
-	 * disabled while other subsystems are still depending on it.
+	 * disabled while other subsystems are still depending on it.  The
+	 * css must not actively control resources and be in the vanilla
+	 * state if it's made visible again later.  Controllers which may
+	 * be depended upon should provide ->css_reset() for this purpose.
 	 */
 	for_each_subsys(ss, ssid) {
 		if (!(disable & (1 << ssid)))
 			continue;
 
 		cgroup_for_each_live_child(child, cgrp) {
-			if (css_disable & (1 << ssid))
-				kill_css(cgroup_css(child, ss));
-			else
+			struct cgroup_subsys_state *css = cgroup_css(child, ss);
+
+			if (css_disable & (1 << ssid)) {
+				kill_css(css);
+			} else {
 				cgroup_clear_dir(child, 1 << ssid);
+				if (ss->css_reset)
+					ss->css_reset(css);
+			}
 		}
 	}