[PATCH v2 3/4] cgroup: Allow reenabling of controller in bypass mode

[Waiman Long <longman@redhat.com>]

Controllers set to bypass mode in the parent's "cgroup.subtree_control"
can now be optionally enabled by writing the controller name with the
'+' prefix to "cgroup.controllers". Using the '#' prefix will reset it
back to the bypass state.

This capability increases the flexibility each controller has in
shaping the effective cgroup hierarchy to best suit its need.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/cgroup-v2.txt |  23 +++++++++-
 include/linux/cgroup-defs.h |   7 +++
 kernel/cgroup/cgroup.c      | 109 ++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 134 insertions(+), 5 deletions(-)

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index f17a74b..efb69c4 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -395,6 +395,18 @@ prefixed controller interface files from C and D.  This means that the
 controller interface files - anything which doesn't start with
 "cgroup." are owned by the parent rather than the cgroup itself.
 
+Once a controller is put into bypass mode in "cgroup.subtree_control",
+the cgroup's children can optionally enable this controller by writing
+the controller name with the '+ prefix into "cgroup.controllers".
+In this case, the controller interface files are considered to be
+owned by the child cgroup itself, not by its parent.  Therefore,
+setting the bypass mode in "cgroup.subtree_control" means delegating
+the authority of enabling or disabling the controller interface files
+to its children.  Writing the controller name with the '#' prefix into
+"cgroup.controllers" resets the state back to bypass mode.  The state
+of a controller cannot be changed if it is enabled or bypassed in its
+"cgroup.subtree_control".
+
 
 Cgroup Hierarchy
 ~~~~~~~~~~~~~~~~
@@ -859,11 +871,18 @@ All cgroup core files are prefixed with "cgroup."
 	should be granted along with the containing directory.
 
   cgroup.controllers
-	A read-only space separated values file which exists on all
+	A read-write space separated values file which exists on all
 	cgroups.
 
 	It shows space separated list of all controllers available to
-	the cgroup.  The controllers are not ordered.
+	the cgroup.  Controller names with '#' prefix are in bypass
+	mode.  The controllers are not ordered.
+
+	When a controller is set into bypass mode in its parent's
+	"cgroup.subtree_control", its name prefixed with '+' or '#'
+	can be written to enable it or reset it back to bypass mode
+	respectively.  Controllers not in bypass mode are not allowed
+	to be written.
 
   cgroup.subtree_control
 	A read-write space separated values file which exists on all
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 3cac6d0..25c2ac8 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -308,6 +308,13 @@ struct cgroup {
 	u16 old_subtree_ss_mask;
 	u16 old_subtree_bypass;
 
+	/*
+	 * The bitmask of subsystems that are set in its parent's
+	 * ->subtree_bypass and explictly enabled in this cgroup.
+	 */
+	u16 enable_ss_mask;
+	u16 old_enable_ss_mask;
+
 	/* Private pointers for each registered subsystem */
 	struct cgroup_subsys_state __rcu *subsys[CGROUP_SUBSYS_COUNT];
 
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 1e7feae..358d8b3 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -420,7 +420,7 @@ static u16 cgroup_control(struct cgroup *cgrp, bool show_bypass)
 	u16 root_ss_mask = cgrp->root->subsys_mask;
 
 	if (parent) {
-		u16 ss_mask = parent->subtree_control;
+		u16 ss_mask = parent->subtree_control|cgrp->enable_ss_mask;
 
 		if (show_bypass)
 			ss_mask |= parent->subtree_bypass;
@@ -443,7 +443,7 @@ static u16 cgroup_ss_mask(struct cgroup *cgrp, bool show_bypass)
 	struct cgroup *parent = cgroup_parent(cgrp);
 
 	if (parent) {
-		u16 ss_mask = parent->subtree_ss_mask;
+		u16 ss_mask = parent->subtree_ss_mask|cgrp->enable_ss_mask;
 
 
 		if (show_bypass)
@@ -2811,6 +2811,7 @@ static void cgroup_save_control(struct cgroup *cgrp)
 		dsct->old_subtree_control = dsct->subtree_control;
 		dsct->old_subtree_ss_mask = dsct->subtree_ss_mask;
 		dsct->old_subtree_bypass  = dsct->subtree_bypass;
+		dsct->old_enable_ss_mask  = dsct->enable_ss_mask;
 	}
 }
 
@@ -2854,6 +2855,7 @@ static void cgroup_restore_control(struct cgroup *cgrp)
 		dsct->subtree_control = dsct->old_subtree_control;
 		dsct->subtree_ss_mask = dsct->old_subtree_ss_mask;
 		dsct->subtree_bypass  = dsct->old_subtree_bypass;
+		dsct->enable_ss_mask  = dsct->old_enable_ss_mask;
 	}
 }
 
@@ -3124,7 +3126,8 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 
 
 	cgroup_for_each_live_child(child, cgrp)
-		child_enable |= child->subtree_control|child->subtree_bypass;
+		child_enable |= child->subtree_control|child->subtree_bypass|
+				child->enable_ss_mask;
 
 	/*
 	 * Cannot change the state of a controller if enabled in children.
@@ -3157,6 +3160,105 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	return ret ?: nbytes;
 }
 
+/*
+ * Change bypass status of controllers for a cgroup in the default hierarchy.
+ */
+static ssize_t cgroup_controllers_write(struct kernfs_open_file *of,
+					char *buf, size_t nbytes,
+					loff_t off)
+{
+	u16 enable = 0, bypass = 0;
+	struct cgroup *cgrp, *parent;
+	struct cgroup_subsys *ss;
+	char *tok;
+	int ssid, ret;
+
+	/*
+	 * Parse input - space separated list of subsystem names prefixed
+	 * with either + or #.
+	 */
+	buf = strstrip(buf);
+	while ((tok = strsep(&buf, " "))) {
+		if (tok[0] == '\0')
+			continue;
+		do_each_subsys_mask(ss, ssid, ~cgrp_dfl_inhibit_ss_mask) {
+			if (!cgroup_ssid_enabled(ssid) ||
+			    strcmp(tok + 1, ss->name))
+				continue;
+
+			if (*tok == '+') {
+				enable |= 1 << ssid;
+				bypass &= ~(1 << ssid);
+			} else if (*tok == '#') {
+				bypass |= 1 << ssid;
+				enable &= ~(1 << ssid);
+			} else {
+				return -EINVAL;
+			}
+			break;
+		} while_each_subsys_mask();
+		if (ssid == CGROUP_SUBSYS_COUNT)
+			return -EINVAL;
+	}
+
+	cgrp = cgroup_kn_lock_live(of->kn, true);
+	if (!cgrp)
+		return -ENODEV;
+
+	/*
+	 * Write to root cgroup's controllers file is not allowed.
+	 */
+	parent = cgroup_parent(cgrp);
+	if (!parent) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
+
+	/*
+	 * Only controllers set into bypass mode in the parent cgroup
+	 * can be specified here.
+	 */
+	if (~parent->subtree_bypass & (enable|bypass)) {
+		ret = -ENOENT;
+		goto out_unlock;
+	}
+
+	/*
+	 * Mask off irrelevant bits.
+	 */
+	enable &= ~cgrp->enable_ss_mask;
+	bypass &=  cgrp->enable_ss_mask;
+
+	if (!(enable|bypass)) {
+		ret = 0;
+		goto out_unlock;
+	}
+
+	/*
+	 * We cannot change the bypass state of a controller that is enabled
+	 * in subtree_control.
+	 */
+	if ((cgrp->subtree_control|cgrp->subtree_bypass) & (enable|bypass)) {
+		ret = -EBUSY;
+		goto out_unlock;
+	}
+
+	/* Save and update control masks and prepare csses */
+	cgroup_save_control(cgrp);
+
+	cgrp->enable_ss_mask |= enable;
+	cgrp->enable_ss_mask &= ~bypass;
+
+	ret = cgroup_apply_control(cgrp);
+	cgroup_finalize_control(cgrp, ret);
+	kernfs_activate(cgrp->kn);
+	ret = 0;
+
+out_unlock:
+	cgroup_kn_unlock(of->kn);
+	return ret ?: nbytes;
+}
+
 static int cgroup_enable_threaded(struct cgroup *cgrp)
 {
 	struct cgroup *parent = cgroup_parent(cgrp);
@@ -4322,6 +4424,7 @@ static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
 	{
 		.name = "cgroup.controllers",
 		.seq_show = cgroup_controllers_show,
+		.write = cgroup_controllers_write,
 	},
 	{
 		.name = "cgroup.subtree_control",
-- 
1.8.3.1