Merge branch 'for-4.14' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup updates from Tejun Heo:
 "Several notable changes this cycle:

   - Thread mode was merged. This will be used for cgroup2 support for
     CPU and possibly other controllers. Unfortunately, CPU controller
     cgroup2 support didn't make this pull request but most contentions
     have been resolved and the support is likely to be merged before
     the next merge window.

   - cgroup.stat now shows the number of descendant cgroups.

   - cpuset now can enable the easier-to-configure v2 behavior on v1
     hierarchy"

* 'for-4.14' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup: (21 commits)
  cpuset: Allow v2 behavior in v1 cgroup
  cgroup: Add mount flag to enable cpuset to use v2 behavior in v1 cgroup
  cgroup: remove unneeded checks
  cgroup: misc changes
  cgroup: short-circuit cset_cgroup_from_root() on the default hierarchy
  cgroup: re-use the parent pointer in cgroup_destroy_locked()
  cgroup: add cgroup.stat interface with basic hierarchy stats
  cgroup: implement hierarchy limits
  cgroup: keep track of number of descent cgroups
  cgroup: add comment to cgroup_enable_threaded()
  cgroup: remove unnecessary empty check when enabling threaded mode
  cgroup: update debug controller to print out thread mode information
  cgroup: implement cgroup v2 thread support
  cgroup: implement CSS_TASK_ITER_THREADED
  cgroup: introduce cgroup->dom_cgrp and threaded css_set handling
  cgroup: add @flags to css_task_iter_start() and implement CSS_TASK_ITER_PROCS
  cgroup: reorganize cgroup.procs / task write path
  cgroup: replace css_set walking populated test with testing cgrp->nr_populated_csets
  cgroup: distinguish local and children populated states
  cgroup: remove now unused list_head @pending in cgroup_apply_cftypes()
  ...

13 files changed, 1194 insertions, 272 deletions

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index bde177103567..dc44785dc0fa 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -18,7 +18,9 @@ v1 is available under Documentation/cgroup-v1/.
      1-2. What is cgroup?
    2. Basic Operations
      2-1. Mounting
-     2-2. Organizing Processes
+     2-2. Organizing Processes and Threads
+       2-2-1. Processes
+       2-2-2. Threads
      2-3. [Un]populated Notification
      2-4. Controlling Controllers
        2-4-1. Enabling and Disabling
@@ -167,8 +169,11 @@ cgroup v2 currently supports the following mount options.
 	Delegation section for details.
 
 
-Organizing Processes
---------------------
+Organizing Processes and Threads
+--------------------------------
+
+Processes
+~~~~~~~~~
 
 Initially, only the root cgroup exists to which all processes belong.
 A child cgroup can be created by creating a sub-directory::
@@ -219,6 +224,105 @@ is removed subsequently, " (deleted)" is appended to the path::
   0::/test-cgroup/test-cgroup-nested (deleted)
 
 
+Threads
+~~~~~~~
+
+cgroup v2 supports thread granularity for a subset of controllers to
+support use cases requiring hierarchical resource distribution across
+the threads of a group of processes.  By default, all threads of a
+process belong to the same cgroup, which also serves as the resource
+domain to host resource consumptions which are not specific to a
+process or thread.  The thread mode allows threads to be spread across
+a subtree while still maintaining the common resource domain for them.
+
+Controllers which support thread mode are called threaded controllers.
+The ones which don't are called domain controllers.
+
+Marking a cgroup threaded makes it join the resource domain of its
+parent as a threaded cgroup.  The parent may be another threaded
+cgroup whose resource domain is further up in the hierarchy.  The root
+of a threaded subtree, that is, the nearest ancestor which is not
+threaded, is called threaded domain or thread root interchangeably and
+serves as the resource domain for the entire subtree.
+
+Inside a threaded subtree, threads of a process can be put in
+different cgroups and are not subject to the no internal process
+constraint - threaded controllers can be enabled on non-leaf cgroups
+whether they have threads in them or not.
+
+As the threaded domain cgroup hosts all the domain resource
+consumptions of the subtree, it is considered to have internal
+resource consumptions whether there are processes in it or not and
+can't have populated child cgroups which aren't threaded.  Because the
+root cgroup is not subject to no internal process constraint, it can
+serve both as a threaded domain and a parent to domain cgroups.
+
+The current operation mode or type of the cgroup is shown in the
+"cgroup.type" file which indicates whether the cgroup is a normal
+domain, a domain which is serving as the domain of a threaded subtree,
+or a threaded cgroup.
+
+On creation, a cgroup is always a domain cgroup and can be made
+threaded by writing "threaded" to the "cgroup.type" file.  The
+operation is single direction::
+
+  # echo threaded > cgroup.type
+
+Once threaded, the cgroup can't be made a domain again.  To enable the
+thread mode, the following conditions must be met.
+
+- As the cgroup will join the parent's resource domain.  The parent
+  must either be a valid (threaded) domain or a threaded cgroup.
+
+- When the parent is an unthreaded domain, it must not have any domain
+  controllers enabled or populated domain children.  The root is
+  exempt from this requirement.
+
+Topology-wise, a cgroup can be in an invalid state.  Please consider
+the following toplogy::
+
+  A (threaded domain) - B (threaded) - C (domain, just created)
+
+C is created as a domain but isn't connected to a parent which can
+host child domains.  C can't be used until it is turned into a
+threaded cgroup.  "cgroup.type" file will report "domain (invalid)" in
+these cases.  Operations which fail due to invalid topology use
+EOPNOTSUPP as the errno.
+
+A domain cgroup is turned into a threaded domain when one of its child
+cgroup becomes threaded or threaded controllers are enabled in the
+"cgroup.subtree_control" file while there are processes in the cgroup.
+A threaded domain reverts to a normal domain when the conditions
+clear.
+
+When read, "cgroup.threads" contains the list of the thread IDs of all
+threads in the cgroup.  Except that the operations are per-thread
+instead of per-process, "cgroup.threads" has the same format and
+behaves the same way as "cgroup.procs".  While "cgroup.threads" can be
+written to in any cgroup, as it can only move threads inside the same
+threaded domain, its operations are confined inside each threaded
+subtree.
+
+The threaded domain cgroup serves as the resource domain for the whole
+subtree, and, while the threads can be scattered across the subtree,
+all the processes are considered to be in the threaded domain cgroup.
+"cgroup.procs" in a threaded domain cgroup contains the PIDs of all
+processes in the subtree and is not readable in the subtree proper.
+However, "cgroup.procs" can be written to from anywhere in the subtree
+to migrate all threads of the matching process to the cgroup.
+
+Only threaded controllers can be enabled in a threaded subtree.  When
+a threaded controller is enabled inside a threaded subtree, it only
+accounts for and controls resource consumptions associated with the
+threads in the cgroup and its descendants.  All consumptions which
+aren't tied to a specific thread belong to the threaded domain cgroup.
+
+Because a threaded subtree is exempt from no internal process
+constraint, a threaded controller must be able to handle competition
+between threads in a non-leaf cgroup and its child cgroups.  Each
+threaded controller defines how such competitions are handled.
+
+
 [Un]populated Notification
 --------------------------
 
@@ -302,15 +406,15 @@ disabled if one or more children have it enabled.
 No Internal Process Constraint
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Non-root cgroups can only distribute resources to their children when
-they don't have any processes of their own.  In other words, only
-cgroups which don't contain any processes can have controllers enabled
-in their "cgroup.subtree_control" files.
+Non-root cgroups can distribute domain resources to their children
+only when they don't have any processes of their own.  In other words,
+only domain cgroups which don't contain any processes can have domain
+controllers enabled in their "cgroup.subtree_control" files.
 
-This guarantees that, when a controller is looking at the part of the
-hierarchy which has it enabled, processes are always only on the
-leaves.  This rules out situations where child cgroups compete against
-internal processes of the parent.
+This guarantees that, when a domain controller is looking at the part
+of the hierarchy which has it enabled, processes are always only on
+the leaves.  This rules out situations where child cgroups compete
+against internal processes of the parent.
 
 The root cgroup is exempt from this restriction.  Root contains
 processes and anonymous resource consumption which can't be associated
@@ -334,10 +438,10 @@ Model of Delegation
 ~~~~~~~~~~~~~~~~~~~
 
 A cgroup can be delegated in two ways.  First, to a less privileged
-user by granting write access of the directory and its "cgroup.procs"
-and "cgroup.subtree_control" files to the user.  Second, if the
-"nsdelegate" mount option is set, automatically to a cgroup namespace
-on namespace creation.
+user by granting write access of the directory and its "cgroup.procs",
+"cgroup.threads" and "cgroup.subtree_control" files to the user.
+Second, if the "nsdelegate" mount option is set, automatically to a
+cgroup namespace on namespace creation.
 
 Because the resource control interface files in a given directory
 control the distribution of the parent's resources, the delegatee
@@ -644,6 +748,29 @@ Core Interface Files
 
 All cgroup core files are prefixed with "cgroup."
 
+  cgroup.type
+
+	A read-write single value file which exists on non-root
+	cgroups.
+
+	When read, it indicates the current type of the cgroup, which
+	can be one of the following values.
+
+	- "domain" : A normal valid domain cgroup.
+
+	- "domain threaded" : A threaded domain cgroup which is
+          serving as the root of a threaded subtree.
+
+	- "domain invalid" : A cgroup which is in an invalid state.
+	  It can't be populated or have controllers enabled.  It may
+	  be allowed to become a threaded cgroup.
+
+	- "threaded" : A threaded cgroup which is a member of a
+          threaded subtree.
+
+	A cgroup can be turned into a threaded cgroup by writing
+	"threaded" to this file.
+
   cgroup.procs
 	A read-write new-line separated values file which exists on
 	all cgroups.
@@ -658,9 +785,6 @@ All cgroup core files are prefixed with "cgroup."
 	the PID to the cgroup.  The writer should match all of the
 	following conditions.
 
-	- Its euid is either root or must match either uid or suid of
-          the target process.
-
 	- It must have write access to the "cgroup.procs" file.
 
 	- It must have write access to the "cgroup.procs" file of the
@@ -669,6 +793,35 @@ All cgroup core files are prefixed with "cgroup."
 	When delegating a sub-hierarchy, write access to this file
 	should be granted along with the containing directory.
 
+	In a threaded cgroup, reading this file fails with EOPNOTSUPP
+	as all the processes belong to the thread root.  Writing is
+	supported and moves every thread of the process to the cgroup.
+
+  cgroup.threads
+	A read-write new-line separated values file which exists on
+	all cgroups.
+
+	When read, it lists the TIDs of all threads which belong to
+	the cgroup one-per-line.  The TIDs are not ordered and the
+	same TID may show up more than once if the thread got moved to
+	another cgroup and then back or the TID got recycled while
+	reading.
+
+	A TID can be written to migrate the thread associated with the
+	TID to the cgroup.  The writer should match all of the
+	following conditions.
+
+	- It must have write access to the "cgroup.threads" file.
+
+	- The cgroup that the thread is currently in must be in the
+          same resource domain as the destination cgroup.
+
+	- It must have write access to the "cgroup.procs" file of the
+	  common ancestor of the source and destination cgroups.
+
+	When delegating a sub-hierarchy, write access to this file
+	should be granted along with the containing directory.
+
   cgroup.controllers
 	A read-only space separated values file which exists on all
 	cgroups.
@@ -701,6 +854,38 @@ All cgroup core files are prefixed with "cgroup."
 		1 if the cgroup or its descendants contains any live
 		processes; otherwise, 0.
 
+  cgroup.max.descendants
+	A read-write single value files.  The default is "max".
+
+	Maximum allowed number of descent cgroups.
+	If the actual number of descendants is equal or larger,
+	an attempt to create a new cgroup in the hierarchy will fail.
+
+  cgroup.max.depth
+	A read-write single value files.  The default is "max".
+
+	Maximum allowed descent depth below the current cgroup.
+	If the actual descent depth is equal or larger,
+	an attempt to create a new child cgroup will fail.
+
+  cgroup.stat
+	A read-only flat-keyed file with the following entries:
+
+	  nr_descendants
+		Total number of visible descendant cgroups.
+
+	  nr_dying_descendants
+		Total number of dying descendant cgroups. A cgroup becomes
+		dying after being deleted by a user. The cgroup will remain
+		in dying state for some time undefined time (which can depend
+		on system load) before being completely destroyed.
+
+		A process can't enter a dying cgroup under any circumstances,
+		a dying cgroup can't revive.
+
+		A dying cgroup can consume system resources not exceeding
+		limits, which were active at the moment of cgroup deletion.
+
 
 Controllers
 ===========
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 09f4c7df1478..ade4a78a54c2 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -74,6 +74,11 @@ enum {
 	 * aren't writeable from inside the namespace.
 	 */
 	CGRP_ROOT_NS_DELEGATE	= (1 << 3),
+
+	/*
+	 * Enable cpuset controller in v1 cgroup to use v2 behavior.
+	 */
+	CGRP_ROOT_CPUSET_V2_MODE = (1 << 4),
 };
 
 /* cftype->flags */
@@ -172,6 +177,14 @@ struct css_set {
 	/* reference count */
 	refcount_t refcount;
 
+	/*
+	 * For a domain cgroup, the following points to self.  If threaded,
+	 * to the matching cset of the nearest domain ancestor.  The
+	 * dom_cset provides access to the domain cgroup and its csses to
+	 * which domain level resource consumptions should be charged.
+	 */
+	struct css_set *dom_cset;
+
 	/* the default cgroup associated with this css_set */
 	struct cgroup *dfl_cgrp;
 
@@ -200,6 +213,10 @@ struct css_set {
 	 */
 	struct list_head e_cset_node[CGROUP_SUBSYS_COUNT];
 
+	/* all threaded csets whose ->dom_cset points to this cset */
+	struct list_head threaded_csets;
+	struct list_head threaded_csets_node;
+
 	/*
 	 * List running through all cgroup groups in the same hash
 	 * slot. Protected by css_set_lock
@@ -261,13 +278,35 @@ struct cgroup {
 	 */
 	int level;
 
+	/* Maximum allowed descent tree depth */
+	int max_depth;
+
+	/*
+	 * Keep track of total numbers of visible and dying descent cgroups.
+	 * Dying cgroups are cgroups which were deleted by a user,
+	 * but are still existing because someone else is holding a reference.
+	 * max_descendants is a maximum allowed number of descent cgroups.
+	 */
+	int nr_descendants;
+	int nr_dying_descendants;
+	int max_descendants;
+
 	/*
 	 * Each non-empty css_set associated with this cgroup contributes
-	 * one to populated_cnt.  All children with non-zero popuplated_cnt
-	 * of their own contribute one.  The count is zero iff there's no
-	 * task in this cgroup or its subtree.
+	 * one to nr_populated_csets.  The counter is zero iff this cgroup
+	 * doesn't have any tasks.
+	 *
+	 * All children which have non-zero nr_populated_csets and/or
+	 * nr_populated_children of their own contribute one to either
+	 * nr_populated_domain_children or nr_populated_threaded_children
+	 * depending on their type.  Each counter is zero iff all cgroups
+	 * of the type in the subtree proper don't have any tasks.
 	 */
-	int populated_cnt;
+	int nr_populated_csets;
+	int nr_populated_domain_children;
+	int nr_populated_threaded_children;
+
+	int nr_threaded_children;	/* # of live threaded child cgroups */
 
 	struct kernfs_node *kn;		/* cgroup kernfs entry */
 	struct cgroup_file procs_file;	/* handle for "cgroup.procs" */
@@ -306,6 +345,15 @@ struct cgroup {
 	struct list_head e_csets[CGROUP_SUBSYS_COUNT];
 
 	/*
+	 * If !threaded, self.  If threaded, it points to the nearest
+	 * domain ancestor.  Inside a threaded subtree, cgroups are exempt
+	 * from process granularity and no-internal-task constraint.
+	 * Domain level resource consumptions which aren't tied to a
+	 * specific task are charged to the dom_cgrp.
+	 */
+	struct cgroup *dom_cgrp;
+
+	/*
 	 * list of pidlists, up to two for each namespace (one for procs, one
 	 * for tasks); created on demand.
 	 */
@@ -492,6 +540,18 @@ struct cgroup_subsys {
 	bool implicit_on_dfl:1;
 
 	/*
+	 * If %true, the controller, supports threaded mode on the default
+	 * hierarchy.  In a threaded subtree, both process granularity and
+	 * no-internal-process constraint are ignored and a threaded
+	 * controllers should be able to handle that.
+	 *
+	 * Note that as an implicit controller is automatically enabled on
+	 * all cgroups on the default hierarchy, it should also be
+	 * threaded.  implicit && !threaded is not supported.
+	 */
+	bool threaded: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
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 710a005c6b7a..085056e562b1 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -36,18 +36,28 @@
 #define CGROUP_WEIGHT_DFL		100
 #define CGROUP_WEIGHT_MAX		10000
 
+/* walk only threadgroup leaders */
+#define CSS_TASK_ITER_PROCS		(1U << 0)
+/* walk all threaded css_sets in the domain */
+#define CSS_TASK_ITER_THREADED		(1U << 1)
+
 /* a css_task_iter should be treated as an opaque object */
 struct css_task_iter {
 	struct cgroup_subsys		*ss;
+	unsigned int			flags;
 
 	struct list_head		*cset_pos;
 	struct list_head		*cset_head;
 
+	struct list_head		*tcset_pos;
+	struct list_head		*tcset_head;
+
 	struct list_head		*task_pos;
 	struct list_head		*tasks_head;
 	struct list_head		*mg_tasks_head;
 
 	struct css_set			*cur_cset;
+	struct css_set			*cur_dcset;
 	struct task_struct		*cur_task;
 	struct list_head		iters_node;	/* css_set->task_iters */
 };
@@ -129,7 +139,7 @@ struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset,
 struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset,
 					struct cgroup_subsys_state **dst_cssp);
 
-void css_task_iter_start(struct cgroup_subsys_state *css,
+void css_task_iter_start(struct cgroup_subsys_state *css, unsigned int flags,
 			 struct css_task_iter *it);
 struct task_struct *css_task_iter_next(struct css_task_iter *it);
 void css_task_iter_end(struct css_task_iter *it);
@@ -388,6 +398,16 @@ static inline void css_put_many(struct cgroup_subsys_state *css, unsigned int n)
 		percpu_ref_put_many(&css->refcnt, n);
 }
 
+static inline void cgroup_get(struct cgroup *cgrp)
+{
+	css_get(&cgrp->self);
+}
+
+static inline bool cgroup_tryget(struct cgroup *cgrp)
+{
+	return css_tryget(&cgrp->self);
+}
+
 static inline void cgroup_put(struct cgroup *cgrp)
 {
 	css_put(&cgrp->self);
@@ -500,6 +520,20 @@ static inline struct cgroup *task_cgroup(struct task_struct *task,
 	return task_css(task, subsys_id)->cgroup;
 }
 
+static inline struct cgroup *task_dfl_cgroup(struct task_struct *task)
+{
+	return task_css_set(task)->dfl_cgrp;
+}
+
+static inline struct cgroup *cgroup_parent(struct cgroup *cgrp)
+{
+	struct cgroup_subsys_state *parent_css = cgrp->self.parent;
+
+	if (parent_css)
+		return container_of(parent_css, struct cgroup, self);
+	return NULL;
+}
+
 /**
  * cgroup_is_descendant - test ancestry
  * @cgrp: the cgroup to be tested
@@ -537,7 +571,8 @@ static inline bool task_under_cgroup_hierarchy(struct task_struct *task,
 /* no synchronization, the result can only be used as a hint */
 static inline bool cgroup_is_populated(struct cgroup *cgrp)
 {
-	return cgrp->populated_cnt;
+	return cgrp->nr_populated_csets + cgrp->nr_populated_domain_children +
+		cgrp->nr_populated_threaded_children;
 }
 
 /* returns ino associated with a cgroup */
diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 8b4c3c2f2509..5151ff256c29 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -156,6 +156,8 @@ static inline void get_css_set(struct css_set *cset)
 
 bool cgroup_ssid_enabled(int ssid);
 bool cgroup_on_dfl(const struct cgroup *cgrp);
+bool cgroup_is_thread_root(struct cgroup *cgrp);
+bool cgroup_is_threaded(struct cgroup *cgrp);
 
 struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root);
 struct cgroup *task_cgroup_from_root(struct task_struct *task,
@@ -173,7 +175,7 @@ struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,
 			       struct cgroup_root *root, unsigned long magic,
 			       struct cgroup_namespace *ns);
 
-bool cgroup_may_migrate_to(struct cgroup *dst_cgrp);
+int cgroup_migrate_vet_dst(struct cgroup *dst_cgrp);
 void cgroup_migrate_finish(struct cgroup_mgctx *mgctx);
 void cgroup_migrate_add_src(struct css_set *src_cset, struct cgroup *dst_cgrp,
 			    struct cgroup_mgctx *mgctx);
@@ -183,10 +185,10 @@ int cgroup_migrate(struct task_struct *leader, bool threadgroup,
 
 int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 		       bool threadgroup);
-ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
-			     size_t nbytes, loff_t off, bool threadgroup);
-ssize_t cgroup_procs_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
-			   loff_t off);
+struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup)
+	__acquires(&cgroup_threadgroup_rwsem);
+void cgroup_procs_write_finish(struct task_struct *task)
+	__releases(&cgroup_threadgroup_rwsem);
 
 void cgroup_lock_and_drain_offline(struct cgroup *cgrp);
 
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 7bf4b1533f34..024085daab1a 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -99,8 +99,9 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
 	if (cgroup_on_dfl(to))
 		return -EINVAL;
 
-	if (!cgroup_may_migrate_to(to))
-		return -EBUSY;
+	ret = cgroup_migrate_vet_dst(to);
+	if (ret)
+		return ret;
 
 	mutex_lock(&cgroup_mutex);
 
@@ -121,7 +122,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
 	 * ->can_attach() fails.
 	 */
 	do {
-		css_task_iter_start(&from->self, &it);
+		css_task_iter_start(&from->self, 0, &it);
 		task = css_task_iter_next(&it);
 		if (task)
 			get_task_struct(task);
@@ -373,7 +374,7 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
 	if (!array)
 		return -ENOMEM;
 	/* now, populate the array */
-	css_task_iter_start(&cgrp->self, &it);
+	css_task_iter_start(&cgrp->self, 0, &it);
 	while ((tsk = css_task_iter_next(&it))) {
 		if (unlikely(n == length))
 			break;
@@ -510,10 +511,58 @@ static int cgroup_pidlist_show(struct seq_file *s, void *v)
 	return 0;
 }
 
-static ssize_t cgroup_tasks_write(struct kernfs_open_file *of,
-				  char *buf, size_t nbytes, loff_t off)
+static ssize_t __cgroup1_procs_write(struct kernfs_open_file *of,
+				     char *buf, size_t nbytes, loff_t off,
+				     bool threadgroup)
 {
-	return __cgroup_procs_write(of, buf, nbytes, off, false);
+	struct cgroup *cgrp;
+	struct task_struct *task;
+	const struct cred *cred, *tcred;
+	ssize_t ret;
+
+	cgrp = cgroup_kn_lock_live(of->kn, false);
+	if (!cgrp)
+		return -ENODEV;
+
+	task = cgroup_procs_write_start(buf, threadgroup);
+	ret = PTR_ERR_OR_ZERO(task);
+	if (ret)
+		goto out_unlock;
+
+	/*
+	 * Even if we're attaching all tasks in the thread group, we only
+	 * need to check permissions on one of them.
+	 */
+	cred = current_cred();
+	tcred = get_task_cred(task);
+	if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
+	    !uid_eq(cred->euid, tcred->uid) &&
+	    !uid_eq(cred->euid, tcred->suid))
+		ret = -EACCES;
+	put_cred(tcred);
+	if (ret)
+		goto out_finish;
+
+	ret = cgroup_attach_task(cgrp, task, threadgroup);
+
+out_finish:
+	cgroup_procs_write_finish(task);
+out_unlock:
+	cgroup_kn_unlock(of->kn);
+
+	return ret ?: nbytes;
+}
+
+static ssize_t cgroup1_procs_write(struct kernfs_open_file *of,
+				   char *buf, size_t nbytes, loff_t off)
+{
+	return __cgroup1_procs_write(of, buf, nbytes, off, true);
+}
+
+static ssize_t cgroup1_tasks_write(struct kernfs_open_file *of,
+				   char *buf, size_t nbytes, loff_t off)
+{
+	return __cgroup1_procs_write(of, buf, nbytes, off, false);
 }
 
 static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
@@ -592,7 +641,7 @@ struct cftype cgroup1_base_files[] = {
 		.seq_stop = cgroup_pidlist_stop,
 		.seq_show = cgroup_pidlist_show,
 		.private = CGROUP_FILE_PROCS,
-		.write = cgroup_procs_write,
+		.write = cgroup1_procs_write,
 	},
 	{
 		.name = "cgroup.clone_children",
@@ -611,7 +660,7 @@ struct cftype cgroup1_base_files[] = {
 		.seq_stop = cgroup_pidlist_stop,
 		.seq_show = cgroup_pidlist_show,
 		.private = CGROUP_FILE_TASKS,
-		.write = cgroup_tasks_write,
+		.write = cgroup1_tasks_write,
 	},
 	{
 		.name = "notify_on_release",
@@ -701,7 +750,7 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
 	}
 	rcu_read_unlock();
 
-	css_task_iter_start(&cgrp->self, &it);
+	css_task_iter_start(&cgrp->self, 0, &it);
 	while ((tsk = css_task_iter_next(&it))) {
 		switch (tsk->state) {
 		case TASK_RUNNING:
@@ -846,6 +895,8 @@ static int cgroup1_show_options(struct seq_file *seq, struct kernfs_root *kf_roo
 		seq_puts(seq, ",noprefix");
 	if (root->flags & CGRP_ROOT_XATTR)
 		seq_puts(seq, ",xattr");
+	if (root->flags & CGRP_ROOT_CPUSET_V2_MODE)
+		seq_puts(seq, ",cpuset_v2_mode");
 
 	spin_lock(&release_agent_path_lock);
 	if (strlen(root->release_agent_path))
@@ -900,6 +951,10 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 			opts->cpuset_clone_children = true;
 			continue;
 		}
+		if (!strcmp(token, "cpuset_v2_mode")) {
+			opts->flags |= CGRP_ROOT_CPUSET_V2_MODE;
+			continue;
+		}
 		if (!strcmp(token, "xattr")) {
 			opts->flags |= CGRP_ROOT_XATTR;
 			continue;
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index f64fc967a9ef..4f2196a00953 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -162,6 +162,9 @@ static u16 cgrp_dfl_inhibit_ss_mask;
 /* some controllers are implicitly enabled on the default hierarchy */
 static u16 cgrp_dfl_implicit_ss_mask;
 
+/* some controllers can be threaded on the default hierarchy */
+static u16 cgrp_dfl_threaded_ss_mask;
+
 /* The list of hierarchy roots */
 LIST_HEAD(cgroup_roots);
 static int cgroup_root_count;
@@ -316,13 +319,87 @@ static void cgroup_idr_remove(struct idr *idr, int id)
 	spin_unlock_bh(&cgroup_idr_lock);
 }
 
-static struct cgroup *cgroup_parent(struct cgroup *cgrp)
+static bool cgroup_has_tasks(struct cgroup *cgrp)
 {
-	struct cgroup_subsys_state *parent_css = cgrp->self.parent;
+	return cgrp->nr_populated_csets;
+}
 
-	if (parent_css)
-		return container_of(parent_css, struct cgroup, self);
-	return NULL;
+bool cgroup_is_threaded(struct cgroup *cgrp)
+{
+	return cgrp->dom_cgrp != cgrp;
+}
+
+/* can @cgrp host both domain and threaded children? */
+static bool cgroup_is_mixable(struct cgroup *cgrp)
+{
+	/*
+	 * Root isn't under domain level resource control exempting it from
+	 * the no-internal-process constraint, so it can serve as a thread
+	 * root and a parent of resource domains at the same time.
+	 */
+	return !cgroup_parent(cgrp);
+}
+
+/* can @cgrp become a thread root? should always be true for a thread root */
+static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
+{
+	/* mixables don't care */
+	if (cgroup_is_mixable(cgrp))
+		return true;
+
+	/* domain roots can't be nested under threaded */
+	if (cgroup_is_threaded(cgrp))
+		return false;
+
+	/* can only have either domain or threaded children */
+	if (cgrp->nr_populated_domain_children)
+		return false;
+
+	/* and no domain controllers can be enabled */
+	if (cgrp->subtree_control & ~cgrp_dfl_threaded_ss_mask)
+		return false;
+
+	return true;
+}
+
+/* is @cgrp root of a threaded subtree? */
+bool cgroup_is_thread_root(struct cgroup *cgrp)
+{
+	/* thread root should be a domain */
+	if (cgroup_is_threaded(cgrp))
+		return false;
+
+	/* a domain w/ threaded children is a thread root */
+	if (cgrp->nr_threaded_children)
+		return true;
+
+	/*
+	 * A domain which has tasks and explicit threaded controllers
+	 * enabled is a thread root.
+	 */
+	if (cgroup_has_tasks(cgrp) &&
+	    (cgrp->subtree_control & cgrp_dfl_threaded_ss_mask))
+		return true;
+
+	return false;
+}
+
+/* a domain which isn't connected to the root w/o brekage can't be used */
+static bool cgroup_is_valid_domain(struct cgroup *cgrp)
+{
+	/* the cgroup itself can be a thread root */
+	if (cgroup_is_threaded(cgrp))
+		return false;
+
+	/* but the ancestors can't be unless mixable */
+	while ((cgrp = cgroup_parent(cgrp))) {
+		if (!cgroup_is_mixable(cgrp) && cgroup_is_thread_root(cgrp))
+			return false;
+		if (cgroup_is_threaded(cgrp))
+			return false;
+	}
+
+	return true;
 }
 
 /* subsystems visibly enabled on a cgroup */
@@ -331,8 +408,14 @@ static u16 cgroup_control(struct cgroup *cgrp)
 	struct cgroup *parent = cgroup_parent(cgrp);
 	u16 root_ss_mask = cgrp->root->subsys_mask;
 
-	if (parent)
-		return parent->subtree_control;
+	if (parent) {
+		u16 ss_mask = parent->subtree_control;
+
+		/* threaded cgroups can only have threaded controllers */
+		if (cgroup_is_threaded(cgrp))
+			ss_mask &= cgrp_dfl_threaded_ss_mask;
+		return ss_mask;
+	}
 
 	if (cgroup_on_dfl(cgrp))
 		root_ss_mask &= ~(cgrp_dfl_inhibit_ss_mask |
@@ -345,8 +428,14 @@ static u16 cgroup_ss_mask(struct cgroup *cgrp)
 {
 	struct cgroup *parent = cgroup_parent(cgrp);
 
-	if (parent)
-		return parent->subtree_ss_mask;
+	if (parent) {
+		u16 ss_mask = parent->subtree_ss_mask;
+
+		/* threaded cgroups can only have threaded controllers */
+		if (cgroup_is_threaded(cgrp))
+			ss_mask &= cgrp_dfl_threaded_ss_mask;
+		return ss_mask;
+	}
 
 	return cgrp->root->subsys_mask;
 }
@@ -436,22 +525,12 @@ out_unlock:
 	return css;
 }
 
-static void __maybe_unused cgroup_get(struct cgroup *cgrp)
-{
-	css_get(&cgrp->self);
-}
-
 static void cgroup_get_live(struct cgroup *cgrp)
 {
 	WARN_ON_ONCE(cgroup_is_dead(cgrp));
 	css_get(&cgrp->self);
 }
 
-static bool cgroup_tryget(struct cgroup *cgrp)
-{
-	return css_tryget(&cgrp->self);
-}
-
 struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
 {
 	struct cgroup *cgrp = of->kn->parent->priv;
@@ -560,9 +639,11 @@ EXPORT_SYMBOL_GPL(of_css);
  */
 struct css_set init_css_set = {
 	.refcount		= REFCOUNT_INIT(1),
+	.dom_cset		= &init_css_set,
 	.tasks			= LIST_HEAD_INIT(init_css_set.tasks),
 	.mg_tasks		= LIST_HEAD_INIT(init_css_set.mg_tasks),
 	.task_iters		= LIST_HEAD_INIT(init_css_set.task_iters),
+	.threaded_csets		= LIST_HEAD_INIT(init_css_set.threaded_csets),
 	.cgrp_links		= LIST_HEAD_INIT(init_css_set.cgrp_links),
 	.mg_preload_node	= LIST_HEAD_INIT(init_css_set.mg_preload_node),
 	.mg_node		= LIST_HEAD_INIT(init_css_set.mg_node),
@@ -570,6 +651,11 @@ struct css_set init_css_set = {
 
 static int css_set_count	= 1;	/* 1 for init_css_set */
 
+static bool css_set_threaded(struct css_set *cset)
+{
+	return cset->dom_cset != cset;
+}
+
 /**
  * css_set_populated - does a css_set contain any tasks?
  * @cset: target css_set
@@ -587,39 +673,48 @@ static bool css_set_populated(struct css_set *cset)
 }
 
 /**
- * cgroup_update_populated - updated populated count of a cgroup
+ * cgroup_update_populated - update the populated count of a cgroup
  * @cgrp: the target cgroup
  * @populated: inc or dec populated count
  *
  * One of the css_sets associated with @cgrp is either getting its first
- * task or losing the last.  Update @cgrp->populated_cnt accordingly.  The
- * count is propagated towards root so that a given cgroup's populated_cnt
- * is zero iff the cgroup and all its descendants don't contain any tasks.
+ * task or losing the last.  Update @cgrp->nr_populated_* accordingly.  The
+ * count is propagated towards root so that a given cgroup's
+ * nr_populated_children is zero iff none of its descendants contain any
+ * tasks.
  *
- * @cgrp's interface file "cgroup.populated" is zero if
- * @cgrp->populated_cnt is zero and 1 otherwise.  When @cgrp->populated_cnt
- * changes from or to zero, userland is notified that the content of the
- * interface file has changed.  This can be used to detect when @cgrp and
- * its descendants become populated or empty.
+ * @cgrp's interface file "cgroup.populated" is zero if both
+ * @cgrp->nr_populated_csets and @cgrp->nr_populated_children are zero and
+ * 1 otherwise.  When the sum changes from or to zero, userland is notified
+ * that the content of the interface file has changed.  This can be used to
+ * detect when @cgrp and its descendants become populated or empty.
  */
 static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 {
+	struct cgroup *child = NULL;
+	int adj = populated ? 1 : -1;
+
 	lockdep_assert_held(&css_set_lock);
 
 	do {
-		bool trigger;
+		bool was_populated = cgroup_is_populated(cgrp);
 
-		if (populated)
-			trigger = !cgrp->populated_cnt++;
-		else
-			trigger = !--cgrp->populated_cnt;
+		if (!child) {
+			cgrp->nr_populated_csets += adj;
+		} else {
+			if (cgroup_is_threaded(child))
+				cgrp->nr_populated_threaded_children += adj;
+			else
+				cgrp->nr_populated_domain_children += adj;
+		}
 
-		if (!trigger)
+		if (was_populated == cgroup_is_populated(cgrp))
 			break;
 
 		cgroup1_check_for_release(cgrp);
 		cgroup_file_notify(&cgrp->events_file);
 
+		child = cgrp;
 		cgrp = cgroup_parent(cgrp);
 	} while (cgrp);
 }
@@ -630,7 +725,7 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
  * @populated: whether @cset is populated or depopulated
  *
  * @cset is either getting the first task or losing the last.  Update the
- * ->populated_cnt of all associated cgroups accordingly.
+ * populated counters of all associated cgroups accordingly.
  */
 static void css_set_update_populated(struct css_set *cset, bool populated)
 {
@@ -653,7 +748,7 @@ static void css_set_update_populated(struct css_set *cset, bool populated)
  * css_set, @from_cset can be NULL.  If @task is being disassociated
  * instead of moved, @to_cset can be NULL.
  *
- * This function automatically handles populated_cnt updates and
+ * This function automatically handles populated counter updates and
  * css_task_iter adjustments but the caller is responsible for managing
  * @from_cset and @to_cset's reference counts.
  */
@@ -737,6 +832,8 @@ void put_css_set_locked(struct css_set *cset)
 	if (!refcount_dec_and_test(&cset->refcount))
 		return;
 
+	WARN_ON_ONCE(!list_empty(&cset->threaded_csets));
+
 	/* This css_set is dead. unlink it and release cgroup and css refs */
 	for_each_subsys(ss, ssid) {
 		list_del(&cset->e_cset_node[ssid]);
@@ -753,6 +850,11 @@ void put_css_set_locked(struct css_set *cset)
 		kfree(link);
 	}
 
+	if (css_set_threaded(cset)) {
+		list_del(&cset->threaded_csets_node);
+		put_css_set_locked(cset->dom_cset);
+	}
+
 	kfree_rcu(cset, rcu_head);
 }
 
@@ -771,6 +873,7 @@ static bool compare_css_sets(struct css_set *cset,
 			     struct cgroup *new_cgrp,
 			     struct cgroup_subsys_state *template[])
 {
+	struct cgroup *new_dfl_cgrp;
 	struct list_head *l1, *l2;
 
 	/*
@@ -781,6 +884,16 @@ static bool compare_css_sets(struct css_set *cset,
 	if (memcmp(template, cset->subsys, sizeof(cset->subsys)))
 		return false;
 
+
+	/* @cset's domain should match the default cgroup's */
+	if (cgroup_on_dfl(new_cgrp))
+		new_dfl_cgrp = new_cgrp;
+	else
+		new_dfl_cgrp = old_cset->dfl_cgrp;
+
+	if (new_dfl_cgrp->dom_cgrp != cset->dom_cset->dfl_cgrp)
+		return false;
+
 	/*
 	 * Compare cgroup pointers in order to distinguish between
 	 * different cgroups in hierarchies.  As different cgroups may
@@ -988,9 +1101,11 @@ static struct css_set *find_css_set(struct css_set *old_cset,
 	}
 
 	refcount_set(&cset->refcount, 1);
+	cset->dom_cset = cset;
 	INIT_LIST_HEAD(&cset->tasks);
 	INIT_LIST_HEAD(&cset->mg_tasks);
 	INIT_LIST_HEAD(&cset->task_iters);
+	INIT_LIST_HEAD(&cset->threaded_csets);
 	INIT_HLIST_NODE(&cset->hlist);
 	INIT_LIST_HEAD(&cset->cgrp_links);
 	INIT_LIST_HEAD(&cset->mg_preload_node);
@@ -1028,6 +1143,28 @@ static struct css_set *find_css_set(struct css_set *old_cset,
 
 	spin_unlock_irq(&css_set_lock);
 
+	/*
+	 * If @cset should be threaded, look up the matching dom_cset and
+	 * link them up.  We first fully initialize @cset then look for the
+	 * dom_cset.  It's simpler this way and safe as @cset is guaranteed
+	 * to stay empty until we return.
+	 */
+	if (cgroup_is_threaded(cset->dfl_cgrp)) {
+		struct css_set *dcset;
+
+		dcset = find_css_set(cset, cset->dfl_cgrp->dom_cgrp);
+		if (!dcset) {
+			put_css_set(cset);
+			return NULL;
+		}
+
+		spin_lock_irq(&css_set_lock);
+		cset->dom_cset = dcset;
+		list_add_tail(&cset->threaded_csets_node,
+			      &dcset->threaded_csets);
+		spin_unlock_irq(&css_set_lock);
+	}
+
 	return cset;
 }
 
@@ -1155,6 +1292,8 @@ static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
 
 	if (cset == &init_css_set) {
 		res = &root->cgrp;
+	} else if (root == &cgrp_dfl_root) {
+		res = cset->dfl_cgrp;
 	} else {
 		struct cgrp_cset_link *link;
 
@@ -1670,6 +1809,9 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
 	mutex_init(&cgrp->pidlist_mutex);
 	cgrp->self.cgroup = cgrp;
 	cgrp->self.flags |= CSS_ONLINE;
+	cgrp->dom_cgrp = cgrp;
+	cgrp->max_descendants = INT_MAX;
+	cgrp->max_depth = INT_MAX;
 
 	for_each_subsys(ss, ssid)
 		INIT_LIST_HEAD(&cgrp->e_csets[ssid]);
@@ -2172,17 +2314,40 @@ out_release_tset:
 }
 
 /**
- * cgroup_may_migrate_to - verify whether a cgroup can be migration destination
+ * cgroup_migrate_vet_dst - verify whether a cgroup can be migration destination
  * @dst_cgrp: destination cgroup to test
  *
- * On the default hierarchy, except for the root, subtree_control must be
- * zero for migration destination cgroups with tasks so that child cgroups
- * don't compete against tasks.
+ * On the default hierarchy, except for the mixable, (possible) thread root
+ * and threaded cgroups, subtree_control must be zero for migration
+ * destination cgroups with tasks so that child cgroups don't compete
+ * against tasks.
  */
-bool cgroup_may_migrate_to(struct cgroup *dst_cgrp)
+int cgroup_migrate_vet_dst(struct cgroup *dst_cgrp)
 {
-	return !cgroup_on_dfl(dst_cgrp) || !cgroup_parent(dst_cgrp) ||
-		!dst_cgrp->subtree_control;
+	/* v1 doesn't have any restriction */
+	if (!cgroup_on_dfl(dst_cgrp))
+		return 0;
+
+	/* verify @dst_cgrp can host resources */
+	if (!cgroup_is_valid_domain(dst_cgrp->dom_cgrp))
+		return -EOPNOTSUPP;
+
+	/* mixables don't care */
+	if (cgroup_is_mixable(dst_cgrp))
+		return 0;
+
+	/*
+	 * If @dst_cgrp is already or can become a thread root or is
+	 * threaded, it doesn't matter.
+	 */
+	if (cgroup_can_be_thread_root(dst_cgrp) || cgroup_is_threaded(dst_cgrp))
+		return 0;
+
+	/* apply no-internal-process constraint */
+	if (dst_cgrp->subtree_control)
+		return -EBUSY;
+
+	return 0;
 }
 
 /**
@@ -2387,8 +2552,9 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 	struct task_struct *task;
 	int ret;
 
-	if (!cgroup_may_migrate_to(dst_cgrp))
-		return -EBUSY;
+	ret = cgroup_migrate_vet_dst(dst_cgrp);
+	if (ret)
+		return ret;
 
 	/* look up all src csets */
 	spin_lock_irq(&css_set_lock);
@@ -2415,96 +2581,23 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 	return ret;
 }
 
-static int cgroup_procs_write_permission(struct task_struct *task,
-					 struct cgroup *dst_cgrp,
-					 struct kernfs_open_file *of)
-{
-	struct super_block *sb = of->file->f_path.dentry->d_sb;
-	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
-	struct cgroup *root_cgrp = ns->root_cset->dfl_cgrp;
-	struct cgroup *src_cgrp, *com_cgrp;
-	struct inode *inode;
-	int ret;
-
-	if (!cgroup_on_dfl(dst_cgrp)) {
-		const struct cred *cred = current_cred();
-		const struct cred *tcred = get_task_cred(task);
-
-		/*
-		 * even if we're attaching all tasks in the thread group,
-		 * we only need to check permissions on one of them.
-		 */
-		if (uid_eq(cred->euid, GLOBAL_ROOT_UID) ||
-		    uid_eq(cred->euid, tcred->uid) ||
-		    uid_eq(cred->euid, tcred->suid))
-			ret = 0;
-		else
-			ret = -EACCES;
-
-		put_cred(tcred);
-		return ret;
-	}
-
-	/* find the source cgroup */
-	spin_lock_irq(&css_set_lock);
-	src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
-	spin_unlock_irq(&css_set_lock);
-
-	/* and the common ancestor */
-	com_cgrp = src_cgrp;
-	while (!cgroup_is_descendant(dst_cgrp, com_cgrp))
-		com_cgrp = cgroup_parent(com_cgrp);
-
-	/* %current should be authorized to migrate to the common ancestor */
-	inode = kernfs_get_inode(sb, com_cgrp->procs_file.kn);
-	if (!inode)
-		return -ENOMEM;
-
-	ret = inode_permission(inode, MAY_WRITE);
-	iput(inode);
-	if (ret)
-		return ret;
-
-	/*
-	 * If namespaces are delegation boundaries, %current must be able
-	 * to see both source and destination cgroups from its namespace.
-	 */
-	if ((cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE) &&
-	    (!cgroup_is_descendant(src_cgrp, root_cgrp) ||
-	     !cgroup_is_descendant(dst_cgrp, root_cgrp)))
-		return -ENOENT;
-
-	return 0;
-}
-
-/*
- * Find the task_struct of the task to attach by vpid and pass it along to the
- * function to attach either it or all tasks in its threadgroup. Will lock
- * cgroup_mutex and threadgroup.
- */
-ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
-			     size_t nbytes, loff_t off, bool threadgroup)
+struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup)
+	__acquires(&cgroup_threadgroup_rwsem)
 {
 	struct task_struct *tsk;
-	struct cgroup_subsys *ss;
-	struct cgroup *cgrp;
 	pid_t pid;
-	int ssid, ret;
 
 	if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
-		return -EINVAL;
-
-	cgrp = cgroup_kn_lock_live(of->kn, false);
-	if (!cgrp)
-		return -ENODEV;
+		return ERR_PTR(-EINVAL);
 
 	percpu_down_write(&cgroup_threadgroup_rwsem);
+
 	rcu_read_lock();
 	if (pid) {
 		tsk = find_task_by_vpid(pid);
 		if (!tsk) {
-			ret = -ESRCH;
-			goto out_unlock_rcu;
+			tsk = ERR_PTR(-ESRCH);
+			goto out_unlock_threadgroup;
 		}
 	} else {
 		tsk = current;
@@ -2520,35 +2613,33 @@ ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
 	 * cgroup with no rt_runtime allocated.  Just say no.
 	 */
 	if (tsk->no_cgroup_migration || (tsk->flags & PF_NO_SETAFFINITY)) {
-		ret = -EINVAL;
-		goto out_unlock_rcu;
+		tsk = ERR_PTR(-EINVAL);
+		goto out_unlock_threadgroup;
 	}
 
 	get_task_struct(tsk);
+	goto out_unlock_rcu;
+
+out_unlock_threadgroup:
+	percpu_up_write(&cgroup_threadgroup_rwsem);
+out_unlock_rcu:
 	rcu_read_unlock();
+	return tsk;
+}
 
-	ret = cgroup_procs_write_permission(tsk, cgrp, of);
-	if (!ret)
-		ret = cgroup_attach_task(cgrp, tsk, threadgroup);
+void cgroup_procs_write_finish(struct task_struct *task)
+	__releases(&cgroup_threadgroup_rwsem)
+{
+	struct cgroup_subsys *ss;
+	int ssid;
 
-	put_task_struct(tsk);
-	goto out_unlock_threadgroup;
+	/* release reference from cgroup_procs_write_start() */
+	put_task_struct(task);
 
-out_unlock_rcu:
-	rcu_read_unlock();
-out_unlock_threadgroup:
 	percpu_up_write(&cgroup_threadgroup_rwsem);
 	for_each_subsys(ss, ssid)
 		if (ss->post_attach)
 			ss->post_attach();
-	cgroup_kn_unlock(of->kn);
-	return ret ?: nbytes;
-}
-
-ssize_t cgroup_procs_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
-			   loff_t off)
-{
-	return __cgroup_procs_write(of, buf, nbytes, off, true);
 }
 
 static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)
@@ -2891,6 +2982,46 @@ static void cgroup_finalize_control(struct cgroup *cgrp, int ret)
 	cgroup_apply_control_disable(cgrp);
 }
 
+static int cgroup_vet_subtree_control_enable(struct cgroup *cgrp, u16 enable)
+{
+	u16 domain_enable = enable & ~cgrp_dfl_threaded_ss_mask;
+
+	/* if nothing is getting enabled, nothing to worry about */
+	if (!enable)
+		return 0;
+
+	/* can @cgrp host any resources? */
+	if (!cgroup_is_valid_domain(cgrp->dom_cgrp))
+		return -EOPNOTSUPP;
+
+	/* mixables don't care */
+	if (cgroup_is_mixable(cgrp))
+		return 0;
+
+	if (domain_enable) {
+		/* can't enable domain controllers inside a thread subtree */
+		if (cgroup_is_thread_root(cgrp) || cgroup_is_threaded(cgrp))
+			return -EOPNOTSUPP;
+	} else {
+		/*
+		 * Threaded controllers can handle internal competitions
+		 * and are always allowed inside a (prospective) thread
+		 * subtree.
+		 */
+		if (cgroup_can_be_thread_root(cgrp) || cgroup_is_threaded(cgrp))
+			return 0;
+	}
+
+	/*
+	 * Controllers can't be enabled for a cgroup with tasks to avoid
+	 * child cgroups competing against tasks.
+	 */
+	if (cgroup_has_tasks(cgrp))
+		return -EBUSY;
+
+	return 0;
+}
+
 /* change the enabled child controllers for a cgroup in the default hierarchy */
 static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 					    char *buf, size_t nbytes,
@@ -2966,33 +3097,9 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 		goto out_unlock;
 	}
 
-	/*
-	 * Except for the root, subtree_control must be zero for a cgroup
-	 * with tasks so that child cgroups don't compete against tasks.
-	 */
-	if (enable && cgroup_parent(cgrp)) {
-		struct cgrp_cset_link *link;
-
-		/*
-		 * Because namespaces pin csets too, @cgrp->cset_links
-		 * might not be empty even when @cgrp is empty.  Walk and
-		 * verify each cset.
-		 */
-		spin_lock_irq(&css_set_lock);
-
-		ret = 0;
-		list_for_each_entry(link, &cgrp->cset_links, cset_link) {
-			if (css_set_populated(link->cset)) {
-				ret = -EBUSY;
-				break;
-			}
-		}
-
-		spin_unlock_irq(&css_set_lock);
-
-		if (ret)
-			goto out_unlock;
-	}
+	ret = cgroup_vet_subtree_control_enable(cgrp, enable);
+	if (ret)
+		goto out_unlock;
 
 	/* save and update control masks and prepare csses */
 	cgroup_save_control(cgrp);
@@ -3011,6 +3118,172 @@ out_unlock:
 	return ret ?: nbytes;
 }
 
+/**
+ * cgroup_enable_threaded - make @cgrp threaded
+ * @cgrp: the target cgroup
+ *
+ * Called when "threaded" is written to the cgroup.type interface file and
+ * tries to make @cgrp threaded and join the parent's resource domain.
+ * This function is never called on the root cgroup as cgroup.type doesn't
+ * exist on it.
+ */
+static int cgroup_enable_threaded(struct cgroup *cgrp)
+{
+	struct cgroup *parent = cgroup_parent(cgrp);
+	struct cgroup *dom_cgrp = parent->dom_cgrp;
+	int ret;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	/* noop if already threaded */
+	if (cgroup_is_threaded(cgrp))
+		return 0;
+
+	/* we're joining the parent's domain, ensure its validity */
+	if (!cgroup_is_valid_domain(dom_cgrp) ||
+	    !cgroup_can_be_thread_root(dom_cgrp))
+		return -EOPNOTSUPP;
+
+	/*
+	 * The following shouldn't cause actual migrations and should
+	 * always succeed.
+	 */
+	cgroup_save_control(cgrp);
+
+	cgrp->dom_cgrp = dom_cgrp;
+	ret = cgroup_apply_control(cgrp);
+	if (!ret)
+		parent->nr_threaded_children++;
+	else
+		cgrp->dom_cgrp = cgrp;
+
+	cgroup_finalize_control(cgrp, ret);
+	return ret;
+}
+
+static int cgroup_type_show(struct seq_file *seq, void *v)
+{
+	struct cgroup *cgrp = seq_css(seq)->cgroup;
+
+	if (cgroup_is_threaded(cgrp))
+		seq_puts(seq, "threaded\n");
+	else if (!cgroup_is_valid_domain(cgrp))
+		seq_puts(seq, "domain invalid\n");
+	else if (cgroup_is_thread_root(cgrp))
+		seq_puts(seq, "domain threaded\n");
+	else
+		seq_puts(seq, "domain\n");
+
+	return 0;
+}
+
+static ssize_t cgroup_type_write(struct kernfs_open_file *of, char *buf,
+				 size_t nbytes, loff_t off)
+{
+	struct cgroup *cgrp;
+	int ret;
+
+	/* only switching to threaded mode is supported */
+	if (strcmp(strstrip(buf), "threaded"))
+		return -EINVAL;
+
+	cgrp = cgroup_kn_lock_live(of->kn, false);
+	if (!cgrp)
+		return -ENOENT;
+
+	/* threaded can only be enabled */
+	ret = cgroup_enable_threaded(cgrp);
+
+	cgroup_kn_unlock(of->kn);
+	return ret ?: nbytes;
+}
+
+static int cgroup_max_descendants_show(struct seq_file *seq, void *v)
+{
+	struct cgroup *cgrp = seq_css(seq)->cgroup;
+	int descendants = READ_ONCE(cgrp->max_descendants);
+
+	if (descendants == INT_MAX)
+		seq_puts(seq, "max\n");
+	else
+		seq_printf(seq, "%d\n", descendants);
+
+	return 0;
+}
+
+static ssize_t cgroup_max_descendants_write(struct kernfs_open_file *of,
+					   char *buf, size_t nbytes, loff_t off)
+{
+	struct cgroup *cgrp;
+	int descendants;
+	ssize_t ret;
+
+	buf = strstrip(buf);
+	if (!strcmp(buf, "max")) {
+		descendants = INT_MAX;
+	} else {
+		ret = kstrtoint(buf, 0, &descendants);
+		if (ret)
+			return ret;
+	}
+
+	if (descendants < 0)
+		return -ERANGE;
+
+	cgrp = cgroup_kn_lock_live(of->kn, false);
+	if (!cgrp)
+		return -ENOENT;
+
+	cgrp->max_descendants = descendants;
+
+	cgroup_kn_unlock(of->kn);
+
+	return nbytes;
+}
+
+static int cgroup_max_depth_show(struct seq_file *seq, void *v)
+{
+	struct cgroup *cgrp = seq_css(seq)->cgroup;
+	int depth = READ_ONCE(cgrp->max_depth);
+
+	if (depth == INT_MAX)
+		seq_puts(seq, "max\n");
+	else
+		seq_printf(seq, "%d\n", depth);
+
+	return 0;
+}
+
+static ssize_t cgroup_max_depth_write(struct kernfs_open_file *of,
+				      char *buf, size_t nbytes, loff_t off)
+{
+	struct cgroup *cgrp;
+	ssize_t ret;
+	int depth;
+
+	buf = strstrip(buf);
+	if (!strcmp(buf, "max")) {
+		depth = INT_MAX;
+	} else {
+		ret = kstrtoint(buf, 0, &depth);
+		if (ret)
+			return ret;
+	}
+
+	if (depth < 0)
+		return -ERANGE;
+
+	cgrp = cgroup_kn_lock_live(of->kn, false);
+	if (!cgrp)
+		return -ENOENT;
+
+	cgrp->max_depth = depth;
+
+	cgroup_kn_unlock(of->kn);
+
+	return nbytes;
+}
+
 static int cgroup_events_show(struct seq_file *seq, void *v)
 {
 	seq_printf(seq, "populated %d\n",
@@ -3018,6 +3291,18 @@ static int cgroup_events_show(struct seq_file *seq, void *v)
 	return 0;
 }
 
+static int cgroup_stat_show(struct seq_file *seq, void *v)
+{
+	struct cgroup *cgroup = seq_css(seq)->cgroup;
+
+	seq_printf(seq, "nr_descendants %d\n",
+		   cgroup->nr_descendants);
+	seq_printf(seq, "nr_dying_descendants %d\n",
+		   cgroup->nr_dying_descendants);
+
+	return 0;
+}
+
 static int cgroup_file_open(struct kernfs_open_file *of)
 {
 	struct cftype *cft = of->kn->priv;
@@ -3234,7 +3519,6 @@ restart:
 
 static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
 {
-	LIST_HEAD(pending);
 	struct cgroup_subsys *ss = cfts[0].ss;
 	struct cgroup *root = &ss->root->cgrp;
 	struct cgroup_subsys_state *css;
@@ -3659,6 +3943,58 @@ bool css_has_online_children(struct cgroup_subsys_state *css)
 	return ret;
 }
 
+static struct css_set *css_task_iter_next_css_set(struct css_task_iter *it)
+{
+	struct list_head *l;
+	struct cgrp_cset_link *link;
+	struct css_set *cset;
+
+	lockdep_assert_held(&css_set_lock);
+
+	/* find the next threaded cset */
+	if (it->tcset_pos) {
+		l = it->tcset_pos->next;
+
+		if (l != it->tcset_head) {
+			it->tcset_pos = l;
+			return container_of(l, struct css_set,
+					    threaded_csets_node);
+		}
+
+		it->tcset_pos = NULL;
+	}
+
+	/* find the next cset */
+	l = it->cset_pos;
+	l = l->next;
+	if (l == it->cset_head) {
+		it->cset_pos = NULL;
+		return NULL;
+	}
+
+	if (it->ss) {
+		cset = container_of(l, struct css_set, e_cset_node[it->ss->id]);
+	} else {
+		link = list_entry(l, struct cgrp_cset_link, cset_link);
+		cset = link->cset;
+	}
+
+	it->cset_pos = l;
+
+	/* initialize threaded css_set walking */
+	if (it->flags & CSS_TASK_ITER_THREADED) {
+		if (it->cur_dcset)
+			put_css_set_locked(it->cur_dcset);
+		it->cur_dcset = cset;
+		get_css_set(cset);
+
+		it->tcset_head = &cset->threaded_csets;
+		it->tcset_pos = &cset->threaded_csets;
+	}
+
+	return cset;
+}
+
 /**
  * css_task_iter_advance_css_set - advance a task itererator to the next css_set
  * @it: the iterator to advance
@@ -3667,32 +4003,19 @@ bool css_has_online_children(struct cgroup_subsys_state *css)
  */
 static void css_task_iter_advance_css_set(struct css_task_iter *it)
 {
-	struct list_head *l = it->cset_pos;
-	struct cgrp_cset_link *link;
 	struct css_set *cset;
 
 	lockdep_assert_held(&css_set_lock);
 
 	/* Advance to the next non-empty css_set */
 	do {
-		l = l->next;
-		if (l == it->cset_head) {
-			it->cset_pos = NULL;
+		cset = css_task_iter_next_css_set(it);
+		if (!cset) {
 			it->task_pos = NULL;
 			return;
 		}
-
-		if (it->ss) {
-			cset = container_of(l, struct css_set,
-					    e_cset_node[it->ss->id]);
-		} else {
-			link = list_entry(l, struct cgrp_cset_link, cset_link);
-			cset = link->cset;
-		}
 	} while (!css_set_populated(cset));
 
-	it->cset_pos = l;
-
 	if (!list_empty(&cset->tasks))
 		it->task_pos = cset->tasks.next;
 	else
@@ -3732,6 +4055,7 @@ static void css_task_iter_advance(struct css_task_iter *it)
 	lockdep_assert_held(&css_set_lock);
 	WARN_ON_ONCE(!l);
 
+repeat:
 	/*
 	 * Advance iterator to find next entry.  cset->tasks is consumed
 	 * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
@@ -3746,11 +4070,18 @@ static void css_task_iter_advance(struct css_task_iter *it)
 		css_task_iter_advance_css_set(it);
 	else
 		it->task_pos = l;
+
+	/* if PROCS, skip over tasks which aren't group leaders */
+	if ((it->flags & CSS_TASK_ITER_PROCS) && it->task_pos &&
+	    !thread_group_leader(list_entry(it->task_pos, struct task_struct,
+					    cg_list)))
+		goto repeat;
 }
 
 /**
  * css_task_iter_start - initiate task iteration
  * @css: the css to walk tasks of
+ * @flags: CSS_TASK_ITER_* flags
  * @it: the task iterator to use
  *
  * Initiate iteration through the tasks of @css.  The caller can call
@@ -3758,7 +4089,7 @@ static void css_task_iter_advance(struct css_task_iter *it)
  * returns NULL.  On completion of iteration, css_task_iter_end() must be
  * called.
  */
-void css_task_iter_start(struct cgroup_subsys_state *css,
+void css_task_iter_start(struct cgroup_subsys_state *css, unsigned int flags,
 			 struct css_task_iter *it)
 {
 	/* no one should try to iterate before mounting cgroups */
@@ -3769,6 +4100,7 @@ void css_task_iter_start(struct cgroup_subsys_state *css,
 	spin_lock_irq(&css_set_lock);
 
 	it->ss = css->ss;
+	it->flags = flags;
 
 	if (it->ss)
 		it->cset_pos = &css->cgroup->e_csets[css->ss->id];
@@ -3826,6 +4158,9 @@ void css_task_iter_end(struct css_task_iter *it)
 		spin_unlock_irq(&css_set_lock);
 	}
 
+	if (it->cur_dcset)
+		put_css_set(it->cur_dcset);
+
 	if (it->cur_task)
 		put_task_struct(it->cur_task);
 }
@@ -3842,16 +4177,12 @@ static void *cgroup_procs_next(struct seq_file *s, void *v, loff_t *pos)
 {
 	struct kernfs_open_file *of = s->private;
 	struct css_task_iter *it = of->priv;
-	struct task_struct *task;
 
-	do {
-		task = css_task_iter_next(it);
-	} while (task && !thread_group_leader(task));
-
-	return task;
+	return css_task_iter_next(it);
 }
 
-static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
+static void *__cgroup_procs_start(struct seq_file *s, loff_t *pos,
+				  unsigned int iter_flags)
 {
 	struct kernfs_open_file *of = s->private;
 	struct cgroup *cgrp = seq_css(s)->cgroup;
@@ -3869,24 +4200,169 @@ static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
 		if (!it)
 			return ERR_PTR(-ENOMEM);
 		of->priv = it;
-		css_task_iter_start(&cgrp->self, it);
+		css_task_iter_start(&cgrp->self, iter_flags, it);
 	} else if (!(*pos)++) {
 		css_task_iter_end(it);
-		css_task_iter_start(&cgrp->self, it);
+		css_task_iter_start(&cgrp->self, iter_flags, it);
 	}
 
 	return cgroup_procs_next(s, NULL, NULL);
 }
 
+static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
+{
+	struct cgroup *cgrp = seq_css(s)->cgroup;
+
+	/*
+	 * All processes of a threaded subtree belong to the domain cgroup
+	 * of the subtree.  Only threads can be distributed across the
+	 * subtree.  Reject reads on cgroup.procs in the subtree proper.
+	 * They're always empty anyway.
+	 */
+	if (cgroup_is_threaded(cgrp))
+		return ERR_PTR(-EOPNOTSUPP);
+
+	return __cgroup_procs_start(s, pos, CSS_TASK_ITER_PROCS |
+					    CSS_TASK_ITER_THREADED);
+}
+
 static int cgroup_procs_show(struct seq_file *s, void *v)
 {
-	seq_printf(s, "%d\n", task_tgid_vnr(v));
+	seq_printf(s, "%d\n", task_pid_vnr(v));
 	return 0;
 }
 
+static int cgroup_procs_write_permission(struct cgroup *src_cgrp,
+					 struct cgroup *dst_cgrp,
+					 struct super_block *sb)
+{
+	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
+	struct cgroup *com_cgrp = src_cgrp;
+	struct inode *inode;
+	int ret;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	/* find the common ancestor */
+	while (!cgroup_is_descendant(dst_cgrp, com_cgrp))
+		com_cgrp = cgroup_parent(com_cgrp);
+
+	/* %current should be authorized to migrate to the common ancestor */
+	inode = kernfs_get_inode(sb, com_cgrp->procs_file.kn);
+	if (!inode)
+		return -ENOMEM;
+
+	ret = inode_permission(inode, MAY_WRITE);
+	iput(inode);
+	if (ret)
+		return ret;
+
+	/*
+	 * If namespaces are delegation boundaries, %current must be able
+	 * to see both source and destination cgroups from its namespace.
+	 */
+	if ((cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE) &&
+	    (!cgroup_is_descendant(src_cgrp, ns->root_cset->dfl_cgrp) ||
+	     !cgroup_is_descendant(dst_cgrp, ns->root_cset->dfl_cgrp)))
+		return -ENOENT;
+
+	return 0;
+}
+
+static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
+				  char *buf, size_t nbytes, loff_t off)
+{
+	struct cgroup *src_cgrp, *dst_cgrp;
+	struct task_struct *task;
+	ssize_t ret;
+
+	dst_cgrp = cgroup_kn_lock_live(of->kn, false);
+	if (!dst_cgrp)
+		return -ENODEV;
+
+	task = cgroup_procs_write_start(buf, true);
+	ret = PTR_ERR_OR_ZERO(task);
+	if (ret)
+		goto out_unlock;
+
+	/* find the source cgroup */
+	spin_lock_irq(&css_set_lock);
+	src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
+	spin_unlock_irq(&css_set_lock);
+
+	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
+					    of->file->f_path.dentry->d_sb);
+	if (ret)
+		goto out_finish;
+
+	ret = cgroup_attach_task(dst_cgrp, task, true);
+
+out_finish:
+	cgroup_procs_write_finish(task);
+out_unlock:
+	cgroup_kn_unlock(of->kn);
+
+	return ret ?: nbytes;
+}
+
+static void *cgroup_threads_start(struct seq_file *s, loff_t *pos)
+{
+	return __cgroup_procs_start(s, pos, 0);
+}
+
+static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
+				    char *buf, size_t nbytes, loff_t off)
+{
+	struct cgroup *src_cgrp, *dst_cgrp;
+	struct task_struct *task;
+	ssize_t ret;
+
+	buf = strstrip(buf);
+
+	dst_cgrp = cgroup_kn_lock_live(of->kn, false);
+	if (!dst_cgrp)
+		return -ENODEV;
+
+	task = cgroup_procs_write_start(buf, false);
+	ret = PTR_ERR_OR_ZERO(task);
+	if (ret)
+		goto out_unlock;
+
+	/* find the source cgroup */
+	spin_lock_irq(&css_set_lock);
+	src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
+	spin_unlock_irq(&css_set_lock);
+
+	/* thread migrations follow the cgroup.procs delegation rule */
+	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
+					    of->file->f_path.dentry->d_sb);
+	if (ret)
+		goto out_finish;
+
+	/* and must be contained in the same domain */
+	ret = -EOPNOTSUPP;
+	if (src_cgrp->dom_cgrp != dst_cgrp->dom_cgrp)
+		goto out_finish;
+
+	ret = cgroup_attach_task(dst_cgrp, task, false);
+
+out_finish:
+	cgroup_procs_write_finish(task);
+out_unlock:
+	cgroup_kn_unlock(of->kn);
+
+	return ret ?: nbytes;
+}
+
 /* cgroup core interface files for the default hierarchy */
 static struct cftype cgroup_base_files[] = {
 	{
+		.name = "cgroup.type",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.seq_show = cgroup_type_show,
+		.write = cgroup_type_write,
+	},
+	{
 		.name = "cgroup.procs",
 		.flags = CFTYPE_NS_DELEGATABLE,
 		.file_offset = offsetof(struct cgroup, procs_file),
@@ -3897,6 +4373,14 @@ static struct cftype cgroup_base_files[] = {
 		.write = cgroup_procs_write,
 	},
 	{
+		.name = "cgroup.threads",
+		.release = cgroup_procs_release,
+		.seq_start = cgroup_threads_start,
+		.seq_next = cgroup_procs_next,
+		.seq_show = cgroup_procs_show,
+		.write = cgroup_threads_write,
+	},
+	{
 		.name = "cgroup.controllers",
 		.seq_show = cgroup_controllers_show,
 	},
@@ -3912,6 +4396,20 @@ static struct cftype cgroup_base_files[] = {
 		.file_offset = offsetof(struct cgroup, events_file),
 		.seq_show = cgroup_events_show,
 	},
+	{
+		.name = "cgroup.max.descendants",
+		.seq_show = cgroup_max_descendants_show,
+		.write = cgroup_max_descendants_write,
+	},
+	{
+		.name = "cgroup.max.depth",
+		.seq_show = cgroup_max_depth_show,
+		.write = cgroup_max_depth_write,
+	},
+	{
+		.name = "cgroup.stat",
+		.seq_show = cgroup_stat_show,
+	},
 	{ }	/* terminate */
 };
 
@@ -4011,9 +4509,15 @@ static void css_release_work_fn(struct work_struct *work)
 		if (ss->css_released)
 			ss->css_released(css);
 	} else {
+		struct cgroup *tcgrp;
+
 		/* cgroup release path */
 		trace_cgroup_release(cgrp);
 
+		for (tcgrp = cgroup_parent(cgrp); tcgrp;
+		     tcgrp = cgroup_parent(tcgrp))
+			tcgrp->nr_dying_descendants--;
+
 		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
 		cgrp->id = -1;
 
@@ -4209,9 +4713,13 @@ static struct cgroup *cgroup_create(struct cgroup *parent)
 	cgrp->root = root;
 	cgrp->level = level;
 
-	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp))
+	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
 		cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
 
+		if (tcgrp != cgrp)
+			tcgrp->nr_descendants++;
+	}
+
 	if (notify_on_release(parent))
 		set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
 
@@ -4252,6 +4760,29 @@ out_free_cgrp:
 	return ERR_PTR(ret);
 }
 
+static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
+{
+	struct cgroup *cgroup;
+	int ret = false;
+	int level = 1;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	for (cgroup = parent; cgroup; cgroup = cgroup_parent(cgroup)) {
+		if (cgroup->nr_descendants >= cgroup->max_descendants)
+			goto fail;
+
+		if (level > cgroup->max_depth)
+			goto fail;
+
+		level++;
+	}
+
+	ret = true;
+fail:
+	return ret;
+}
+
 int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 {
 	struct cgroup *parent, *cgrp;
@@ -4266,6 +4797,11 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 	if (!parent)
 		return -ENODEV;
 
+	if (!cgroup_check_hierarchy_limits(parent)) {
+		ret = -EAGAIN;
+		goto out_unlock;
+	}
+
 	cgrp = cgroup_create(parent);
 	if (IS_ERR(cgrp)) {
 		ret = PTR_ERR(cgrp);
@@ -4417,6 +4953,7 @@ static void kill_css(struct cgroup_subsys_state *css)
 static int cgroup_destroy_locked(struct cgroup *cgrp)
 	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
 {
+	struct cgroup *tcgrp, *parent = cgroup_parent(cgrp);
 	struct cgroup_subsys_state *css;
 	struct cgrp_cset_link *link;
 	int ssid;
@@ -4461,7 +4998,15 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	 */
 	kernfs_remove(cgrp->kn);
 
-	cgroup1_check_for_release(cgroup_parent(cgrp));
+	if (parent && cgroup_is_threaded(cgrp))
+		parent->nr_threaded_children--;
+
+	for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) {
+		tcgrp->nr_descendants--;
+		tcgrp->nr_dying_descendants++;
+	}
+
+	cgroup1_check_for_release(parent);
 
 	/* put the base reference */
 	percpu_ref_kill(&cgrp->self.refcnt);
@@ -4656,11 +5201,17 @@ int __init cgroup_init(void)
 
 		cgrp_dfl_root.subsys_mask |= 1 << ss->id;
 
+		/* implicit controllers must be threaded too */
+		WARN_ON(ss->implicit_on_dfl && !ss->threaded);
+
 		if (ss->implicit_on_dfl)
 			cgrp_dfl_implicit_ss_mask |= 1 << ss->id;
 		else if (!ss->dfl_cftypes)
 			cgrp_dfl_inhibit_ss_mask |= 1 << ss->id;
 
+		if (ss->threaded)
+			cgrp_dfl_threaded_ss_mask |= 1 << ss->id;
+
 		if (ss->dfl_cftypes == ss->legacy_cftypes) {
 			WARN_ON(cgroup_add_cftypes(ss, ss->dfl_cftypes));
 		} else {
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e7485786db9b..67230ecf2ce1 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -301,6 +301,16 @@ static DECLARE_WORK(cpuset_hotplug_work, cpuset_hotplug_workfn);
 static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
 
 /*
+ * Cgroup v2 behavior is used when on default hierarchy or the
+ * cgroup_v2_mode flag is set.
+ */
+static inline bool is_in_v2_mode(void)
+{
+	return cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
+	      (cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE);
+}
+
+/*
  * This is ugly, but preserves the userspace API for existing cpuset
  * users. If someone tries to mount the "cpuset" filesystem, we
  * silently switch it to mount "cgroup" instead
@@ -490,8 +500,7 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
 
 	/* On legacy hiearchy, we must be a subset of our parent cpuset. */
 	ret = -EACCES;
-	if (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
-	    !is_cpuset_subset(trial, par))
+	if (!is_in_v2_mode() && !is_cpuset_subset(trial, par))
 		goto out;
 
 	/*
@@ -870,7 +879,7 @@ static void update_tasks_cpumask(struct cpuset *cs)
 	struct css_task_iter it;
 	struct task_struct *task;
 
-	css_task_iter_start(&cs->css, &it);
+	css_task_iter_start(&cs->css, 0, &it);
 	while ((task = css_task_iter_next(&it)))
 		set_cpus_allowed_ptr(task, cs->effective_cpus);
 	css_task_iter_end(&it);
@@ -904,8 +913,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
 		 * If it becomes empty, inherit the effective mask of the
 		 * parent, which is guaranteed to have some CPUs.
 		 */
-		if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
-		    cpumask_empty(new_cpus))
+		if (is_in_v2_mode() && cpumask_empty(new_cpus))
 			cpumask_copy(new_cpus, parent->effective_cpus);
 
 		/* Skip the whole subtree if the cpumask remains the same. */
@@ -922,7 +930,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
 		cpumask_copy(cp->effective_cpus, new_cpus);
 		spin_unlock_irq(&callback_lock);
 
-		WARN_ON(!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
+		WARN_ON(!is_in_v2_mode() &&
 			!cpumask_equal(cp->cpus_allowed, cp->effective_cpus));
 
 		update_tasks_cpumask(cp);
@@ -1100,7 +1108,7 @@ static void update_tasks_nodemask(struct cpuset *cs)
 	 * It's ok if we rebind the same mm twice; mpol_rebind_mm()
 	 * is idempotent.  Also migrate pages in each mm to new nodes.
 	 */
-	css_task_iter_start(&cs->css, &it);
+	css_task_iter_start(&cs->css, 0, &it);
 	while ((task = css_task_iter_next(&it))) {
 		struct mm_struct *mm;
 		bool migrate;
@@ -1158,8 +1166,7 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
 		 * If it becomes empty, inherit the effective mask of the
 		 * parent, which is guaranteed to have some MEMs.
 		 */
-		if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
-		    nodes_empty(*new_mems))
+		if (is_in_v2_mode() && nodes_empty(*new_mems))
 			*new_mems = parent->effective_mems;
 
 		/* Skip the whole subtree if the nodemask remains the same. */
@@ -1176,7 +1183,7 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
 		cp->effective_mems = *new_mems;
 		spin_unlock_irq(&callback_lock);
 
-		WARN_ON(!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
+		WARN_ON(!is_in_v2_mode() &&
 			!nodes_equal(cp->mems_allowed, cp->effective_mems));
 
 		update_tasks_nodemask(cp);
@@ -1293,7 +1300,7 @@ static void update_tasks_flags(struct cpuset *cs)
 	struct css_task_iter it;
 	struct task_struct *task;
 
-	css_task_iter_start(&cs->css, &it);
+	css_task_iter_start(&cs->css, 0, &it);
 	while ((task = css_task_iter_next(&it)))
 		cpuset_update_task_spread_flag(cs, task);
 	css_task_iter_end(&it);
@@ -1468,7 +1475,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 
 	/* allow moving tasks into an empty cpuset if on default hierarchy */
 	ret = -ENOSPC;
-	if (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
+	if (!is_in_v2_mode() &&
 	    (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed)))
 		goto out_unlock;
 
@@ -1987,7 +1994,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
 	cpuset_inc();
 
 	spin_lock_irq(&callback_lock);
-	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys)) {
+	if (is_in_v2_mode()) {
 		cpumask_copy(cs->effective_cpus, parent->effective_cpus);
 		cs->effective_mems = parent->effective_mems;
 	}
@@ -2064,7 +2071,7 @@ static void cpuset_bind(struct cgroup_subsys_state *root_css)
 	mutex_lock(&cpuset_mutex);
 	spin_lock_irq(&callback_lock);
 
-	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys)) {
+	if (is_in_v2_mode()) {
 		cpumask_copy(top_cpuset.cpus_allowed, cpu_possible_mask);
 		top_cpuset.mems_allowed = node_possible_map;
 	} else {
@@ -2258,7 +2265,7 @@ retry:
 	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
 	mems_updated = !nodes_equal(new_mems, cs->effective_mems);
 
-	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys))
+	if (is_in_v2_mode())
 		hotplug_update_tasks(cs, &new_cpus, &new_mems,
 				     cpus_updated, mems_updated);
 	else
@@ -2289,7 +2296,7 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
 	static cpumask_t new_cpus;
 	static nodemask_t new_mems;
 	bool cpus_updated, mems_updated;
-	bool on_dfl = cgroup_subsys_on_dfl(cpuset_cgrp_subsys);
+	bool on_dfl = is_in_v2_mode();
 
 	mutex_lock(&cpuset_mutex);
 
diff --git a/kernel/cgroup/debug.c b/kernel/cgroup/debug.c
index dac46af22782..f661b4cc5efd 100644
--- a/kernel/cgroup/debug.c
+++ b/kernel/cgroup/debug.c
@@ -114,27 +114,49 @@ static int cgroup_css_links_read(struct seq_file *seq, void *v)
 {
 	struct cgroup_subsys_state *css = seq_css(seq);
 	struct cgrp_cset_link *link;
-	int dead_cnt = 0, extra_refs = 0;
+	int dead_cnt = 0, extra_refs = 0, threaded_csets = 0;
 
 	spin_lock_irq(&css_set_lock);
+
 	list_for_each_entry(link, &css->cgroup->cset_links, cset_link) {
 		struct css_set *cset = link->cset;
 		struct task_struct *task;
 		int count = 0;
 		int refcnt = refcount_read(&cset->refcount);
 
-		seq_printf(seq, " %d", refcnt);
-		if (refcnt - cset->nr_tasks > 0) {
-			int extra = refcnt - cset->nr_tasks;
-
-			seq_printf(seq, " +%d", extra);
-			/*
-			 * Take out the one additional reference in
-			 * init_css_set.
-			 */
-			if (cset == &init_css_set)
-				extra--;
-			extra_refs += extra;
+		/*
+		 * Print out the proc_cset and threaded_cset relationship
+		 * and highlight difference between refcount and task_count.
+		 */
+		seq_printf(seq, "css_set %pK", cset);
+		if (rcu_dereference_protected(cset->dom_cset, 1) != cset) {
+			threaded_csets++;
+			seq_printf(seq, "=>%pK", cset->dom_cset);
+		}
+		if (!list_empty(&cset->threaded_csets)) {
+			struct css_set *tcset;
+			int idx = 0;
+
+			list_for_each_entry(tcset, &cset->threaded_csets,
+					    threaded_csets_node) {
+				seq_puts(seq, idx ? "," : "<=");
+				seq_printf(seq, "%pK", tcset);
+				idx++;
+			}
+		} else {
+			seq_printf(seq, " %d", refcnt);
+			if (refcnt - cset->nr_tasks > 0) {
+				int extra = refcnt - cset->nr_tasks;
+
+				seq_printf(seq, " +%d", extra);
+				/*
+				 * Take out the one additional reference in
+				 * init_css_set.
+				 */
+				if (cset == &init_css_set)
+					extra--;
+				extra_refs += extra;
+			}
 		}
 		seq_puts(seq, "\n");
 
@@ -163,10 +185,12 @@ static int cgroup_css_links_read(struct seq_file *seq, void *v)
 	}
 	spin_unlock_irq(&css_set_lock);
 
-	if (!dead_cnt && !extra_refs)
+	if (!dead_cnt && !extra_refs && !threaded_csets)
 		return 0;
 
 	seq_puts(seq, "\n");
+	if (threaded_csets)
+		seq_printf(seq, "threaded css_sets = %d\n", threaded_csets);
 	if (extra_refs)
 		seq_printf(seq, "extra references = %d\n", extra_refs);
 	if (dead_cnt)
@@ -352,6 +376,7 @@ static int __init enable_cgroup_debug(char *str)
 {
 	debug_cgrp_subsys.dfl_cftypes = debug_files;
 	debug_cgrp_subsys.implicit_on_dfl = true;
+	debug_cgrp_subsys.threaded = true;
 	return 1;
 }
 __setup("cgroup_debug", enable_cgroup_debug);
diff --git a/kernel/cgroup/freezer.c b/kernel/cgroup/freezer.c
index 1b72d56edce5..08236798d173 100644
--- a/kernel/cgroup/freezer.c
+++ b/kernel/cgroup/freezer.c
@@ -268,7 +268,7 @@ static void update_if_frozen(struct cgroup_subsys_state *css)
 	rcu_read_unlock();
 
 	/* are all tasks frozen? */
-	css_task_iter_start(css, &it);
+	css_task_iter_start(css, 0, &it);
 
 	while ((task = css_task_iter_next(&it))) {
 		if (freezing(task)) {
@@ -320,7 +320,7 @@ static void freeze_cgroup(struct freezer *freezer)
 	struct css_task_iter it;
 	struct task_struct *task;
 
-	css_task_iter_start(&freezer->css, &it);
+	css_task_iter_start(&freezer->css, 0, &it);
 	while ((task = css_task_iter_next(&it)))
 		freeze_task(task);
 	css_task_iter_end(&it);
@@ -331,7 +331,7 @@ static void unfreeze_cgroup(struct freezer *freezer)
 	struct css_task_iter it;
 	struct task_struct *task;
 
-	css_task_iter_start(&freezer->css, &it);
+	css_task_iter_start(&freezer->css, 0, &it);
 	while ((task = css_task_iter_next(&it)))
 		__thaw_task(task);
 	css_task_iter_end(&it);
diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 2237201d66d5..9829c67ebc0a 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -345,4 +345,5 @@ struct cgroup_subsys pids_cgrp_subsys = {
 	.free		= pids_free,
 	.legacy_cftypes	= pids_files,
 	.dfl_cftypes	= pids_files,
+	.threaded	= true,
 };
diff --git a/kernel/events/core.c b/kernel/events/core.c
index fb415e3d824b..3e691b75b2db 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11293,5 +11293,6 @@ struct cgroup_subsys perf_event_cgrp_subsys = {
 	 * controller is not mounted on a legacy hierarchy.
 	 */
 	.implicit_on_dfl = true,
+	.threaded	= true,
 };
 #endif /* CONFIG_CGROUP_PERF */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ad15850ee157..6532b219b222 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -919,7 +919,7 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 		struct css_task_iter it;
 		struct task_struct *task;
 
-		css_task_iter_start(&iter->css, &it);
+		css_task_iter_start(&iter->css, 0, &it);
 		while (!ret && (task = css_task_iter_next(&it)))
 			ret = fn(task, arg);
 		css_task_iter_end(&it);
diff --git a/net/core/netclassid_cgroup.c b/net/core/netclassid_cgroup.c
index 029a61ac6cdd..5e4f04004a49 100644
--- a/net/core/netclassid_cgroup.c
+++ b/net/core/netclassid_cgroup.c
@@ -100,7 +100,7 @@ static int write_classid(struct cgroup_subsys_state *css, struct cftype *cft,
 
 	cs->classid = (u32)value;
 
-	css_task_iter_start(css, &it);
+	css_task_iter_start(css, 0, &it);
 	while ((p = css_task_iter_next(&it))) {
 		task_lock(p);
 		iterate_fd(p->files, 0, update_classid_sock,