Memory cgroup hierarchy introduction (v2)

Add support for building hierarchies in resource counters. Cgroups allows us to build a deep hierarchy, but we currently don't link the resource counters belonging to the memory controller control groups, in the same fashion as the corresponding cgroup entries in the cgroup hierarchy. This patch provides the infrastructure for resource counters that have the same hiearchy as their cgroup counter parts.

These set of patches are based on the resource counter hiearchy patches posted by Pavel Emelianov.

NOTE: Building hiearchies is expensive, deeper hierarchies imply charging the all the way up to the root. It is known that hiearchies are expensive, so the user needs to be careful and aware of the trade-offs before creating very deep ones.

Signed-off-by Balbir Singh <[email protected]>

---

 include/linux/res_counter.h |    8 ++++++--
 kernel/res_counter.c        |   42 ++++++++++++++++++++++++++++++++++--------
 mm/memcontrol.c             |    9 ++++++---
 3 files changed, 46 insertions(+), 13 deletions(-)

diff -puN include/linux/res_counter.h~resource-counters-hierarchy-support include/linux/res_counter.h

include/linux/res_counter.h renamed from include/linux/res_counter.h~resource-counters-hierarchy-support +6 -2

@@ -43,6 +43,10 @@ struct res_counter {
 	 * the routines below consider this to be IRQ-safe
 	 */
 	spinlock_t lock;
+	/*
+	 * Parent counter, used for hierarchial resource accounting
+	 */
+	struct res_counter *parent;
 };
 
 /**

@@ -87,7 +91,7 @@ enum {
  * helpers for accounting
  */
 
-void res_counter_init(struct res_counter *counter);
+void res_counter_init(struct res_counter *counter, struct res_counter *parent);
 
 /*
  * charge - try to consume more resource.

@@ -103,7 +107,7 @@ void res_counter_init(struct res_counter
 int __must_check res_counter_charge_locked(struct res_counter *counter,
 		unsigned long val);
 int __must_check res_counter_charge(struct res_counter *counter,
-		unsigned long val);
+		unsigned long val, struct res_counter **limit_fail_at);
 
 /*
  * uncharge - tell that some portion of the resource is released

diff -puN kernel/res_counter.c~resource-counters-hierarchy-support kernel/res_counter.c

kernel/res_counter.c renamed from kernel/res_counter.c~resource-counters-hierarchy-support +19 -5

@@ -15,10 +15,11 @@
 #include <linux/uaccess.h>
 #include <linux/mm.h>
 
-void res_counter_init(struct res_counter *counter)
+void res_counter_init(struct res_counter *counter, struct res_counter *parent)
 {
 	spin_lock_init(&counter->lock);
 	counter->limit = (unsigned long long)LLONG_MAX;
+	counter->parent = parent;
 }
 
 int res_counter_charge_locked(struct res_counter *counter, unsigned long val)

@@ -34,14 +35,34 @@ int res_counter_charge_locked(struct res
 	return 0;
 }
 
-int res_counter_charge(struct res_counter *counter, unsigned long val)
+int res_counter_charge(struct res_counter *counter, unsigned long val,
+			struct res_counter **limit_fail_at)
 {
 	int ret;
 	unsigned long flags;
+	struct res_counter *c, *u;
 
-	spin_lock_irqsave(&counter->lock, flags);
-	ret = res_counter_charge_locked(counter, val);
-	spin_unlock_irqrestore(&counter->lock, flags);
+	*limit_fail_at = NULL;
+	local_irq_save(flags);
+	for (c = counter; c != NULL; c = c->parent) {
+		spin_lock(&c->lock);
+		ret = res_counter_charge_locked(c, val);
+		spin_unlock(&c->lock);
+		if (ret < 0) {
+			*limit_fail_at = c;
+			goto undo;
+		}
+	}
+	ret = 0;
+	goto done;
+undo:

kernel/res_counter.c renamed from kernel/res_counter.c~resource-counters-hierarchy-support +8 -3

@@ -56,10 +77,15 @@ void res_counter_uncharge_locked(struct 
 void res_counter_uncharge(struct res_counter *counter, unsigned long val)
 {
 	unsigned long flags;
+	struct res_counter *c;
 
-	spin_lock_irqsave(&counter->lock, flags);
-	res_counter_uncharge_locked(counter, val);
-	spin_unlock_irqrestore(&counter->lock, flags);
+	local_irq_save(flags);
+	for (c = counter; c != NULL; c = c->parent) {
+		spin_lock(&c->lock);
+		res_counter_uncharge_locked(c, val);
+		spin_unlock(&c->lock);
+	}
+	local_irq_restore(flags);
 }
 
 

diff -puN mm/memcontrol.c~resource-counters-hierarchy-support mm/memcontrol.c

mm/memcontrol.c renamed from mm/memcontrol.c~resource-counters-hierarchy-support +6 -3

@@ -485,6 +485,7 @@ int mem_cgroup_try_charge(struct mm_stru
 {
 	struct mem_cgroup *mem;
 	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
+	struct res_counter *fail_res;
 	/*
 	 * We always charge the cgroup the mm_struct belongs to.
 	 * The mm_struct's mem_cgroup changes on task migration if the

@@ -510,7 +511,7 @@ int mem_cgroup_try_charge(struct mm_stru
 	}
 
 
-	while (unlikely(res_counter_charge(&mem->res, PAGE_SIZE))) {
+	while (unlikely(res_counter_charge(&mem->res, PAGE_SIZE, &fail_res))) {
 		if (!(gfp_mask & __GFP_WAIT))
 			goto nomem;
 

@@ -1175,18 +1176,20 @@ static void mem_cgroup_free(struct mem_c
 static struct cgroup_subsys_state *
 mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 {
-	struct mem_cgroup *mem;
+	struct mem_cgroup *mem, *parent;
 	int node;
 
 	if (unlikely((cont->parent) == NULL)) {
 		mem = &init_mem_cgroup;
+		parent = NULL;
 	} else {
 		mem = mem_cgroup_alloc();
+		parent = mem_cgroup_from_cont(cont->parent);
 		if (!mem)
 			return ERR_PTR(-ENOMEM);
 	}
 
-	res_counter_init(&mem->res);
+	res_counter_init(&mem->res, parent ? &parent->res : NULL);
 
 	for_each_node_state(node, N_POSSIBLE)
 		if (alloc_mem_cgroup_per_zone_info(mem, node))

_

Signature · 6 lines

	Balbir
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Quoted 10 lines · On Sat, 08 Nov 2008 14:40:47 +0530 Balbir Singh <[email protected]>…

Add support for building hierarchies in resource counters. Cgroups allows us to build a deep hierarchy, but we currently don't link the resource counters belonging to the memory controller control groups, in the same fashion as the corresponding cgroup entries in the cgroup hierarchy. This patch provides the infrastructure for resource counters that have the same hiearchy as their cgroup counter parts.

These set of patches are based on the resource counter hiearchy patches posted by Pavel Emelianov.

Quoted 5 lines

NOTE: Building hiearchies is expensive, deeper hierarchies imply charging the all the way up to the root. It is known that hiearchies are expensive, so the user needs to be careful and aware of the trade-offs before creating very deep ones.

Do you have numbers ?

This patch introduces hierarchical reclaim. When an ancestor goes over its limit, the charging routine points to the parent that is above its limit. The reclaim process then starts from the last scanned child of the ancestor and reclaims until the ancestor goes below its limit.

Signed-off-by Balbir Singh <[email protected]>

---

 mm/memcontrol.c |  152 +++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 128 insertions(+), 24 deletions(-)

diff -puN mm/memcontrol.c~memcg-hierarchical-reclaim mm/memcontrol.c

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-hierarchical-reclaim +40 -0

@@ -132,6 +132,11 @@ struct mem_cgroup {
 	 * statistics.
 	 */
 	struct mem_cgroup_stat stat;
+	/*
+	 * While reclaiming in a hiearchy, we cache the last child we
+	 * reclaimed from.
+	 */
+	struct mem_cgroup *last_scanned_child;
 };
 static struct mem_cgroup init_mem_cgroup;
 

@@ -467,6 +472,124 @@ unsigned long mem_cgroup_isolate_pages(u
 	return nr_taken;
 }
 
+static struct mem_cgroup *
+mem_cgroup_from_res_counter(struct res_counter *counter)
+{
+	return container_of(counter, struct mem_cgroup, res);
+}
+
+/*
+ * Dance down the hierarchy if needed to reclaim memory. We remember the
+ * last child we reclaimed from, so that we don't end up penalizing
+ * one child extensively based on its position in the children list.
+ *
+ * root_mem is the original ancestor that we've been reclaim from.
+ */
+static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *mem,
+						struct mem_cgroup *root_mem,
+						gfp_t gfp_mask)
+{
+	struct cgroup *cg_current, *cgroup;
+	struct mem_cgroup *mem_child;
+	int ret = 0;
+
+	/*
+	 * Reclaim unconditionally and don't check for return value.
+	 * We need to reclaim in the current group and down the tree.
+	 * One might think about checking for children before reclaiming,
+	 * but there might be left over accounting, even after children
+	 * have left.
+	 */
+	try_to_free_mem_cgroup_pages(mem, gfp_mask);
+
+	if (res_counter_check_under_limit(&root_mem->res))
+		return 0;
+
+	if (list_empty(&mem->css.cgroup->children))
+		return 0;

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-hierarchical-reclaim +78 -24

+	cg_current = cgroup;
+	cgroup_lock();
+
+	do {
+		struct list_head *next;
+
+		mem_child = mem_cgroup_from_cont(cgroup);
+		cgroup_unlock();
+
+		ret = mem_cgroup_hierarchical_reclaim(mem_child, root_mem,
+							gfp_mask);
+		mem->last_scanned_child = mem_child;
+
+		cgroup_lock();
+		if (res_counter_check_under_limit(&root_mem->res)) {
+			ret = 0;
+			goto done;
+		}
+
+		/*
+		 * Since we gave up the lock, it is time to
+		 * start from last cgroup
+		 */
+		cgroup = mem->last_scanned_child->css.cgroup;
+		next = cgroup->sibling.next;
+
+		if (next == &cg_current->parent->children)
+			cgroup = list_first_entry(&mem->css.cgroup->children,
+							struct cgroup, sibling);
+		else
+			cgroup = container_of(next, struct cgroup, sibling);
+	} while (cgroup != cg_current);
+
+done:
+	cgroup_unlock();
+	return ret;
+}
+
+/*
+ * Charge memory cgroup mem and check if it is over its limit. If so, reclaim
+ * from mem.
+ */
+static int mem_cgroup_charge_and_reclaim(struct mem_cgroup *mem, gfp_t gfp_mask)
+{
+	int ret = 0;
+	unsigned long nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
+	struct res_counter *fail_res;
+	struct mem_cgroup *mem_over_limit;
+
+	while (unlikely(res_counter_charge(&mem->res, PAGE_SIZE, &fail_res))) {
+		if (!(gfp_mask & __GFP_WAIT))
+			goto out;
+
+		/*
+		 * Is one of our ancestors over their limit?
+		 */
+		if (fail_res)
+			mem_over_limit = mem_cgroup_from_res_counter(fail_res);
+		else
+			mem_over_limit = mem;
+
+		ret = mem_cgroup_hierarchical_reclaim(mem_over_limit,
+							mem_over_limit,
+							gfp_mask);
+
+		if (!nr_retries--) {
+			mem_cgroup_out_of_memory(mem, gfp_mask);
+			goto out;
+		}
+	}
+out:
+	return ret;
+}
 
 /**
  * mem_cgroup_try_charge - get charge of PAGE_SIZE.

@@ -484,8 +607,7 @@ int mem_cgroup_try_charge(struct mm_stru
 			gfp_t gfp_mask, struct mem_cgroup **memcg)
 {
 	struct mem_cgroup *mem;
-	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
-	struct res_counter *fail_res;
+
 	/*
 	 * We always charge the cgroup the mm_struct belongs to.
 	 * The mm_struct's mem_cgroup changes on task migration if the

@@ -510,29 +632,9 @@ int mem_cgroup_try_charge(struct mm_stru
 		css_get(&mem->css);
 	}
 
+	if (mem_cgroup_charge_and_reclaim(mem, gfp_mask))
+		goto nomem;
 
-	while (unlikely(res_counter_charge(&mem->res, PAGE_SIZE, &fail_res))) {
-		if (!(gfp_mask & __GFP_WAIT))
-			goto nomem;
-
-		if (try_to_free_mem_cgroup_pages(mem, gfp_mask))
-			continue;
-
-		/*
-		 * try_to_free_mem_cgroup_pages() might not give us a full
-		 * picture of reclaim. Some pages are reclaimed and might be
-		 * moved to swap cache or just unmapped from the cgroup.
-		 * Check the limit again to see if the reclaim reduced the
-		 * current usage of the cgroup before giving up
-		 */
-		if (res_counter_check_under_limit(&mem->res))
-			continue;
-
-		if (!nr_retries--) {
-			mem_cgroup_out_of_memory(mem, gfp_mask);
-			goto nomem;
-		}
-	}
 	return 0;
 nomem:
 	css_put(&mem->css);

@@ -1195,6 +1297,8 @@ mem_cgroup_create(struct cgroup_subsys *
 		if (alloc_mem_cgroup_per_zone_info(mem, node))
 			goto free_out;
 
+	mem->last_scanned_child = NULL;
+
 	return &mem->css;
 free_out:
 	for_each_node_state(node, N_POSSIBLE)

_

Signature · 6 lines

	Balbir
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More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Quoted 69 lines · On Sat, 08 Nov 2008 14:41:00 +0530 Balbir Singh <[email protected]>…

This patch introduces hierarchical reclaim. When an ancestor goes over its limit, the charging routine points to the parent that is above its limit. The reclaim process then starts from the last scanned child of the ancestor and reclaims until the ancestor goes below its limit.

Signed-off-by Balbir Singh <[email protected]>

---

 mm/memcontrol.c |  152 +++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 128 insertions(+), 24 deletions(-)

diff -puN mm/memcontrol.c~memcg-hierarchical-reclaim mm/memcontrol.c

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-hierarchical-reclaim +44 -0

@@ -132,6 +132,11 @@ struct mem_cgroup {
 	 * statistics.
 	 */
 	struct mem_cgroup_stat stat;
+	/*
+	 * While reclaiming in a hiearchy, we cache the last child we
+	 * reclaimed from.
+	 */
+	struct mem_cgroup *last_scanned_child;
 };
 static struct mem_cgroup init_mem_cgroup;
 

@@ -467,6 +472,124 @@ unsigned long mem_cgroup_isolate_pages(u
 	return nr_taken;
 }
 
+static struct mem_cgroup *
+mem_cgroup_from_res_counter(struct res_counter *counter)
+{
+	return container_of(counter, struct mem_cgroup, res);
+}
+
+/*
+ * Dance down the hierarchy if needed to reclaim memory. We remember the
+ * last child we reclaimed from, so that we don't end up penalizing
+ * one child extensively based on its position in the children list.
+ *
+ * root_mem is the original ancestor that we've been reclaim from.
+ */
+static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *mem,
+						struct mem_cgroup *root_mem,
+						gfp_t gfp_mask)
+{
+	struct cgroup *cg_current, *cgroup;
+	struct mem_cgroup *mem_child;
+	int ret = 0;
+
+	/*
+	 * Reclaim unconditionally and don't check for return value.
+	 * We need to reclaim in the current group and down the tree.
+	 * One might think about checking for children before reclaiming,
+	 * but there might be left over accounting, even after children
+	 * have left.
+	 */
+	try_to_free_mem_cgroup_pages(mem, gfp_mask);
+
+	if (res_counter_check_under_limit(&root_mem->res))
+		return 0;
+
+	if (list_empty(&mem->css.cgroup->children))
+		return 0;
+
+	/*
+	 * Scan all children under the mem_cgroup mem
+	 */

mm/memcontrol.c unsigned long mem_cgroup_isolate_pages(u · line 519

Quoted 6 lines

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-hierarchical-reclaim +6 -0

+	if (!mem->last_scanned_child)
+		cgroup = list_first_entry(&mem->css.cgroup->children,
+				struct cgroup, sibling);
+	else
+		cgroup = mem->last_scanned_child->css.cgroup;
+

Who guarantee this last_scan_child is accessible at this point ?

Quoted 72 lines · KAMEZAWA Hiroyuki

Quoted 69 lines · On Sat, 08 Nov 2008 14:41:00 +0530 Balbir Singh <[email protected]>…

This patch introduces hierarchical reclaim. When an ancestor goes over its limit, the charging routine points to the parent that is above its limit. The reclaim process then starts from the last scanned child of the ancestor and reclaims until the ancestor goes below its limit.

Signed-off-by Balbir Singh <[email protected]>

---

 mm/memcontrol.c |  152 +++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 128 insertions(+), 24 deletions(-)

diff -puN mm/memcontrol.c~memcg-hierarchical-reclaim mm/memcontrol.c

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-hierarchical-reclaim +44 -0

@@ -132,6 +132,11 @@ struct mem_cgroup {
 	 * statistics.
 	 */
 	struct mem_cgroup_stat stat;
+	/*
+	 * While reclaiming in a hiearchy, we cache the last child we
+	 * reclaimed from.
+	 */
+	struct mem_cgroup *last_scanned_child;
 };
 static struct mem_cgroup init_mem_cgroup;
 

@@ -467,6 +472,124 @@ unsigned long mem_cgroup_isolate_pages(u
 	return nr_taken;
 }
 
+static struct mem_cgroup *
+mem_cgroup_from_res_counter(struct res_counter *counter)
+{
+	return container_of(counter, struct mem_cgroup, res);
+}
+
+/*
+ * Dance down the hierarchy if needed to reclaim memory. We remember the
+ * last child we reclaimed from, so that we don't end up penalizing
+ * one child extensively based on its position in the children list.
+ *
+ * root_mem is the original ancestor that we've been reclaim from.
+ */
+static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *mem,
+						struct mem_cgroup *root_mem,
+						gfp_t gfp_mask)
+{
+	struct cgroup *cg_current, *cgroup;
+	struct mem_cgroup *mem_child;
+	int ret = 0;
+
+	/*
+	 * Reclaim unconditionally and don't check for return value.
+	 * We need to reclaim in the current group and down the tree.
+	 * One might think about checking for children before reclaiming,
+	 * but there might be left over accounting, even after children
+	 * have left.
+	 */
+	try_to_free_mem_cgroup_pages(mem, gfp_mask);
+
+	if (res_counter_check_under_limit(&root_mem->res))
+		return 0;
+
+	if (list_empty(&mem->css.cgroup->children))
+		return 0;
+
+	/*
+	 * Scan all children under the mem_cgroup mem
+	 */

Quoted 9 lines

mm/memcontrol.c unsigned long mem_cgroup_isolate_pages(u · line 519

Quoted 6 lines

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-hierarchical-reclaim +6 -0

+	if (!mem->last_scanned_child)
+		cgroup = list_first_entry(&mem->css.cgroup->children,
+				struct cgroup, sibling);
+	else
+		cgroup = mem->last_scanned_child->css.cgroup;
+

Who guarantee this last_scan_child is accessible at this point ?

Good catch! I'll fix this in mem_cgroup_destroy. It'll need some locking around it as well.

Signature · 1 line

	Balbir

Quoted 1 line

please see mem+swap controller's refcnt-to-memcg for delaying free of memcg. it will be a hint.

mm/memcontrol.c out: · line 1099

Quoted 8 lines

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-add-hierarchy-selector +8 -0

+static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
+					u64 val)
+{
+	int retval = 0;
+	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
+
+	if (val == 1) {
+		if (list_empty(&cont->children))

cgroup_lock should be held before checking cont->children.

mm/memcontrol.c out: · line 1103

Quoted 4 lines

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-add-hierarchy-selector +4 -0

+			mem->use_hierarchy = val;
+		else
+			retval = -EBUSY;
+	} else if (val == 0) {

And code duplicate.

Quoted 11 lines

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-add-hierarchy-selector +10 -0

+		if (list_empty(&cont->children))
+			mem->use_hierarchy nn= val;
+		else
+			retval = -EBUSY;
+	} else
+		retval = -EINVAL;
+
+	return retval;
+}
+

Don't enable multiple hierarchy support by default. This patch introduces a features element that can be set to enable the nested depth hierarchy feature. This feature can only be enabled when the cgroup for which the feature this is enabled, has no children.

Signed-off-by Balbir Singh <[email protected]>

---

 mm/memcontrol.c |   43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff -puN mm/memcontrol.c~memcg-add-hierarchy-selector mm/memcontrol.c

mm/memcontrol.c renamed from mm/memcontrol.c~memcg-add-hierarchy-selector +42 -1

@@ -137,6 +137,11 @@ struct mem_cgroup {
 	 * reclaimed from.
 	 */
 	struct mem_cgroup *last_scanned_child;
+	/*
+	 * Should the accounting and control be hierarchical, per subtree?
+	 */
+	unsigned long use_hierarchy;
+
 };
 static struct mem_cgroup init_mem_cgroup;
 

@@ -1079,6 +1084,33 @@ out:
 	return ret;
 }
 
+static u64 mem_cgroup_hierarchy_read(struct cgroup *cont, struct cftype *cft)
+{
+	return mem_cgroup_from_cont(cont)->use_hierarchy;
+}
+
+static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
+					u64 val)
+{
+	int retval = 0;
+	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
+
+	if (val == 1) {
+		if (list_empty(&cont->children))
+			mem->use_hierarchy = val;
+		else
+			retval = -EBUSY;
+	} else if (val == 0) {
+		if (list_empty(&cont->children))
+			mem->use_hierarchy = val;
+		else
+			retval = -EBUSY;
+	} else
+		retval = -EINVAL;
+
+	return retval;
+}
+
 static u64 mem_cgroup_read(struct cgroup *cont, struct cftype *cft)
 {
 	return res_counter_read_u64(&mem_cgroup_from_cont(cont)->res,

@@ -1213,6 +1245,11 @@ static struct cftype mem_cgroup_files[] 
 		.name = "stat",
 		.read_map = mem_control_stat_show,
 	},
+	{
+		.name = "use_hierarchy",
+		.write_u64 = mem_cgroup_hierarchy_write,
+		.read_u64 = mem_cgroup_hierarchy_read,
+	},
 };
 
 static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)

@@ -1289,9 +1326,13 @@ mem_cgroup_create(struct cgroup_subsys *
 		parent = mem_cgroup_from_cont(cont->parent);
 		if (!mem)
 			return ERR_PTR(-ENOMEM);
+		mem->use_hierarchy = parent->use_hierarchy;
 	}
 
-	res_counter_init(&mem->res, parent ? &parent->res : NULL);
+	if (parent && parent->use_hierarchy)
+		res_counter_init(&mem->res, &parent->res);
+	else
+		res_counter_init(&mem->res, NULL);
 
 	for_each_node_state(node, N_POSSIBLE)
 		if (alloc_mem_cgroup_per_zone_info(mem, node))

_

Signature · 1 line

	Balbir

Quoted 8 lines · On Sat, 08 Nov 2008 14:41:13 +0530 Balbir Singh <[email protected]>…

Don't enable multiple hierarchy support by default. This patch introduces a features element that can be set to enable the nested depth hierarchy feature. This feature can only be enabled when the cgroup for which the feature this is enabled, has no children.

Signed-off-by Balbir Singh <[email protected]>

---

IMHO, permission to this is not enough.

I think following is sane way.
==
When parent->use_hierarchy==1.

	my->use_hierarchy must be "1" and cannot be tunrned to be "0" even if no children.
   When parent->use_hierarchy==0
	my->use_hierarchy can be either of "0" or "1".
	this value can be chagned if we don't have children

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