[PATCH 1/5] cgroup: ID notification call back

[KAMEZAWA Hiroyuki]

CC'ed to Paul Menage and Li Zefan.
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

When cgroup subsystem use ID (ss->use_id==1), each css's ID is assigned
after successful call of ->create(). css_ID is tightly coupled with
css struct itself but it is allocated by ->create() call, IOW,
per-subsystem special allocations.

To know css_id before creation, this patch adds id_attached() callback.
after css_ID allocation. This will be used by memory cgroup's quick lookup
routine.

Maybe you can think of other implementations as
	- pass ID to ->create()
	or
	- add post_create()
	etc...
But when considering dirtiness of codes, this straightforward patch seems
good to me. If someone wants post_create(), this patch can be replaced.

Changelog: 20100820
 - new approarch.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 Documentation/cgroups/cgroups.txt |   10 ++++++++++
 include/linux/cgroup.h            |    1 +
 kernel/cgroup.c                   |    5 ++++-
 3 files changed, 15 insertions(+), 1 deletion(-)

Index: mmotm-0811/Documentation/cgroups/cgroups.txt
===================================================================
--- mmotm-0811.orig/Documentation/cgroups/cgroups.txt
+++ mmotm-0811/Documentation/cgroups/cgroups.txt
@@ -621,6 +621,16 @@ and root cgroup. Currently this will onl
 the default hierarchy (which never has sub-cgroups) and a hierarchy
 that is being created/destroyed (and hence has no sub-cgroups).
 
+void id_attached(struct cgroup_subsys *ss, struct cgroup *root)
+(cgroup_mutex and ss->hierarchy_mutex held by caller)
+(called only when ss->use_id=1)
+
+Called when css_id is attached to css. Because css_id is assigned
+against "css", css_id is not available until ->create() is called.
+If subsystem wants to make use of ID at createtion time, use
+this handler. This handler will be called after css_id is assigned
+to css. Not necessary to be implemented in usual(see memcontrol.c)
+
 4. Questions
 ============
 
Index: mmotm-0811/include/linux/cgroup.h
===================================================================
--- mmotm-0811.orig/include/linux/cgroup.h
+++ mmotm-0811/include/linux/cgroup.h
@@ -475,6 +475,7 @@ struct cgroup_subsys {
 			struct cgroup *cgrp);
 	void (*post_clone)(struct cgroup_subsys *ss, struct cgroup *cgrp);
 	void (*bind)(struct cgroup_subsys *ss, struct cgroup *root);
+	void (*id_attached)(struct cgroup_subsys *ss, struct cgroup *cgrp);
 
 	int subsys_id;
 	int active;
Index: mmotm-0811/kernel/cgroup.c
===================================================================
--- mmotm-0811.orig/kernel/cgroup.c
+++ mmotm-0811/kernel/cgroup.c
@@ -4618,6 +4618,8 @@ static int __init_or_module cgroup_init_
 	newid->stack[0] = newid->id;
 	newid->css = rootcss;
 	rootcss->id = newid;
+	if (ss->id_attached)
+		ss->id_attached(ss, dummytop);
 	return 0;
 }
 
@@ -4646,7 +4648,8 @@ static int alloc_css_id(struct cgroup_su
 	 * see cgroup_populate_dir()
 	 */
 	rcu_assign_pointer(child_css->id, child_id);
-
+	if (ss->id_attached)
+		ss->id_attached(ss, child);
 	return 0;
 }
 

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