cgroup: simplify init_subsys()

We are at system boot and there is only 1 cgroup group (i,e,
init_css_set), so we don't need to run through the css_set linked list. Neither do we need to run through the task list, since no processes have been created yet.

Also referring to a comment in cgroup.h:

struct css_set

 {
        ...
        /*
         * Set of subsystem states, one for each subsystem. This array
         * is immutable after creation apart from the init_css_set
         * during subsystem registration (at boot time).
         */
        struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];

}

Signed-off-by Li Zefan <[email protected]>

---

  Documentation/cgroups.txt |    3 +--
  kernel/cgroup.c           |   35 +++++++++--------------------------
  2 files changed, 10 insertions(+), 28 deletions(-)

diff --git a/Documentation/cgroups.txt b/Documentation/cgroups.txt
index 31d12e2..c298a66 100644
--- a/Documentation/cgroups.txt
+++ b/Documentation/cgroups.txt
@@ -500,8 +500,7 @@ post-attachment activity that requires memory allocations or blocking.

void fork(struct cgroup_subsy *ss, struct task_struct *task)

-Called when a task is forked into a cgroup. Also called during -registration for all existing tasks.
+Called when a task is forked into a cgroup.

void exit(struct cgroup_subsys *ss, struct task_struct *task)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f79e60d..250e28e 100644

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2471,7 +2471,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)

static void __init cgroup_init_subsys(struct cgroup_subsys *ss)

  {
        struct cgroup_subsys_state *css;

- struct list_head *l;

        printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);

@@ -2482,35 +2481,19 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)

        BUG_ON(IS_ERR(css));
        init_cgroup_css(css, ss, dummytop);

- /* Update all cgroup groups to contain a subsys
+ /* Update the init_css_set to contain a subsys
* pointer to this state - since the subsystem is
- * newly registered, all tasks and hence all cgroup
- * groups are in the subsystem's top cgroup. */

 -       write_lock(&css_set_lock);
 -       l = &init_css_set.list;
 -       do {

- struct css_set *cg =
- list_entry(l, struct css_set, list);
- cg->subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
- l = l->next;
- } while (l != &init_css_set.list);
- write_unlock(&css_set_lock);
-

(quoted diff) +1 -1

-       /* If this subsystem requested that it be notified with fork
 -        * events, we should send it one now for every process in the
 -        * system */
 -       if (ss->fork) {
 -               struct task_struct *g, *p;
 -
 -               read_lock(&tasklist_lock);
 -               do_each_thread(g, p) {
 -                       ss->fork(ss, p);
 -               } while_each_thread(g, p);
 -               read_unlock(&tasklist_lock);
 -       }
 
+        * newly registered, all tasks and hence the
 +        * init_css_set is in the subsystem's top cgroup. */
 +       init_css_set.subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
 
        need_forkexit_callback |= ss->fork || ss->exit;
 
 +       /* At system boot, before all subsystems have been
 +        * registered, no tasks have been forked, so we don't
 +        * need to invoke fork callbacks here. */
 +       BUG_ON(!list_empty(&init_task.tasks));
 +
        ss->active = 1;
  }
 
 --
 1.5.4.rc3

We are at system boot and there is only 1 cgroup group (i,e,
init_css_set), so we don't need to run through the css_set linked list. Neither do we need to run through the task list, since no processes have been created yet.

Also referring to a comment in cgroup.h:

struct css_set

 {
        ...
        /*
         * Set of subsystem states, one for each subsystem. This array
         * is immutable after creation apart from the init_css_set
         * during subsystem registration (at boot time).
         */
        struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];

}

Signed-off-by Li Zefan <[email protected]>

---

  Documentation/cgroups.txt |    3 +--
  kernel/cgroup.c           |   35 +++++++++--------------------------
  2 files changed, 10 insertions(+), 28 deletions(-)

diff --git a/Documentation/cgroups.txt b/Documentation/cgroups.txt
index 31d12e2..c298a66 100644
--- a/Documentation/cgroups.txt
+++ b/Documentation/cgroups.txt
@@ -500,8 +500,7 @@ post-attachment activity that requires memory allocations or blocking.

void fork(struct cgroup_subsy *ss, struct task_struct *task)

-Called when a task is forked into a cgroup. Also called during -registration for all existing tasks.
+Called when a task is forked into a cgroup.

void exit(struct cgroup_subsys *ss, struct task_struct *task)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f79e60d..250e28e 100644

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2471,7 +2471,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)

static void __init cgroup_init_subsys(struct cgroup_subsys *ss)

  {
        struct cgroup_subsys_state *css;

- struct list_head *l;

        printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);

@@ -2482,35 +2481,19 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)

        BUG_ON(IS_ERR(css));
        init_cgroup_css(css, ss, dummytop);

- /* Update all cgroup groups to contain a subsys
+ /* Update the init_css_set to contain a subsys
* pointer to this state - since the subsystem is
- * newly registered, all tasks and hence all cgroup
- * groups are in the subsystem's top cgroup. */

 -       write_lock(&css_set_lock);
 -       l = &init_css_set.list;
 -       do {

- struct css_set *cg =
- list_entry(l, struct css_set, list);
- cg->subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
- l = l->next;
- } while (l != &init_css_set.list);
- write_unlock(&css_set_lock);
-

(quoted diff) +1 -1

-       /* If this subsystem requested that it be notified with fork
 -        * events, we should send it one now for every process in the
 -        * system */
 -       if (ss->fork) {
 -               struct task_struct *g, *p;
 -
 -               read_lock(&tasklist_lock);
 -               do_each_thread(g, p) {
 -                       ss->fork(ss, p);
 -               } while_each_thread(g, p);
 -               read_unlock(&tasklist_lock);
 -       }
 
+        * newly registered, all tasks and hence the
 +        * init_css_set is in the subsystem's top cgroup. */
 +       init_css_set.subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
 
        need_forkexit_callback |= ss->fork || ss->exit;
 
 +       /* At system boot, before all subsystems have been
 +        * registered, no tasks have been forked, so we don't
 +        * need to invoke fork callbacks here. */
 +       BUG_ON(!list_empty(&init_task.tasks));
 +
        ss->active = 1;
  }
 
 --
 1.5.4.rc3