[PATCH v2 2/6] sched: split out css_online/css_offline from tg creation/destruction

Li Zefan
2013-01-24T06:31:54Z

This is a preparaton for later patches.

- What do we gain from cpu_cgroup_css_online():

After ss->css_alloc() and before ss->css_online(), there's a small
window that tg->css.cgroup is NULL. With this change, tg won't be seen
before ss->css_online(), where it's added to the global list, so we're
guaranteed we'll never see NULL tg->css.cgroup.

- What do we gain from cpu_cgroup_css_offline():

tg is freed via RCU, so is cgroup. Without this change, This is how
synchronization works:

cgroup_rmdir()
no ss->css_offline()
diput()
syncornize_rcu()
ss->css_free() <-- unregister tg, and free it via call_rcu()
kfree_rcu(cgroup) <-- wait possible refs to cgroup, and free cgroup

We can't just kfree(cgroup), because tg might access tg->css.cgroup.

With this change:

cgroup_rmdir()
ss->css_offline() <-- unregister tg
diput()
synchronize_rcu() <-- wait possible refs to tg and cgroup
ss->css_free() <-- free tg
kfree_rcu(cgroup) <-- free cgroup

As you see, kfree_rcu() is redundant now.

Signed-off-by: Li Zefan <lizefan@huawei.com>
---

v2: changelog rephrased

---
include/linux/sched.h | 3 +++
kernel/sched/auto_group.c | 3 +++
kernel/sched/core.c | 49 +++++++++++++++++++++++++++++++++++++----------
3 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 206bb08..577eb97 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2750,7 +2750,10 @@ extern void normalize_rt_tasks(void);
extern struct task_group root_task_group;

extern struct task_group *sched_create_group(struct task_group *parent);
+extern void sched_online_group(struct task_group *tg,
+ struct task_group *parent);
extern void sched_destroy_group(struct task_group *tg);
+extern void sched_offline_group(struct task_group *tg);
extern void sched_move_task(struct task_struct *tsk);
#ifdef CONFIG_FAIR_GROUP_SCHED
extern int sched_group_set_shares(struct task_group *tg, unsigned long shares);
diff --git a/kernel/sched/auto_group.c b/kernel/sched/auto_group.c
index 0984a21..64de5f8 100644
--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c
@@ -35,6 +35,7 @@ static inline void autogroup_destroy(struct kref *kref)
ag->tg->rt_se = NULL;
ag->tg->rt_rq = NULL;
#endif
+ sched_offline_group(ag->tg);
sched_destroy_group(ag->tg);
}

@@ -76,6 +77,8 @@ static inline struct autogroup *autogroup_create(void)
if (IS_ERR(tg))
goto out_free;

+ sched_online_group(tg, &root_task_group);
+
kref_init(&ag->kref);
init_rwsem(&ag->lock);
ag->id = atomic_inc_return(&autogroup_seq_nr);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 257002c..1061672 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7159,7 +7159,6 @@ static void free_sched_group(struct task_group *tg)
struct task_group *sched_create_group(struct task_group *parent)
{
struct task_group *tg;
- unsigned long flags;

tg = kzalloc(sizeof(*tg), GFP_KERNEL);
if (!tg)
@@ -7171,6 +7170,17 @@ struct task_group *sched_create_group(struct task_group *parent)
if (!alloc_rt_sched_group(tg, parent))
goto err;

+ return tg;
+
+err:
+ free_sched_group(tg);
+ return ERR_PTR(-ENOMEM);
+}
+
+void sched_online_group(struct task_group *tg, struct task_group *parent)
+{
+ unsigned long flags;
+
spin_lock_irqsave(&task_group_lock, flags);
list_add_rcu(&tg->list, &task_groups);

@@ -7180,12 +7190,6 @@ struct task_group *sched_create_group(struct task_group *parent)
INIT_LIST_HEAD(&tg->children);
list_add_rcu(&tg->siblings, &parent->children);
spin_unlock_irqrestore(&task_group_lock, flags);
-
- return tg;
-
-err:
- free_sched_group(tg);
- return ERR_PTR(-ENOMEM);
}

/* rcu callback to free various structures associated with a task group */
@@ -7198,6 +7202,12 @@ static void free_sched_group_rcu(struct rcu_head *rhp)
/* Destroy runqueue etc associated with a task group */
void sched_destroy_group(struct task_group *tg)
{
+ /* wait for possible concurrent references to cfs_rqs complete */
+ call_rcu(&tg->rcu, free_sched_group_rcu);
+}
+
+void sched_offline_group(struct task_group *tg)
+{
unsigned long flags;
int i;

@@ -7209,9 +7219,6 @@ void sched_destroy_group(struct task_group *tg)
list_del_rcu(&tg->list);
list_del_rcu(&tg->siblings);
spin_unlock_irqrestore(&task_group_lock, flags);
-
- /* wait for possible concurrent references to cfs_rqs complete */
- call_rcu(&tg->rcu, free_sched_group_rcu);
}

/* change task's runqueue when it moves between groups.
@@ -7563,6 +7570,19 @@ static struct cgroup_subsys_state *cpu_cgroup_css_alloc(struct cgroup *cgrp)
return &tg->css;
}

+static int cpu_cgroup_css_online(struct cgroup *cgrp)
+{
+ struct task_group *tg = cgroup_tg(cgrp);
+ struct task_group *parent;
+
+ if (!cgrp->parent)
+ return 0;
+
+ parent = cgroup_tg(cgrp->parent);
+ sched_online_group(tg, parent);
+ return 0;
+}
+
static void cpu_cgroup_css_free(struct cgroup *cgrp)
{
struct task_group *tg = cgroup_tg(cgrp);
@@ -7570,6 +7590,13 @@ static void cpu_cgroup_css_free(struct cgroup *cgrp)
sched_destroy_group(tg);
}

+static void cpu_cgroup_css_offline(struct cgroup *cgrp)
+{
+ struct task_group *tg = cgroup_tg(cgrp);
+
+ sched_offline_group(tg);
+}
+
static int cpu_cgroup_can_attach(struct cgroup *cgrp,
struct cgroup_taskset *tset)
{
@@ -7925,6 +7952,8 @@ struct cgroup_subsys cpu_cgroup_subsys = {
.name = "cpu",
.css_alloc = cpu_cgroup_css_alloc,
.css_free = cpu_cgroup_css_free,
+ .css_online = cpu_cgroup_css_online,
+ .css_offline = cpu_cgroup_css_offline,
.can_attach = cpu_cgroup_can_attach,
.attach = cpu_cgroup_attach,
.exit = cpu_cgroup_exit,
--
1.8.0.2