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Commit ace783b9 authored by Li Zefan's avatar Li Zefan Committed by Tejun Heo
Browse files

sched: split out css_online/css_offline from tg creation/destruction 



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: default avatarLi Zefan <lizefan@huawei.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarIngo Molnar <mingo@kernel.org>
parent fe1c06ca

include/linux/sched.h

+3 −0
Original line number Diff line number Diff line
@@ -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);

kernel/sched/auto_group.c

+3 −0
Original line number Diff line number Diff line
@@ -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);

kernel/sched/core.c

+39 −10
Original line number Diff line number Diff line
@@ -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 */
@@ -7197,6 +7201,12 @@ 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,