Commit f6d7d049 authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup: factor out css_set_move_task() 



A task is associated and disassociated with its css_set in three
places - during migration, after a new task is created and when a task
exits.  The first is handled by cgroup_task_migrate() and the latter
two are open-coded.

These are similar operations and spreading them over multiple places
makes it harder to follow and update.  This patch collects all task
css_set [dis]association operations into css_set_move_task().

While css_set_move_task() may check whether populated state needs to
be updated when not strictly necessary, the behavior is essentially
equivalent before and after this patch.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 389b9c1b

kernel/cgroup.c

+56 −48
Original line number Diff line number Diff line
@@ -664,6 +664,52 @@ static void css_set_update_populated(struct css_set *cset, bool populated) 
		cgroup_update_populated(link->cgrp, populated);

}



/**

 * css_set_move_task - move a task from one css_set to another

 * @task: task being moved

 * @from_cset: css_set @task currently belongs to (may be NULL)

 * @to_cset: new css_set @task is being moved to (may be NULL)

 * @use_mg_tasks: move to @to_cset->mg_tasks instead of ->tasks

 *

 * Move @task from @from_cset to @to_cset.  If @task didn't belong to any

 * css_set, @from_cset can be NULL.  If @task is being disassociated

 * instead of moved, @to_cset can be NULL.

 *

 * This function automatically handles populated_cnt updates but the caller

 * is responsible for managing @from_cset and @to_cset's reference counts.

 */

static void css_set_move_task(struct task_struct *task,

			      struct css_set *from_cset, struct css_set *to_cset,

			      bool use_mg_tasks)

{

	lockdep_assert_held(&css_set_rwsem);



	if (from_cset) {

		WARN_ON_ONCE(list_empty(&task->cg_list));

		list_del_init(&task->cg_list);

		if (!css_set_populated(from_cset))

			css_set_update_populated(from_cset, false);

	} else {

		WARN_ON_ONCE(!list_empty(&task->cg_list));

	}



	if (to_cset) {

		/*

		 * We are synchronized through cgroup_threadgroup_rwsem

		 * against PF_EXITING setting such that we can't race

		 * against cgroup_exit() changing the css_set to

		 * init_css_set and dropping the old one.

		 */

		WARN_ON_ONCE(task->flags & PF_EXITING);



		if (!css_set_populated(to_cset))

			css_set_update_populated(to_cset, true);

		rcu_assign_pointer(task->cgroups, to_cset);

		list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :

							     &to_cset->tasks);

	}

}



/*

 * hash table for cgroup groups. This improves the performance to find

 * an existing css_set. This hash doesn't (currently) take into
@@ -2261,47 +2307,6 @@ struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset) 
	return NULL;

}



/**

 * cgroup_task_migrate - move a task from one cgroup to another.

 * @tsk: the task being migrated

 * @new_cset: the new css_set @tsk is being attached to

 *

 * Must be called with cgroup_mutex, threadgroup and css_set_rwsem locked.

 */

static void cgroup_task_migrate(struct task_struct *tsk,

				struct css_set *new_cset)

{

	struct css_set *old_cset;



	lockdep_assert_held(&cgroup_mutex);

	lockdep_assert_held(&css_set_rwsem);



	/*

	 * We are synchronized through cgroup_threadgroup_rwsem against

	 * PF_EXITING setting such that we can't race against cgroup_exit()

	 * changing the css_set to init_css_set and dropping the old one.

	 */

	WARN_ON_ONCE(tsk->flags & PF_EXITING);

	old_cset = task_css_set(tsk);



	if (!css_set_populated(new_cset))

		css_set_update_populated(new_cset, true);



	get_css_set(new_cset);

	rcu_assign_pointer(tsk->cgroups, new_cset);

	list_move_tail(&tsk->cg_list, &new_cset->mg_tasks);



	if (!css_set_populated(old_cset))

		css_set_update_populated(old_cset, false);



	/*

	 * We just gained a reference on old_cset by taking it from the

	 * task. As trading it for new_cset is protected by cgroup_mutex,

	 * we're safe to drop it here; it will be freed under RCU.

	 */

	put_css_set_locked(old_cset);

}



/**

 * cgroup_taskset_migrate - migrate a taskset to a cgroup

 * @tset: taget taskset
@@ -2342,8 +2347,14 @@ static int cgroup_taskset_migrate(struct cgroup_taskset *tset, 
	 */

	down_write(&css_set_rwsem);

	list_for_each_entry(cset, &tset->src_csets, mg_node) {

		list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list)

			cgroup_task_migrate(task, cset->mg_dst_cset);

		list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list) {

			struct css_set *from_cset = task_css_set(task);

			struct css_set *to_cset = cset->mg_dst_cset;



			get_css_set(to_cset);

			css_set_move_task(task, from_cset, to_cset, true);

			put_css_set_locked(from_cset);

		}

	}

	up_write(&css_set_rwsem);
@@ -5478,9 +5489,8 @@ void cgroup_post_fork(struct task_struct *child, 
		down_write(&css_set_rwsem);

		cset = task_css_set(current);

		if (list_empty(&child->cg_list)) {

			rcu_assign_pointer(child->cgroups, cset);

			list_add_tail(&child->cg_list, &cset->tasks);

			get_css_set(cset);

			css_set_move_task(child, NULL, cset, false);

		}

		up_write(&css_set_rwsem);

	}
@@ -5528,9 +5538,7 @@ void cgroup_exit(struct task_struct *tsk) 


	if (!list_empty(&tsk->cg_list)) {

		down_write(&css_set_rwsem);

		list_del_init(&tsk->cg_list);

		if (!css_set_populated(cset))

			css_set_update_populated(cset, false);

		css_set_move_task(tsk, cset, NULL, false);

		up_write(&css_set_rwsem);

		put_cset = true;

	}

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