Commit 4530eddb authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup, memcg, cpuset: implement cgroup_taskset_for_each_leader() 



It wasn't explicitly documented but, when a process is being migrated,
cpuset and memcg depend on cgroup_taskset_first() returning the
threadgroup leader; however, this approach is somewhat ghetto and
would no longer work for the planned multi-process migration.

This patch introduces explicit cgroup_taskset_for_each_leader() which
iterates over only the threadgroup leaders and replaces
cgroup_taskset_first() usages for accessing the leader with it.

This prepares both memcg and cpuset for multi-process migration.  This
patch also updates the documentation for cgroup_taskset_for_each() to
clarify the iteration rules and removes comments mentioning task
ordering in tasksets.

v2: A previous patch which added threadgroup leader test was dropped.
    Patch updated accordingly.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarZefan Li <lizefan@huawei.com>
Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
parent 3df9ca0a

include/linux/cgroup.h

+22 −0
Original line number Diff line number Diff line
@@ -232,11 +232,33 @@ void css_task_iter_end(struct css_task_iter *it); 
 * cgroup_taskset_for_each - iterate cgroup_taskset

 * @task: the loop cursor

 * @tset: taskset to iterate

 *

 * @tset may contain multiple tasks and they may belong to multiple

 * processes.  When there are multiple tasks in @tset, if a task of a

 * process is in @tset, all tasks of the process are in @tset.  Also, all

 * are guaranteed to share the same source and destination csses.

 *

 * Iteration is not in any specific order.

 */

#define cgroup_taskset_for_each(task, tset)				\

	for ((task) = cgroup_taskset_first((tset)); (task);		\

	     (task) = cgroup_taskset_next((tset)))



/**

 * cgroup_taskset_for_each_leader - iterate group leaders in a cgroup_taskset

 * @leader: the loop cursor

 * @tset: takset to iterate

 *

 * Iterate threadgroup leaders of @tset.  For single-task migrations, @tset

 * may not contain any.

 */

#define cgroup_taskset_for_each_leader(leader, tset)			\

	for ((leader) = cgroup_taskset_first((tset)); (leader);		\

	     (leader) = cgroup_taskset_next((tset)))			\

		if ((leader) != (leader)->group_leader)			\

			;						\

		else



/*

 * Inline functions.

 */

kernel/cgroup.c

+0 −11
Original line number Diff line number Diff line
@@ -2217,13 +2217,6 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp, 


	get_css_set(new_cset);

	rcu_assign_pointer(tsk->cgroups, new_cset);



	/*

	 * Use move_tail so that cgroup_taskset_first() still returns the

	 * leader after migration.  This works because cgroup_migrate()

	 * ensures that the dst_cset of the leader is the first on the

	 * tset's dst_csets list.

	 */

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



	/*
@@ -2419,10 +2412,6 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader, 
		if (!cset->mg_src_cgrp)

			goto next;



		/*

		 * cgroup_taskset_first() must always return the leader.

		 * Take care to avoid disturbing the ordering.

		 */

		list_move_tail(&task->cg_list, &cset->mg_tasks);

		if (list_empty(&cset->mg_node))

			list_add_tail(&cset->mg_node, &tset.src_csets);

kernel/cpuset.c

+4 −5
Original line number Diff line number Diff line
@@ -1488,7 +1488,7 @@ static void cpuset_attach(struct cgroup_subsys_state *css, 
	/* static buf protected by cpuset_mutex */

	static nodemask_t cpuset_attach_nodemask_to;

	struct task_struct *task;

	struct task_struct *leader = cgroup_taskset_first(tset);

	struct task_struct *leader;

	struct cpuset *cs = css_cs(css);

	struct cpuset *oldcs = cpuset_attach_old_cs;
@@ -1514,12 +1514,11 @@ static void cpuset_attach(struct cgroup_subsys_state *css, 
	}



	/*

	 * Change mm, possibly for multiple threads in a threadgroup. This

	 * is expensive and may sleep and should be moved outside migration

	 * path proper.

	 * Change mm for all threadgroup leaders. This is expensive and may

	 * sleep and should be moved outside migration path proper.

	 */

	cpuset_attach_nodemask_to = cs->effective_mems;

	if (thread_group_leader(leader)) {

	cgroup_taskset_for_each_leader(leader, tset) {

		struct mm_struct *mm = get_task_mm(leader);



		if (mm) {

mm/memcontrol.c

+15 −2
Original line number Diff line number Diff line
@@ -4828,7 +4828,7 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css, 
{

	struct mem_cgroup *memcg = mem_cgroup_from_css(css);

	struct mem_cgroup *from;

	struct task_struct *p;

	struct task_struct *leader, *p;

	struct mm_struct *mm;

	unsigned long move_flags;

	int ret = 0;
@@ -4842,7 +4842,20 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css, 
	if (!move_flags)

		return 0;



	p = cgroup_taskset_first(tset);

	/*

	 * Multi-process migrations only happen on the default hierarchy

	 * where charge immigration is not used.  Perform charge

	 * immigration if @tset contains a leader and whine if there are

	 * multiple.

	 */

	p = NULL;

	cgroup_taskset_for_each_leader(leader, tset) {

		WARN_ON_ONCE(p);

		p = leader;

	}

	if (!p)

		return 0;



	from = mem_cgroup_from_task(p);



	VM_BUG_ON(from == memcg);

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