cgroup, cpuset: replace move_member_tasks_to_cpuset() with cgroup_transfer_tasks()

When a cpuset becomes empty (no CPU or memory), its tasks are
transferred with the nearest ancestor with execution resources.  This
is implemented using cgroup_scan_tasks() with a callback which grabs
cgroup_mutex and invokes cgroup_attach_task() on each task.

Both cgroup_mutex and cgroup_attach_task() are scheduled to be
unexported.  Implement cgroup_transfer_tasks() in cgroup proper which
is essentially the same as move_member_tasks_to_cpuset() except that
it takes cgroups instead of cpusets and @to comes before @from like
normal functions with those arguments, and replace
move_member_tasks_to_cpuset() with it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 4aee5bdd66c..147d7ccb3b0 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3269,6 +3269,34 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan)
 	return 0;
 }
 
+static void cgroup_transfer_one_task(struct task_struct *task,
+				     struct cgroup_scanner *scan)
+{
+	struct cgroup *new_cgroup = scan->data;
+
+	cgroup_lock();
+	cgroup_attach_task(new_cgroup, task, false);
+	cgroup_unlock();
+}
+
+/**
+ * cgroup_trasnsfer_tasks - move tasks from one cgroup to another
+ * @to: cgroup to which the tasks will be moved
+ * @from: cgroup in which the tasks currently reside
+ */
+int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
+{
+	struct cgroup_scanner scan;
+
+	scan.cg = from;
+	scan.test_task = NULL; /* select all tasks in cgroup */
+	scan.process_task = cgroup_transfer_one_task;
+	scan.heap = NULL;
+	scan.data = to;
+
+	return cgroup_scan_tasks(&scan);
+}
+
 /*
  * Stuff for reading the 'tasks'/'procs' files.
  *