sched: Expose css_tg()

A new BPF extensible sched_class will use css_tg() in the init and exit
paths to visit all task_groups by walking cgroups.

v4: __setscheduler_prio() is already exposed. Dropped from this patch.

v3: Dropped SCHED_CHANGE_BLOCK() as upstream is adding more generic cleanup
    mechanism.

v2: Expose SCHED_CHANGE_BLOCK() too and update the description.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>

kernel/sched/core.c

@@ -8942,11 +8942,6 @@ void sched_move_task(struct task_struct *tsk)
 	}
 }
 
-static inline struct task_group *css_tg(struct cgroup_subsys_state *css)
-{
-	return css ? container_of(css, struct task_group, css) : NULL;
-}
-
 static struct cgroup_subsys_state *
 cpu_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 {

kernel/sched/sched.h

@@ -514,6 +514,11 @@ static inline int walk_tg_tree(tg_visitor down, tg_visitor up, void *data)
 	return walk_tg_tree_from(&root_task_group, down, up, data);
 }
 
+static inline struct task_group *css_tg(struct cgroup_subsys_state *css)
+{
+	return css ? container_of(css, struct task_group, css) : NULL;
+}
+
 extern int tg_nop(struct task_group *tg, void *data);
 
 #ifdef CONFIG_FAIR_GROUP_SCHED