sched/fair: Remove redundant cpu_cgrp_subsys->fork()

We use cpu_cgrp_subsys->fork() to set task group for the new fair task
in cgroup_post_fork().

Since commit b1e82065 ("sched: Fix yet more sched_fork() races")
has already set_task_rq() for the new fair task in sched_cgroup_fork(),
so cpu_cgrp_subsys->fork() can be removed.

  cgroup_can_fork()	--> pin parent's sched_task_group
  sched_cgroup_fork()
    __set_task_cpu()
      set_task_rq()
  cgroup_post_fork()
    ss->fork() := cpu_cgroup_fork()
      sched_change_group(..., TASK_SET_GROUP)
        task_set_group_fair()
          set_task_rq()  --> can be removed

After this patch's change, task_change_group_fair() only need to
care about task cgroup migration, make the code much simplier.
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/20220818124805.601-3-zhouchengming@bytedance.com

kernel/sched/core.c

@@ -481,8 +481,7 @@ sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags) { }
  *				p->se.load, p->rt_priority,
  *				p->dl.dl_{runtime, deadline, period, flags, bw, density}
  *  - sched_setnuma():		p->numa_preferred_nid
- *  - sched_move_task()/
- *    cpu_cgroup_fork():	p->sched_task_group
+ *  - sched_move_task():	p->sched_task_group
  *  - uclamp_update_active()	p->uclamp*
  *
  * p->state <- TASK_*:
@@ -10114,7 +10113,7 @@ void sched_release_group(struct task_group *tg)
 	spin_unlock_irqrestore(&task_group_lock, flags);
 }
 
-static void sched_change_group(struct task_struct *tsk, int type)
+static void sched_change_group(struct task_struct *tsk)
 {
 	struct task_group *tg;
 
@@ -10130,7 +10129,7 @@ static void sched_change_group(struct task_struct *tsk, int type)
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	if (tsk->sched_class->task_change_group)
-		tsk->sched_class->task_change_group(tsk, type);
+		tsk->sched_class->task_change_group(tsk);
 	else
 #endif
 		set_task_rq(tsk, task_cpu(tsk));
@@ -10161,7 +10160,7 @@ void sched_move_task(struct task_struct *tsk)
 	if (running)
 		put_prev_task(rq, tsk);
 
-	sched_change_group(tsk, TASK_MOVE_GROUP);
+	sched_change_group(tsk);
 
 	if (queued)
 		enqueue_task(rq, tsk, queue_flags);
@@ -10239,23 +10238,6 @@ static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
 	sched_unregister_group(tg);
 }
 
-/*
- * This is called before wake_up_new_task(), therefore we really only
- * have to set its group bits, all the other stuff does not apply.
- */
-static void cpu_cgroup_fork(struct task_struct *task)
-{
-	struct rq_flags rf;
-	struct rq *rq;
-
-	rq = task_rq_lock(task, &rf);
-
-	update_rq_clock(rq);
-	sched_change_group(task, TASK_SET_GROUP);
-
-	task_rq_unlock(rq, task, &rf);
-}
-
 static int cpu_cgroup_can_attach(struct cgroup_taskset *tset)
 {
 	struct task_struct *task;
@@ -11121,7 +11103,6 @@ struct cgroup_subsys cpu_cgrp_subsys = {
 	.css_released	= cpu_cgroup_css_released,
 	.css_free	= cpu_cgroup_css_free,
 	.css_extra_stat_show = cpu_extra_stat_show,
-	.fork		= cpu_cgroup_fork,
 	.can_attach	= cpu_cgroup_can_attach,
 	.attach		= cpu_cgroup_attach,
 	.legacy_cftypes	= cpu_legacy_files,

kernel/sched/fair.c

@@ -11657,15 +11657,7 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
 }
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
-static void task_set_group_fair(struct task_struct *p)
-{
-	struct sched_entity *se = &p->se;
-
-	set_task_rq(p, task_cpu(p));
-	se->depth = se->parent ? se->parent->depth + 1 : 0;
-}
-
-static void task_move_group_fair(struct task_struct *p)
+static void task_change_group_fair(struct task_struct *p)
 {
 	detach_task_cfs_rq(p);
 	set_task_rq(p, task_cpu(p));
@@ -11677,19 +11669,6 @@ static void task_move_group_fair(struct task_struct *p)
 	attach_task_cfs_rq(p);
 }
 
-static void task_change_group_fair(struct task_struct *p, int type)
-{
-	switch (type) {
-	case TASK_SET_GROUP:
-		task_set_group_fair(p);
-		break;
-
-	case TASK_MOVE_GROUP:
-		task_move_group_fair(p);
-		break;
-	}
-}
-
 void free_fair_sched_group(struct task_group *tg)
 {
 	int i;

kernel/sched/sched.h

@@ -2193,11 +2193,8 @@ struct sched_class {
 
 	void (*update_curr)(struct rq *rq);
 
-#define TASK_SET_GROUP		0
-#define TASK_MOVE_GROUP		1
-
 #ifdef CONFIG_FAIR_GROUP_SCHED
-	void (*task_change_group)(struct task_struct *p, int type);
+	void (*task_change_group)(struct task_struct *p);
 #endif
 };