[PATCH v4 7/9] sched/fair: allow changing cgroup of new forked task

[Chengming Zhou]

commit 7dc603c9028e ("sched/fair: Fix PELT integrity for new tasks")
introduce a TASK_NEW state and an unnessary limitation that would fail
when changing cgroup of new forked task.

Because at that time, we can't handle task_change_group_fair() for new
forked fair task which hasn't been woken up by wake_up_new_task(),
which will cause detach on an unattached task sched_avg problem.

This patch delete this unnessary limitation by adding check before do
detach or attach in task_change_group_fair().

So cpu_cgrp_subsys.can_attach() has nothing to do for fair tasks,
only define it in #ifdef CONFIG_RT_GROUP_SCHED.

Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
---
 include/linux/sched.h |  5 ++---
 kernel/sched/core.c   | 30 +++++++-----------------------
 kernel/sched/fair.c   |  7 +++++++
 3 files changed, 16 insertions(+), 26 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 88b8817b827d..b504e55bbf7a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -95,10 +95,9 @@ struct task_group;
 #define TASK_WAKEKILL			0x0100
 #define TASK_WAKING			0x0200
 #define TASK_NOLOAD			0x0400
-#define TASK_NEW			0x0800
 /* RT specific auxilliary flag to mark RT lock waiters */
-#define TASK_RTLOCK_WAIT		0x1000
-#define TASK_STATE_MAX			0x2000
+#define TASK_RTLOCK_WAIT		0x0800
+#define TASK_STATE_MAX			0x1000
 
 /* Convenience macros for the sake of set_current_state: */
 #define TASK_KILLABLE			(TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e74e79f783af..d5faa1700bd7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4500,11 +4500,11 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
 	__sched_fork(clone_flags, p);
 	/*
-	 * We mark the process as NEW here. This guarantees that
+	 * We mark the process as running here. This guarantees that
 	 * nobody will actually run it, and a signal or other external
 	 * event cannot wake it up and insert it on the runqueue either.
 	 */
-	p->__state = TASK_NEW;
+	p->__state = TASK_RUNNING;
 
 	/*
 	 * Make sure we do not leak PI boosting priority to the child.
@@ -4622,7 +4622,6 @@ void wake_up_new_task(struct task_struct *p)
 	struct rq *rq;
 
 	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
-	WRITE_ONCE(p->__state, TASK_RUNNING);
 #ifdef CONFIG_SMP
 	/*
 	 * Fork balancing, do it here and not earlier because:
@@ -10238,36 +10237,19 @@ static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
 	sched_unregister_group(tg);
 }
 
+#ifdef CONFIG_RT_GROUP_SCHED
 static int cpu_cgroup_can_attach(struct cgroup_taskset *tset)
 {
 	struct task_struct *task;
 	struct cgroup_subsys_state *css;
-	int ret = 0;
 
 	cgroup_taskset_for_each(task, css, tset) {
-#ifdef CONFIG_RT_GROUP_SCHED
 		if (!sched_rt_can_attach(css_tg(css), task))
 			return -EINVAL;
-#endif
-		/*
-		 * Serialize against wake_up_new_task() such that if it's
-		 * running, we're sure to observe its full state.
-		 */
-		raw_spin_lock_irq(&task->pi_lock);
-		/*
-		 * Avoid calling sched_move_task() before wake_up_new_task()
-		 * has happened. This would lead to problems with PELT, due to
-		 * move wanting to detach+attach while we're not attached yet.
-		 */
-		if (READ_ONCE(task->__state) == TASK_NEW)
-			ret = -EINVAL;
-		raw_spin_unlock_irq(&task->pi_lock);
-
-		if (ret)
-			break;
 	}
-	return ret;
+	return 0;
 }
+#endif
 
 static void cpu_cgroup_attach(struct cgroup_taskset *tset)
 {
@@ -11103,7 +11085,9 @@ 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,
+#ifdef CONFIG_RT_GROUP_SCHED
 	.can_attach	= cpu_cgroup_can_attach,
+#endif
 	.attach		= cpu_cgroup_attach,
 	.legacy_cftypes	= cpu_legacy_files,
 	.dfl_cftypes	= cpu_files,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4bc76d95a99d..90aba33a3780 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11669,6 +11669,13 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
 #ifdef CONFIG_FAIR_GROUP_SCHED
 static void task_change_group_fair(struct task_struct *p)
 {
+	/*
+	 * We couldn't detach or attach a forked task which
+	 * hasn't been woken up by wake_up_new_task().
+	 */
+	if (!p->on_rq && !se->sum_exec_runtime)
+		return;
+
 	detach_task_cfs_rq(p);
 
 #ifdef CONFIG_SMP
-- 
2.36.1