[PATCH v2 2/6] sched_ext: Inline scx_init_task() and move RESET_RUNNABLE_AT into scx_set_task_state()

[Tejun Heo <tj@kernel.org>]

Prepare for the SCX_TASK_INIT_BEGIN/DEAD work that follows by collapsing the
scx_init_task() helper. Move the SCX_TASK_RESET_RUNNABLE_AT setting into
scx_set_task_state() on the INIT transition (it was set unconditionally at
every INIT site through the scx_init_task() helper), inline scx_init_task()
into scx_fork() and scx_root_enable_workfn(), and drop the helper.

As a side effect, scx_sub_disable() migration sequence now also sets
RESET_RUNNABLE_AT (it previously wrote INIT directly without going through
scx_init_task()). The flag triggers a runnable_at reset on the next
set_task_runnable(), which is harmless on a task that has just been moved
between scheds.

On root-enable, p->scx.flags is written without the task's rq lock. The task
isn't visible to scx yet, and a follow-up patch restores the lock-held
write.

v2: Note p->scx.flags rq-lock relaxation on root-enable path. (Andrea)

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 31 +++++++++----------------------
 1 file changed, 9 insertions(+), 22 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 10c6e0261f11..81841277a54f 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -726,6 +726,7 @@ static void scx_set_task_state(struct task_struct *p, u32 state)
 		break;
 	case SCX_TASK_INIT:
 		warn = prev_state != SCX_TASK_NONE;
+		p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;
 		break;
 	case SCX_TASK_READY:
 		warn = prev_state == SCX_TASK_NONE;
@@ -3585,22 +3586,6 @@ static int __scx_init_task(struct scx_sched *sch, struct task_struct *p, bool fo
 	return 0;
 }
 
-static int scx_init_task(struct scx_sched *sch, struct task_struct *p, bool fork)
-{
-	int ret;
-
-	ret = __scx_init_task(sch, p, fork);
-	if (!ret) {
-		/*
-		 * While @p's rq is not locked. @p is not visible to the rest of
-		 * SCX yet and it's safe to update the flags and state.
-		 */
-		p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;
-		scx_set_task_state(p, SCX_TASK_INIT);
-	}
-	return ret;
-}
-
 static void __scx_enable_task(struct scx_sched *sch, struct task_struct *p)
 {
 	struct rq *rq = task_rq(p);
@@ -3763,10 +3748,11 @@ int scx_fork(struct task_struct *p, struct kernel_clone_args *kargs)
 #else
 		struct scx_sched *sch = scx_root;
 #endif
-		ret = scx_init_task(sch, p, true);
-		if (!ret)
-			scx_set_task_sched(p, sch);
-		return ret;
+		ret = __scx_init_task(sch, p, true);
+		if (unlikely(ret))
+			return ret;
+		scx_set_task_state(p, SCX_TASK_INIT);
+		scx_set_task_sched(p, sch);
 	}
 
 	return 0;
@@ -6897,8 +6883,8 @@ static void scx_root_enable_workfn(struct kthread_work *work)
 
 		scx_task_iter_unlock(&sti);
 
-		ret = scx_init_task(sch, p, false);
-		if (ret) {
+		ret = __scx_init_task(sch, p, false);
+		if (unlikely(ret)) {
 			put_task_struct(p);
 			scx_task_iter_stop(&sti);
 			scx_error(sch, "ops.init_task() failed (%d) for %s[%d]",
@@ -6906,6 +6892,7 @@ static void scx_root_enable_workfn(struct kthread_work *work)
 			goto err_disable_unlock_all;
 		}
 
+		scx_set_task_state(p, SCX_TASK_INIT);
 		scx_set_task_sched(p, sch);
 		scx_set_task_state(p, SCX_TASK_READY);
 
-- 
2.54.0