sched_ext: Replace SCX_TASK_OFF_TASKS flag with SCX_TASK_DEAD state

SCX_TASK_OFF_TASKS marked tasks already through sched_ext_dead() so cgroup
task iteration would skip them. This can be expressed better with a task
state. Replace the flag with SCX_TASK_DEAD.

scx_disable_and_exit_task() resets state to NONE on its way out, so
sched_ext_dead() now sets DEAD after the wrapper returns. The validation
matrix grows NONE -> DEAD, warns on DEAD -> NONE, and tightens READY's
predecessor to INIT or ENABLED so the new DEAD value cannot silently
transition to READY.

Prepares for the following enable vs dead race fix.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 11 insertions, 6 deletions

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 81841277a54f..2fc4a12711f9 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -723,17 +723,22 @@ static void scx_set_task_state(struct task_struct *p, u32 state)
 
 	switch (state) {
 	case SCX_TASK_NONE:
+		warn = prev_state == SCX_TASK_DEAD;
 		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;
+		warn = !(prev_state == SCX_TASK_INIT ||
+			 prev_state == SCX_TASK_ENABLED);
 		break;
 	case SCX_TASK_ENABLED:
 		warn = prev_state != SCX_TASK_READY;
 		break;
+	case SCX_TASK_DEAD:
+		warn = prev_state != SCX_TASK_NONE;
+		break;
 	default:
 		WARN_ONCE(1, "sched_ext: Invalid task state %d -> %d for %s[%d]",
 			  prev_state, state, p->comm, p->pid);
@@ -972,11 +977,11 @@ static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
 		/*
 		 * cgroup_task_dead() removes the dead tasks from cset->tasks
 		 * after sched_ext_dead() and cgroup iteration may see tasks
-		 * which already finished sched_ext_dead(). %SCX_TASK_OFF_TASKS
-		 * is set by sched_ext_dead() under @p's rq lock. Test it to
+		 * which already finished sched_ext_dead(). %SCX_TASK_DEAD is
+		 * set by sched_ext_dead() under @p's rq lock. Test it to
 		 * avoid visiting tasks which are already dead from SCX POV.
 		 */
-		if (p->scx.flags & SCX_TASK_OFF_TASKS) {
+		if (scx_get_task_state(p) == SCX_TASK_DEAD) {
 			__scx_task_iter_rq_unlock(iter);
 			continue;
 		}
@@ -3847,7 +3852,7 @@ void sched_ext_dead(struct task_struct *p)
 	 * @p is off scx_tasks and wholly ours. scx_root_enable()'s READY ->
 	 * ENABLED transitions can't race us. Disable ops for @p.
 	 *
-	 * %SCX_TASK_OFF_TASKS synchronizes against cgroup task iteration - see
+	 * %SCX_TASK_DEAD synchronizes against cgroup task iteration - see
 	 * scx_task_iter_next_locked(). NONE tasks need no marking: cgroup
 	 * iteration is only used from sub-sched paths, which require root
 	 * enabled. Root enable transitions every live task to at least READY.
@@ -3858,7 +3863,7 @@ void sched_ext_dead(struct task_struct *p)
 
 		rq = task_rq_lock(p, &rf);
 		scx_disable_and_exit_task(scx_task_sched(p), p);
-		p->scx.flags |= SCX_TASK_OFF_TASKS;
+		scx_set_task_state(p, SCX_TASK_DEAD);
 		task_rq_unlock(rq, p, &rf);
 	}
 }