[PATCH 3/6] sched_ext: Replace SCX_TASK_OFF_TASKS flag with SCX_TASK_DEAD state

[Tejun Heo <tj@kernel.org>]

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>
---
 include/linux/sched/ext.h |  9 +++++----
 kernel/sched/ext.c        | 17 +++++++++++------
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index adb9a4de068a..9f1a326ad03e 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -101,24 +101,25 @@ enum scx_ent_flags {
 	SCX_TASK_DEQD_FOR_SLEEP	= 1 << 3, /* last dequeue was for SLEEP */
 	SCX_TASK_SUB_INIT	= 1 << 4, /* task being initialized for a sub sched */
 	SCX_TASK_IMMED		= 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
-	SCX_TASK_OFF_TASKS	= 1 << 6, /* removed from scx_tasks by sched_ext_dead() */
 
 	/*
-	 * Bits 8 and 9 are used to carry task state:
+	 * Bits 8 to 10 are used to carry task state:
 	 *
 	 * NONE		ops.init_task() not called yet
 	 * INIT		ops.init_task() succeeded, but task can be cancelled
 	 * READY	fully initialized, but not in sched_ext
 	 * ENABLED	fully initialized and in sched_ext
+	 * DEAD		terminal state set by sched_ext_dead()
 	 */
-	SCX_TASK_STATE_SHIFT	= 8,	  /* bits 8 and 9 are used to carry task state */
-	SCX_TASK_STATE_BITS	= 2,
+	SCX_TASK_STATE_SHIFT	= 8,
+	SCX_TASK_STATE_BITS	= 3,
 	SCX_TASK_STATE_MASK	= ((1 << SCX_TASK_STATE_BITS) - 1) << SCX_TASK_STATE_SHIFT,
 
 	SCX_TASK_NONE		= 0 << SCX_TASK_STATE_SHIFT,
 	SCX_TASK_INIT		= 1 << SCX_TASK_STATE_SHIFT,
 	SCX_TASK_READY		= 2 << SCX_TASK_STATE_SHIFT,
 	SCX_TASK_ENABLED	= 3 << SCX_TASK_STATE_SHIFT,
+	SCX_TASK_DEAD		= 4 << SCX_TASK_STATE_SHIFT,
 
 	/*
 	 * Bits 12 and 13 are used to carry reenqueue reason. In addition to
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);
 	}
 }
-- 
2.54.0