[PATCH 4/4] sched_ext: Don't enable non-ext tasks in the sub-sched task loops

[Tejun Heo]

Root enable and scx_post_fork() enable a task only if it's on the ext class.
Tasks on other classes, possible under an SCX_OPS_SWITCH_PARTIAL root, are
left READY and enabled by switching_to_scx() when they switch over. The sub
enable-commit pass and the sub-disable re-home loop enable unconditionally,
so a fair-class READY task in the subtree becomes ENABLED while not on
sched_ext. A later switch to SCHED_EXT then trips the task state validation
WARN (ENABLED with the previous state not READY) and calls ops.enable() a
second time.

Gate scx_enable_task() on the task's class in both loops.

Fixes: 337ec00b1d9c ("sched_ext: Implement cgroup sub-sched enabling and disabling")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/ext/ext.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 98dd7df88db4..18183062f751 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -6010,15 +6010,22 @@ static void scx_sub_disable(struct scx_sched *sch)
 			/*
 			 * $p is initialized for $parent and still attached to
 			 * @sch. Disable and exit for @sch, switch over to
-			 * $parent, override the state to READY to account for
-			 * $p having already been initialized, and then enable.
+			 * $parent and override the state to READY to account
+			 * for $p having already been initialized.
 			 */
 			scx_disable_and_exit_task(sch, p);
 			scx_set_task_state(p, SCX_TASK_INIT_BEGIN);
 			scx_set_task_state(p, SCX_TASK_INIT);
 			scx_set_task_sched(p, parent);
 			scx_set_task_state(p, SCX_TASK_READY);
-			scx_enable_task(parent, p);
+
+			/*
+			 * A task on a non-ext class, possible under an
+			 * %SCX_OPS_SWITCH_PARTIAL root, stays READY and is
+			 * enabled by switching_to_scx() if it switches over.
+			 */
+			if (p->sched_class == &ext_sched_class)
+				scx_enable_task(parent, p);
 		}
 
 		task_rq_unlock(rq, p, &rf);
@@ -7726,10 +7733,14 @@ static void scx_sub_enable_workfn(struct kthread_work *work)
 
 			/*
 			 * $p is now only initialized for @sch and READY, which
-			 * is what we want. Assign it to @sch and enable.
+			 * is what we want. Assign it to @sch and, if it's on
+			 * the ext class, enable. A non-ext task, possible under
+			 * an %SCX_OPS_SWITCH_PARTIAL root, stays READY and is
+			 * enabled by switching_to_scx() if it switches over.
 			 */
 			scx_set_task_sched(p, sch);
-			scx_enable_task(sch, p);
+			if (p->sched_class == &ext_sched_class)
+				scx_enable_task(sch, p);
 
 			p->scx.flags &= ~SCX_TASK_SUB_INIT;
 		}
-- 
2.55.0