[PATCH] sched/core: Report failed rt migrations to non-root cgroup without rt bandwidth under RT_GROUP_SCHED

[Aaron Tomlin <atomlin@atomlin.com>]

Following on from commit c7461cca91 ("cgroup, docs: Be explicit about
independence of RT_GROUP_SCHED and non-cpu controllers"), this patch
introduces an explicit error message that informs the user why the
task migration failed. Now user-mode has a clear and actionable reason
for the failure, greatly assisting with debugging.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 kernel/sched/core.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 62b3416f5e43..b9d7689d21e4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9185,8 +9185,18 @@ static int cpu_cgroup_can_attach(struct cgroup_taskset *tset)
 		goto scx_check;
 
 	cgroup_taskset_for_each(task, css, tset) {
-		if (!sched_rt_can_attach(css_tg(css), task))
+		struct task_group *tg = css_tg(css);
+
+		if (!sched_rt_can_attach(tg, task)) {
+			if (tg != &root_task_group) {
+				pr_err_ratelimited("cgroup: cannot attach "
+						"cpu controller. Task "
+						"%s:%d is not in the root "
+						"cgroup.", task->comm,
+						task_pid_nr(task));
+			}
 			return -EINVAL;
+		}
 	}
 scx_check:
 #endif /* CONFIG_RT_GROUP_SCHED */
-- 
2.49.0