sched/rt: Skip group schedulable check with rt_group_sched=0

[ Upstream commit 8b016dc ]

The warning from the commit 87f1fb7 ("sched: Add RT_GROUP WARN
checks for non-root task_groups") is wrong -- it assumes that only
task_groups with rt_rq are traversed, however, the schedulability check
would iterate all task_groups even when rt_group_sched=0 is disabled at
boot time but some non-root task_groups exist.

The schedulability check is supposed to validate:
  a) that children don't overcommit its parent,
  b) no RT task group overcommits global RT limit.
but with rt_group_sched=0 there is no (non-trivial) hierarchy of RT groups,
therefore skip the validation altogether. Otherwise, writes to the
global sched_rt_runtime_us knob will be rejected with incorrect
validation error.

This fix is immaterial with CONFIG_RT_GROUP_SCHED=n.

Fixes: 87f1fb7 ("sched: Add RT_GROUP WARN checks for non-root task_groups")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260323-sched-rert_groups-v3-1-1e7d5ed6b249@suse.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Werkov

kernel/sched/rt.c

@@ -2685,9 +2685,6 @@ static int tg_rt_schedulable(struct task_group *tg, void *data)
 	    tg->rt_bandwidth.rt_runtime && tg_has_rt_tasks(tg))
 		return -EBUSY;
 
-	if (WARN_ON(!rt_group_sched_enabled() && tg != &root_task_group))
-		return -EBUSY;
-
 	total = to_ratio(period, runtime);
 
 	/*
@@ -2831,6 +2828,8 @@ long sched_group_rt_period(struct task_group *tg)
 static int sched_rt_global_constraints(void)
 {
 	int ret = 0;
+	if (!rt_group_sched_enabled())
+		return ret;
 
 	mutex_lock(&rt_constraints_mutex);
 	ret = __rt_schedulable(NULL, 0, 0);