[tip: sched/core] sched/rt: Skip group schedulable check with rt_group_sched=0

[tip-bot2 for Michal Koutný]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     8b016dcec9365675be81d26be88f2c09cf983bd4
Gitweb:        https://git.kernel.org/tip/8b016dcec9365675be81d26be88f2c09cf983bd4
Author:        Michal Koutný <mkoutny@xxxxxxxx>
AuthorDate:    Mon, 23 Mar 2026 13:39:37 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 08 Apr 2026 13:11:44 +02:00

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

The warning from the commit 87f1fb77d87a6 ("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: 87f1fb77d87a6 ("sched: Add RT_GROUP WARN checks for non-root task_groups")
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260323-sched-rert_groups-v3-1-1e7d5ed6b249@xxxxxxxx
---
 kernel/sched/rt.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index a48e867..893e54d 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2690,9 +2690,6 @@ 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);
 
 	/*
@@ -2836,6 +2833,8 @@ 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);