Commit 8b016dce authored by Michal Koutný's avatar Michal Koutný Committed by Peter Zijlstra
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sched/rt: Skip group schedulable check with rt_group_sched=0 



The warning from the commit 87f1fb77 ("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: 87f1fb77 ("sched: Add RT_GROUP WARN checks for non-root task_groups")
Signed-off-by: default avatarMichal Koutný <mkoutny@suse.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260323-sched-rert_groups-v3-1-1e7d5ed6b249@suse.com
parent 556146ce

kernel/sched/rt.c

+2 −3
Original line number Diff line number Diff line
@@ -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);