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Message-ID: <20250731105543.40832-19-yurand2000@gmail.com>
Date: Thu, 31 Jul 2025 12:55:36 +0200
From: Yuri Andriaccio <yurand2000@...il.com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org,
Luca Abeni <luca.abeni@...tannapisa.it>,
Yuri Andriaccio <yuri.andriaccio@...tannapisa.it>
Subject: [RFC PATCH v2 18/25] sched/rt: Zero rt-cgroups default bandwidth
Set the default rt-cgroups runtime to zero, otherwise a cgroup-v1 kernel will
not be able to start SCHED_DEADLINE tasks. The bandwidth for rt-cgroups must
then be manually assigned after the kernel boots.
Allow zeroing the runtime of the root control group. This runtime only affects
the available bandwidth of the rt-cgroup hierarchy but not the SCHED_FIFO /
SCHED_RR tasks on the global runqueue.
Notes:
Disabling the root control group bandwidth should not cause any side effect, as
SCHED_FIFO / SCHED_RR tasks do not depend on it since the introduction of
fair_servers.
Signed-off-by: Yuri Andriaccio <yurand2000@...il.com>
---
kernel/sched/core.c | 4 ++--
kernel/sched/rt.c | 13 +++++--------
kernel/sched/syscalls.c | 2 +-
3 files changed, 8 insertions(+), 11 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 85950e10bb1..3ac65c6af70 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8719,7 +8719,7 @@ void __init sched_init(void)
#ifdef CONFIG_RT_GROUP_SCHED
init_dl_bandwidth(&root_task_group.dl_bandwidth,
- global_rt_period(), global_rt_runtime());
+ global_rt_period(), 0);
#endif /* CONFIG_RT_GROUP_SCHED */
#ifdef CONFIG_CGROUP_SCHED
@@ -9348,7 +9348,7 @@ 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))
+ if (rt_task(task) && !sched_rt_can_attach(css_tg(css), task))
return -EINVAL;
}
scx_check:
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index f37ac9100d1..75a6860c2e2 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2122,13 +2122,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
static DEFINE_MUTEX(rt_constraints_mutex);
int i, err = 0;
- /*
- * Disallowing the root group RT runtime is BAD, it would disallow the
- * kernel creating (and or operating) RT threads.
- */
- if (tg == &root_task_group && rt_runtime == 0)
- return -EINVAL;
-
/* No period doesn't make any sense. */
if (rt_period == 0)
return -EINVAL;
@@ -2215,8 +2208,12 @@ static int sched_rt_global_constraints(void)
int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk)
{
+ /* Allow executing in the root cgroup regardless of allowed bandwidth */
+ if (tg == &root_task_group)
+ return 1;
+
/* Don't accept real-time tasks when there is no way for them to run */
- if (rt_group_sched_enabled() && rt_task(tsk) && tg->dl_bandwidth.dl_runtime == 0)
+ if (rt_group_sched_enabled() && tg->dl_bandwidth.dl_runtime == 0)
return 0;
return 1;
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 7c1f7649477..71f20be6f29 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -633,7 +633,7 @@ int __sched_setscheduler(struct task_struct *p,
*/
if (rt_group_sched_enabled() &&
dl_bandwidth_enabled() && rt_policy(policy) &&
- task_group(p)->dl_bandwidth.dl_runtime == 0 &&
+ !sched_rt_can_attach(task_group(p), p) &&
!task_group_is_autogroup(task_group(p))) {
retval = -EPERM;
goto unlock;
--
2.50.1
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