sched: Add commadline option for RT_GROUP_SCHED toggling

Only simple implementation with a static key wrapper, it will be wired
in later.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250310170442.504716-5-mkoutny@suse.com

Author: Werkov

Documentation/admin-guide/kernel-parameters.txt

@@ -6280,6 +6280,11 @@
 			Memory area to be used by remote processor image,
 			managed by CMA.
 
+	rt_group_sched=	[KNL] Enable or disable SCHED_RR/FIFO group scheduling
+			when CONFIG_RT_GROUP_SCHED=y. Defaults to
+			!CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED.
+			Format: <bool>
+
 	rw		[KNL] Mount root device read-write on boot
 
 	S		[KNL] Run init in single mode

init/Kconfig

@@ -1082,6 +1082,17 @@ config RT_GROUP_SCHED
 	  realtime bandwidth for them.
 	  See Documentation/scheduler/sched-rt-group.rst for more information.
 
+config RT_GROUP_SCHED_DEFAULT_DISABLED
+	bool "Require boot parameter to enable group scheduling for SCHED_RR/FIFO"
+	depends on RT_GROUP_SCHED
+	default n
+	help
+	  When set, the RT group scheduling is disabled by default. The option
+	  is in inverted form so that mere RT_GROUP_SCHED enables the group
+	  scheduling.
+
+	  Say N if unsure.
+
 config EXT_GROUP_SCHED
 	bool
 	depends on SCHED_CLASS_EXT && CGROUP_SCHED

kernel/sched/core.c

@@ -9892,6 +9892,31 @@ static struct cftype cpu_legacy_files[] = {
 	{ }	/* Terminate */
 };
 
+#ifdef CONFIG_RT_GROUP_SCHED
+# ifdef CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED
+DEFINE_STATIC_KEY_FALSE(rt_group_sched);
+# else
+DEFINE_STATIC_KEY_TRUE(rt_group_sched);
+# endif
+
+static int __init setup_rt_group_sched(char *str)
+{
+	long val;
+
+	if (kstrtol(str, 0, &val) || val < 0 || val > 1) {
+		pr_warn("Unable to set rt_group_sched\n");
+		return 1;
+	}
+	if (val)
+		static_branch_enable(&rt_group_sched);
+	else
+		static_branch_disable(&rt_group_sched);
+
+	return 1;
+}
+__setup("rt_group_sched=", setup_rt_group_sched);
+#endif /* CONFIG_RT_GROUP_SCHED */
+
 static int cpu_extra_stat_show(struct seq_file *sf,
 			       struct cgroup_subsys_state *css)
 {

kernel/sched/sched.h

@@ -1500,6 +1500,23 @@ static inline bool sched_group_cookie_match(struct rq *rq,
 }
 
 #endif /* !CONFIG_SCHED_CORE */
+#ifdef CONFIG_RT_GROUP_SCHED
+# ifdef CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED
+DECLARE_STATIC_KEY_FALSE(rt_group_sched);
+static inline bool rt_group_sched_enabled(void)
+{
+	return static_branch_unlikely(&rt_group_sched);
+}
+# else
+DECLARE_STATIC_KEY_TRUE(rt_group_sched);
+static inline bool rt_group_sched_enabled(void)
+{
+	return static_branch_likely(&rt_group_sched);
+}
+# endif /* CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED */
+#else
+# define rt_group_sched_enabled()	false
+#endif /* CONFIG_RT_GROUP_SCHED */
 
 static inline void lockdep_assert_rq_held(struct rq *rq)
 {