sched/smp: Always define sched_domains_mutex_lock()/unlock(), def_root_domain and sched_domains_mutex

Simplify the scheduler by making CONFIG_SMP=y primitives and data
structures unconditional.

Unconditionally build kernel/sched/topology.c and the main sched-domains
locking primitives.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20250528080924.2273858-20-mingo@kernel.org

include/linux/sched.h

@@ -395,15 +395,10 @@ enum uclamp_id {
 	UCLAMP_CNT
 };
 
-#ifdef CONFIG_SMP
 extern struct root_domain def_root_domain;
 extern struct mutex sched_domains_mutex;
 extern void sched_domains_mutex_lock(void);
 extern void sched_domains_mutex_unlock(void);
-#else
-static inline void sched_domains_mutex_lock(void) { }
-static inline void sched_domains_mutex_unlock(void) { }
-#endif
 
 struct sched_param {
 	int sched_priority;

kernel/sched/build_utility.c

@@ -83,9 +83,10 @@
 #ifdef CONFIG_SMP
 # include "cpupri.c"
 # include "stop_task.c"
-# include "topology.c"
 #endif
 
+#include "topology.c"
+
 #ifdef CONFIG_SCHED_CORE
 # include "core_sched.c"
 #endif

kernel/sched/topology.c

@@ -17,6 +17,8 @@ void sched_domains_mutex_unlock(void)
 	mutex_unlock(&sched_domains_mutex);
 }
 
+#ifdef CONFIG_SMP
+
 /* Protected by sched_domains_mutex: */
 static cpumask_var_t sched_domains_tmpmask;
 static cpumask_var_t sched_domains_tmpmask2;
@@ -2842,3 +2844,5 @@ void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
 	partition_sched_domains_locked(ndoms_new, doms_new, dattr_new);
 	sched_domains_mutex_unlock();
 }
+
+#endif /* CONFIG_SMP */