sched/smp: Make SMP unconditional

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

Introduce transitory wrappers for functionality not yet converted to SMP.

Note that this patch is pretty large, because there's no clear separation
between various aspects of the SMP scheduler, it's basically a huge block
of #ifdef CONFIG_SMP. A fair amount of it has to be switched on for it to
boot and work on UP systems.

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/20250528080924.2273858-21-mingo@kernel.org

4 files changed, 2 insertions, 42 deletions

diff --git a/include/linux/sched/deadline.h b/include/linux/sched/deadline.h
index f9aabbc9d22e..c40115d4e34d 100644
--- a/include/linux/sched/deadline.h
+++ b/include/linux/sched/deadline.h
@@ -29,15 +29,11 @@ static inline bool dl_time_before(u64 a, u64 b)
 	return (s64)(a - b) < 0;
 }
 
-#ifdef CONFIG_SMP
-
 struct root_domain;
 extern void dl_add_task_root_domain(struct task_struct *p);
 extern void dl_clear_root_domain(struct root_domain *rd);
 extern void dl_clear_root_domain_cpu(int cpu);
 
-#endif /* CONFIG_SMP */
-
 extern u64 dl_cookie;
 extern bool dl_bw_visited(int cpu, u64 cookie);
 
diff --git a/include/linux/sched/idle.h b/include/linux/sched/idle.h
index 439f6029d3b9..8465ff1f20d1 100644
--- a/include/linux/sched/idle.h
+++ b/include/linux/sched/idle.h
@@ -11,11 +11,7 @@ enum cpu_idle_type {
 	CPU_MAX_IDLE_TYPES
 };
 
-#ifdef CONFIG_SMP
 extern void wake_up_if_idle(int cpu);
-#else
-static inline void wake_up_if_idle(int cpu) { }
-#endif
 
 /*
  * Idle thread specific functions to determine the need_resched
diff --git a/include/linux/sched/nohz.h b/include/linux/sched/nohz.h
index 6d67e9a5af6b..0db7f67935fe 100644
--- a/include/linux/sched/nohz.h
+++ b/include/linux/sched/nohz.h
@@ -6,7 +6,7 @@
  * This is the interface between the scheduler and nohz/dynticks:
  */
 
-#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+#ifdef CONFIG_NO_HZ_COMMON
 extern void nohz_balance_enter_idle(int cpu);
 extern int get_nohz_timer_target(void);
 #else
@@ -23,7 +23,7 @@ static inline void calc_load_nohz_remote(struct rq *rq) { }
 static inline void calc_load_nohz_stop(void) { }
 #endif /* CONFIG_NO_HZ_COMMON */
 
-#if defined(CONFIG_NO_HZ_COMMON) && defined(CONFIG_SMP)
+#ifdef CONFIG_NO_HZ_COMMON
 extern void wake_up_nohz_cpu(int cpu);
 #else
 static inline void wake_up_nohz_cpu(int cpu) { }
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 198bb5cc1774..e54e7fa76ba6 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -9,7 +9,6 @@
 /*
  * sched-domains (multiprocessor balancing) declarations:
  */
-#ifdef CONFIG_SMP
 
 /* Generate SD flag indexes */
 #define SD_FLAG(name, mflags) __##name,
@@ -200,37 +199,6 @@ extern void sched_update_asym_prefer_cpu(int cpu, int old_prio, int new_prio);
 
 # define SD_INIT_NAME(type)		.name = #type
 
-#else /* CONFIG_SMP */
-
-struct sched_domain_attr;
-
-static inline void
-partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
-			struct sched_domain_attr *dattr_new)
-{
-}
-
-static inline bool cpus_equal_capacity(int this_cpu, int that_cpu)
-{
-	return true;
-}
-
-static inline bool cpus_share_cache(int this_cpu, int that_cpu)
-{
-	return true;
-}
-
-static inline bool cpus_share_resources(int this_cpu, int that_cpu)
-{
-	return true;
-}
-
-static inline void sched_update_asym_prefer_cpu(int cpu, int old_prio, int new_prio)
-{
-}
-
-#endif	/* !CONFIG_SMP */
-
 #if defined(CONFIG_ENERGY_MODEL) && defined(CONFIG_CPU_FREQ_GOV_SCHEDUTIL)
 extern void rebuild_sched_domains_energy(void);
 #else