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

6 files changed, 2 insertions, 93 deletions

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index b0af8d4ef6e6..1fad1c8a4c76 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -369,8 +369,6 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier,
 
 #endif
 
-#ifdef CONFIG_SMP
-
 /*
  * Migrate-Disable and why it is undesired.
  *
@@ -429,13 +427,6 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier,
 extern void migrate_disable(void);
 extern void migrate_enable(void);
 
-#else
-
-static inline void migrate_disable(void) { }
-static inline void migrate_enable(void) { }
-
-#endif /* CONFIG_SMP */
-
 /**
  * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
  *
diff --git a/include/linux/sched.h b/include/linux/sched.h
index aa54d75034ea..376befdec4b0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -599,7 +599,6 @@ struct sched_entity {
 	unsigned long			runnable_weight;
 #endif
 
-#ifdef CONFIG_SMP
 	/*
 	 * Per entity load average tracking.
 	 *
@@ -607,7 +606,6 @@ struct sched_entity {
 	 * collide with read-mostly values above.
 	 */
 	struct sched_avg		avg;
-#endif
 };
 
 struct sched_rt_entity {
@@ -837,7 +835,6 @@ struct task_struct {
 	struct alloc_tag		*alloc_tag;
 #endif
 
-#ifdef CONFIG_SMP
 	int				on_cpu;
 	struct __call_single_node	wake_entry;
 	unsigned int			wakee_flips;
@@ -853,7 +850,6 @@ struct task_struct {
 	 */
 	int				recent_used_cpu;
 	int				wake_cpu;
-#endif
 	int				on_rq;
 
 	int				prio;
@@ -912,9 +908,7 @@ struct task_struct {
 	cpumask_t			*user_cpus_ptr;
 	cpumask_t			cpus_mask;
 	void				*migration_pending;
-#ifdef CONFIG_SMP
 	unsigned short			migration_disabled;
-#endif
 	unsigned short			migration_flags;
 
 #ifdef CONFIG_PREEMPT_RCU
@@ -946,10 +940,8 @@ struct task_struct {
 	struct sched_info		sched_info;
 
 	struct list_head		tasks;
-#ifdef CONFIG_SMP
 	struct plist_node		pushable_tasks;
 	struct rb_node			pushable_dl_tasks;
-#endif
 
 	struct mm_struct		*mm;
 	struct mm_struct		*active_mm;
@@ -1843,7 +1835,6 @@ extern int cpuset_cpumask_can_shrink(const struct cpumask *cur, const struct cpu
 extern int task_can_attach(struct task_struct *p);
 extern int dl_bw_alloc(int cpu, u64 dl_bw);
 extern void dl_bw_free(int cpu, u64 dl_bw);
-#ifdef CONFIG_SMP
 
 /* do_set_cpus_allowed() - consider using set_cpus_allowed_ptr() instead */
 extern void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask);
@@ -1861,33 +1852,6 @@ extern void release_user_cpus_ptr(struct task_struct *p);
 extern int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask);
 extern void force_compatible_cpus_allowed_ptr(struct task_struct *p);
 extern void relax_compatible_cpus_allowed_ptr(struct task_struct *p);
-#else
-static inline void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
-{
-}
-static inline int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
-{
-	/* Opencoded cpumask_test_cpu(0, new_mask) to avoid dependency on cpumask.h */
-	if ((*cpumask_bits(new_mask) & 1) == 0)
-		return -EINVAL;
-	return 0;
-}
-static inline int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, int node)
-{
-	if (src->user_cpus_ptr)
-		return -EINVAL;
-	return 0;
-}
-static inline void release_user_cpus_ptr(struct task_struct *p)
-{
-	WARN_ON(p->user_cpus_ptr);
-}
-
-static inline int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
-{
-	return 0;
-}
-#endif
 
 extern int yield_to(struct task_struct *p, bool preempt);
 extern void set_user_nice(struct task_struct *p, long nice);
@@ -1976,11 +1940,7 @@ extern int wake_up_state(struct task_struct *tsk, unsigned int state);
 extern int wake_up_process(struct task_struct *tsk);
 extern void wake_up_new_task(struct task_struct *tsk);
 
-#ifdef CONFIG_SMP
 extern void kick_process(struct task_struct *tsk);
-#else
-static inline void kick_process(struct task_struct *tsk) { }
-#endif
 
 extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec);
 #define set_task_comm(tsk, from) ({			\
@@ -2225,7 +2185,6 @@ extern long sched_getaffinity(pid_t pid, struct cpumask *mask);
 #define TASK_SIZE_OF(tsk)	TASK_SIZE
 #endif
 
-#ifdef CONFIG_SMP
 static inline bool owner_on_cpu(struct task_struct *owner)
 {
 	/*
@@ -2237,7 +2196,6 @@ static inline bool owner_on_cpu(struct task_struct *owner)
 
 /* Returns effective CPU energy utilization, as seen by the scheduler */
 unsigned long sched_cpu_util(int cpu);
-#endif /* CONFIG_SMP */
 
 #ifdef CONFIG_SCHED_CORE
 extern void sched_core_free(struct task_struct *tsk);
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