sched/rt: Provide migrate_disable/enable() inlines

Code which solely needs to prevent migration of a task uses
preempt_disable()/enable() pairs. This is the only reliable way to do so
as setting the task affinity to a single CPU can be undone by a
setaffinity operation from a different task/process.

RT provides a seperate migrate_disable/enable() mechanism which does not
disable preemption to achieve the semantic requirements of a (almost) fully
preemptible kernel.

As it is unclear from looking at a given code path whether the intention is
to disable preemption or migration, introduce migrate_disable/enable()
inline functions which can be used to annotate code which merely needs to
disable migration. Map them to preempt_disable/enable() for now. The RT
substitution will be provided later.

Code which is annotated that way documents that it has no requirement to
protect against reentrancy of a preempting task. Either this is not
required at all or the call sites are already serialized by other means.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/878slclv1u.fsf@nanos.tec.linutronix.de

include/linux/preempt.h

@@ -322,4 +322,34 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier,
 
 #endif
 
+/**
+ * migrate_disable - Prevent migration of the current task
+ *
+ * Maps to preempt_disable() which also disables preemption. Use
+ * migrate_disable() to annotate that the intent is to prevent migration,
+ * but not necessarily preemption.
+ *
+ * Can be invoked nested like preempt_disable() and needs the corresponding
+ * number of migrate_enable() invocations.
+ */
+static __always_inline void migrate_disable(void)
+{
+	preempt_disable();
+}
+
+/**
+ * migrate_enable - Allow migration of the current task
+ *
+ * Counterpart to migrate_disable().
+ *
+ * As migrate_disable() can be invoked nested, only the outermost invocation
+ * reenables migration.
+ *
+ * Currently mapped to preempt_enable().
+ */
+static __always_inline void migrate_enable(void)
+{
+	preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */