sched/smt: Expose sched_smt_present static key

Make the scheduler's 'sched_smt_present' static key globaly available, so
it can be used in the x86 speculation control code.

Provide a query function and a stub for the CONFIG_SMP=n case.

Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Ingo Molnar <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Jiri Kosina <[e-mail]>
Cc: Tom Lendacky <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Tim Chen <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Casey Schaufler <[e-mail]>
Cc: Asit Mallick <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Jon Masters <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Greg KH <[e-mail]>
Cc: Dave Stewart <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: [e-mail]
Link: https://lkml.kernel.org/r/[e-mail]

include/linux/sched/smt.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_SCHED_SMT_H
+#define _LINUX_SCHED_SMT_H
+
+#include <linux/static_key.h>
+
+#ifdef CONFIG_SCHED_SMT
+extern struct static_key_false sched_smt_present;
+
+static __always_inline bool sched_smt_active(void)
+{
+	return static_branch_likely(&sched_smt_present);
+}
+#else
+static inline bool sched_smt_active(void) { return false; }
+#endif
+
+#endif

kernel/sched/sched.h

···
 #include <linux/sched/prio.h>
 #include <linux/sched/rt.h>
 #include <linux/sched/signal.h>
+#include <linux/sched/smt.h>
 #include <linux/sched/stat.h>
 #include <linux/sched/sysctl.h>
 #include <linux/sched/task.h>
···
 
 
 #ifdef CONFIG_SCHED_SMT
-
-extern struct static_key_false sched_smt_present;
-
 extern void __update_idle_core(struct rq *rq);
 
 static inline void update_idle_core(struct rq *rq)