sched: Make pushable_tasks CONFIG_SMP dependant

As noted by Peter Zijlstra at https://lkml.org/lkml/2010/11/10/391
(while reviewing other stuff, though), tracking pushable tasks
only makes sense on SMP systems.
Signed-off-by: Dario Faggioli <raistlin@linux.it>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1291143093.2697.298.camel@Palantir>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/init_task.h

@@ -12,6 +12,13 @@
 #include <linux/securebits.h>
 #include <net/net_namespace.h>
 
+#ifdef CONFIG_SMP
+# define INIT_PUSHABLE_TASKS(tsk)					\
+	.pushable_tasks = PLIST_NODE_INIT(tsk.pushable_tasks, MAX_PRIO),
+#else
+# define INIT_PUSHABLE_TASKS(tsk)
+#endif
+
 extern struct files_struct init_files;
 extern struct fs_struct init_fs;
 
@@ -137,7 +144,7 @@ extern struct cred init_cred;
 		.nr_cpus_allowed = NR_CPUS,				\
 	},								\
 	.tasks		= LIST_HEAD_INIT(tsk.tasks),			\
-	.pushable_tasks = PLIST_NODE_INIT(tsk.pushable_tasks, MAX_PRIO), \
+	INIT_PUSHABLE_TASKS(tsk)					\
 	.ptraced	= LIST_HEAD_INIT(tsk.ptraced),			\
 	.ptrace_entry	= LIST_HEAD_INIT(tsk.ptrace_entry),		\
 	.real_parent	= &tsk,						\

include/linux/sched.h

@@ -1240,7 +1240,9 @@ struct task_struct {
 #endif
 
 	struct list_head tasks;
+#ifdef CONFIG_SMP
 	struct plist_node pushable_tasks;
+#endif
 
 	struct mm_struct *mm, *active_mm;
 #if defined(SPLIT_RSS_COUNTING)

kernel/sched.c

@@ -2595,7 +2595,9 @@ void sched_fork(struct task_struct *p, int clone_flags)
 	/* Want to start with kernel preemption disabled. */
 	task_thread_info(p)->preempt_count = 1;
 #endif
+#ifdef CONFIG_SMP
 	plist_node_init(&p->pushable_tasks, MAX_PRIO);
+#endif
 
 	put_cpu();
 }