[PATCH] Tweak idle thread setup semantics

This patch tweaks idle thread setup semantics a bit: instead of setting
NEED_RESCHED in init_idle(), we do an explicit schedule() before calling
into cpu_idle().

This patch, while having no negative side-effects, enables wider use of
cond_resched()s.  (which might happen in the stock kernel too, but it's
particulary important for voluntary-preempt)
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

init/main.c

@@ -383,6 +383,13 @@ static void noinline rest_init(void)
 	numa_default_policy();
 	unlock_kernel();
 	preempt_enable_no_resched();
+
+	/*
+	 * The boot idle thread must execute schedule()
+	 * at least one to get things moving:
+	 */
+	schedule();
+
 	cpu_idle();
 } 
 

kernel/sched.c

@@ -4166,6 +4166,14 @@ void show_state(void)
 	read_unlock(&tasklist_lock);
 }
 
+/**
+ * init_idle - set up an idle thread for a given CPU
+ * @idle: task in question
+ * @cpu: cpu the idle task belongs to
+ *
+ * NOTE: this function does not set the idle thread's NEED_RESCHED
+ * flag, to make booting more robust.
+ */
 void __devinit init_idle(task_t *idle, int cpu)
 {
 	runqueue_t *rq = cpu_rq(cpu);
@@ -4183,7 +4191,6 @@ void __devinit init_idle(task_t *idle, int cpu)
 #if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
 	idle->oncpu = 1;
 #endif
-	set_tsk_need_resched(idle);
 	spin_unlock_irqrestore(&rq->lock, flags);
 
 	/* Set the preempt count _outside_ the spinlocks! */