sched: optimize cond_resched()

Optimize cond_resched() by removing one conditional.

Currently cond_resched() checks system_state ==
SYSTEM_RUNNING in order to avoid scheduling before the
scheduler is running.

We can however, as per suggestion of Matt, use
PREEMPT_ACTIVE to accomplish that very same.
Suggested-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -501,8 +501,11 @@ struct task_cputime {
 /*
  * Disable preemption until the scheduler is running.
  * Reset by start_kernel()->sched_init()->init_idle().
+ *
+ * We include PREEMPT_ACTIVE to avoid cond_resched() from working
+ * before the scheduler is active -- see should_resched().
  */
-#define INIT_PREEMPT_COUNT	(1)
+#define INIT_PREEMPT_COUNT	(1 + PREEMPT_ACTIVE)
 
 /**
  * struct thread_group_cputimer - thread group interval timer counts

kernel/sched.c

@@ -6541,6 +6541,11 @@ SYSCALL_DEFINE0(sched_yield)
 	return 0;
 }
 
+static inline int should_resched(void)
+{
+	return need_resched() && !(preempt_count() & PREEMPT_ACTIVE);
+}
+
 static void __cond_resched(void)
 {
 #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
@@ -6560,8 +6565,7 @@ static void __cond_resched(void)
 
 int __sched _cond_resched(void)
 {
-	if (need_resched() && !(preempt_count() & PREEMPT_ACTIVE) &&
-					system_state == SYSTEM_RUNNING) {
+	if (should_resched()) {
 		__cond_resched();
 		return 1;
 	}
@@ -6579,12 +6583,12 @@ EXPORT_SYMBOL(_cond_resched);
  */
 int cond_resched_lock(spinlock_t *lock)
 {
-	int resched = need_resched() && system_state == SYSTEM_RUNNING;
+	int resched = should_resched();
 	int ret = 0;
 
 	if (spin_needbreak(lock) || resched) {
 		spin_unlock(lock);
-		if (resched && need_resched())
+		if (resched)
 			__cond_resched();
 		else
 			cpu_relax();
@@ -6599,7 +6603,7 @@ int __sched cond_resched_softirq(void)
 {
 	BUG_ON(!in_softirq());
 
-	if (need_resched() && system_state == SYSTEM_RUNNING) {
+	if (should_resched()) {
 		local_bh_enable();
 		__cond_resched();
 		local_bh_disable();