sched: do not do cond_resched() when CONFIG_PREEMPT

Why do we even have cond_resched when real preemption
is on? It seems to be a waste of space and time.

remove cond_resched with CONFIG_PREEMPT on.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/kernel.h

@@ -105,8 +105,8 @@ struct user;
  * supposed to.
  */
 #ifdef CONFIG_PREEMPT_VOLUNTARY
-extern int cond_resched(void);
-# define might_resched() cond_resched()
+extern int _cond_resched(void);
+# define might_resched() _cond_resched()
 #else
 # define might_resched() do { } while (0)
 #endif

include/linux/sched.h

@@ -1885,7 +1885,18 @@ static inline int need_resched(void)
  * cond_resched_lock() will drop the spinlock before scheduling,
  * cond_resched_softirq() will enable bhs before scheduling.
  */
-extern int cond_resched(void);
+#ifdef CONFIG_PREEMPT
+static inline int cond_resched(void)
+{
+	return 0;
+}
+#else
+extern int _cond_resched(void);
+static inline int cond_resched(void)
+{
+	return _cond_resched();
+}
+#endif
 extern int cond_resched_lock(spinlock_t * lock);
 extern int cond_resched_softirq(void);
 

kernel/sched.c

@@ -4678,7 +4678,8 @@ static void __cond_resched(void)
 	} while (need_resched());
 }
 
-int __sched cond_resched(void)
+#if !defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTARY)
+int __sched _cond_resched(void)
 {
 	if (need_resched() && !(preempt_count() & PREEMPT_ACTIVE) &&
 					system_state == SYSTEM_RUNNING) {
@@ -4687,7 +4688,8 @@ int __sched cond_resched(void)
 	}
 	return 0;
 }
-EXPORT_SYMBOL(cond_resched);
+EXPORT_SYMBOL(_cond_resched);
+#endif
 
 /*
  * cond_resched_lock() - if a reschedule is pending, drop the given lock,