sched/core: Avoid _cond_resched() for PREEMPT=y

On fully preemptible kernels _cond_resched() is pointless, so avoid
emitting any code for it.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mikulas Patocka <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Peter Zijlstra

include/linux/sched.h

@@ -3209,7 +3209,11 @@ static inline int signal_pending_state(long state, struct task_struct *p)
  * cond_resched_lock() will drop the spinlock before scheduling,
  * cond_resched_softirq() will enable bhs before scheduling.
  */
+#ifndef CONFIG_PREEMPT
 extern int _cond_resched(void);
+#else
+static inline int _cond_resched(void) { return 0; }
+#endif
 
 #define cond_resched() ({			\
 	___might_sleep(__FILE__, __LINE__, 0);	\

kernel/sched/core.c

@@ -4883,6 +4883,7 @@ SYSCALL_DEFINE0(sched_yield)
 	return 0;
 }
 
+#ifndef CONFIG_PREEMPT
 int __sched _cond_resched(void)
 {
 	if (should_resched(0)) {
@@ -4892,6 +4893,7 @@ int __sched _cond_resched(void)
 	return 0;
 }
 EXPORT_SYMBOL(_cond_resched);
+#endif
 
 /*
  * __cond_resched_lock() - if a reschedule is pending, drop the given lock,