sched/rt: Document scheduler related skip-resched-check sites

Create a distinction between scheduler related preempt_enable_no_resched()
calls and the nearly one hundred other places in the kernel that do not
want to reschedule, for one reason or another.

This distinction matters for -rt, where the scheduler and the non-scheduler
preempt models (and checks) are different. For upstream it's purely
documentational.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/n/tip-gs88fvx2mdv5psnzxnv575ke@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/powerpc/kernel/idle.c

@@ -102,7 +102,7 @@ void cpu_idle(void)
 		rcu_idle_exit();
 		tick_nohz_idle_exit();
 		if (cpu_should_die()) {
-			preempt_enable_no_resched();
+			sched_preempt_enable_no_resched();
 			cpu_die();
 		}
 		schedule_preempt_disabled();

arch/sparc/kernel/process_64.c

@@ -106,7 +106,7 @@ void cpu_idle(void)
 
 #ifdef CONFIG_HOTPLUG_CPU
 		if (cpu_is_offline(cpu)) {
-			preempt_enable_no_resched();
+			sched_preempt_enable_no_resched();
 			cpu_play_dead();
 		}
 #endif

include/linux/preempt.h

@@ -48,12 +48,14 @@ do { \
 	barrier(); \
 } while (0)
 
-#define preempt_enable_no_resched() \
+#define sched_preempt_enable_no_resched() \
 do { \
 	barrier(); \
 	dec_preempt_count(); \
 } while (0)
 
+#define preempt_enable_no_resched()	sched_preempt_enable_no_resched()
+
 #define preempt_enable() \
 do { \
 	preempt_enable_no_resched(); \
@@ -92,6 +94,7 @@ do { \
 #else /* !CONFIG_PREEMPT_COUNT */
 
 #define preempt_disable()		do { } while (0)
+#define sched_preempt_enable_no_resched()	do { } while (0)
 #define preempt_enable_no_resched()	do { } while (0)
 #define preempt_enable()		do { } while (0)
 

kernel/sched/core.c

@@ -3220,7 +3220,7 @@ static void __sched __schedule(void)
 
 	post_schedule(rq);
 
-	preempt_enable_no_resched();
+	sched_preempt_enable_no_resched();
 	if (need_resched())
 		goto need_resched;
 }
@@ -3253,7 +3253,7 @@ EXPORT_SYMBOL(schedule);
  */
 void __sched schedule_preempt_disabled(void)
 {
-	preempt_enable_no_resched();
+	sched_preempt_enable_no_resched();
 	schedule();
 	preempt_disable();
 }
@@ -4486,7 +4486,7 @@ SYSCALL_DEFINE0(sched_yield)
 	__release(rq->lock);
 	spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
 	do_raw_spin_unlock(&rq->lock);
-	preempt_enable_no_resched();
+	sched_preempt_enable_no_resched();
 
 	schedule();
 

kernel/softirq.c

@@ -353,7 +353,7 @@ void irq_exit(void)
 		tick_nohz_irq_exit();
 #endif
 	rcu_irq_exit();
-	preempt_enable_no_resched();
+	sched_preempt_enable_no_resched();
 }
 
 /*
@@ -759,7 +759,7 @@ static int run_ksoftirqd(void * __bind_cpu)
 			if (local_softirq_pending())
 				__do_softirq();
 			local_irq_enable();
-			preempt_enable_no_resched();
+			sched_preempt_enable_no_resched();
 			cond_resched();
 			preempt_disable();
 			rcu_note_context_switch((long)__bind_cpu);