sched: clarify ifdef tangle

- Add some comments to try to make the ifdef puzzle a bit clearer

- Explicitly inline one of the three init_hrtick() implementations.
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1102,7 +1102,7 @@ static void hrtick_start(struct rq *rq, u64 delay)
 	hrtimer_start(&rq->hrtick_timer, ns_to_ktime(delay), HRTIMER_MODE_REL);
 }
 
-static void init_hrtick(void)
+static inline void init_hrtick(void)
 {
 }
 #endif /* CONFIG_SMP */
@@ -1121,7 +1121,7 @@ static void init_rq_hrtick(struct rq *rq)
 	rq->hrtick_timer.function = hrtick;
 	rq->hrtick_timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
 }
-#else
+#else	/* CONFIG_SCHED_HRTICK */
 static inline void hrtick_clear(struct rq *rq)
 {
 }
@@ -1133,7 +1133,7 @@ static inline void init_rq_hrtick(struct rq *rq)
 static inline void init_hrtick(void)
 {
 }
-#endif
+#endif	/* CONFIG_SCHED_HRTICK */
 
 /*
  * resched_task - mark a task 'to be rescheduled now'.