hrtimer: Make hrtimer_cpu_base.next_timer handling unconditional

hrtimer_cpu_base.next_timer stores the pointer to the next expiring timer
in a CPU base.

This pointer cannot be dereferenced and is solely used to check whether a
hrtimer which is removed is the hrtimer which is the first to expire in the
CPU base. If this is the case, then the timer hardware needs to be
reprogrammed to avoid an extra interrupt for nothing.

Again, this is conditional functionality, but there is no compelling reason
to make this conditional. As a preparation, hrtimer_cpu_base.next_timer
needs to be available unconditonally.

Aside of that the upcoming support for softirq based hrtimers requires access
to this pointer unconditionally as well, so our motivation is not entirely
simplicity based.

Make the update of hrtimer_cpu_base.next_timer unconditional and remove the
#ifdef cruft. The impact on CONFIG_HIGH_RES_TIMERS=n && CONFIG_NOHZ=n is
marginal as it's just a store on an already dirtied cacheline.

No functional change.

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: keescook@chromium.org
Link: http://lkml.kernel.org/r/20171221104205.7269-17-anna-maria@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/hrtimer.h

@@ -164,13 +164,13 @@ enum  hrtimer_base_type {
  * @hres_active:	State of high resolution mode
  * @in_hrtirq:		hrtimer_interrupt() is currently executing
  * @hang_detected:	The last hrtimer interrupt detected a hang
- * @next_timer:		Pointer to the first expiring timer
  * @nr_events:		Total number of hrtimer interrupt events
  * @nr_retries:		Total number of hrtimer interrupt retries
  * @nr_hangs:		Total number of hrtimer interrupt hangs
  * @max_hang_time:	Maximum time spent in hrtimer_interrupt
  * @expires_next:	absolute time of the next event, is required for remote
  *			hrtimer enqueue
+ * @next_timer:		Pointer to the first expiring timer
  * @clock_base:		array of clock bases for this cpu
  *
  * Note: next_timer is just an optimization for __remove_hrtimer().
@@ -186,13 +186,13 @@ struct hrtimer_cpu_base {
 #ifdef CONFIG_HIGH_RES_TIMERS
 	unsigned int			in_hrtirq	: 1,
 					hang_detected	: 1;
-	struct hrtimer			*next_timer;
 	unsigned int			nr_events;
 	unsigned short			nr_retries;
 	unsigned short			nr_hangs;
 	unsigned int			max_hang_time;
 #endif
 	ktime_t				expires_next;
+	struct hrtimer			*next_timer;
 	struct hrtimer_clock_base	clock_base[HRTIMER_MAX_CLOCK_BASES];
 } ____cacheline_aligned;
 

kernel/time/hrtimer.c

@@ -459,21 +459,13 @@ __next_base(struct hrtimer_cpu_base *cpu_base, unsigned int *active)
 	while ((base = __next_base((cpu_base), &(active))))
 
 #if defined(CONFIG_NO_HZ_COMMON) || defined(CONFIG_HIGH_RES_TIMERS)
-static inline void hrtimer_update_next_timer(struct hrtimer_cpu_base *cpu_base,
-					     struct hrtimer *timer)
-{
-#ifdef CONFIG_HIGH_RES_TIMERS
-	cpu_base->next_timer = timer;
-#endif
-}
-
 static ktime_t __hrtimer_get_next_event(struct hrtimer_cpu_base *cpu_base)
 {
 	struct hrtimer_clock_base *base;
 	unsigned int active = cpu_base->active_bases;
 	ktime_t expires, expires_next = KTIME_MAX;
 
-	hrtimer_update_next_timer(cpu_base, NULL);
+	cpu_base->next_timer = NULL;
 	for_each_active_base(base, cpu_base, active) {
 		struct timerqueue_node *next;
 		struct hrtimer *timer;
@@ -483,7 +475,7 @@ static ktime_t __hrtimer_get_next_event(struct hrtimer_cpu_base *cpu_base)
 		expires = ktime_sub(hrtimer_get_expires(timer), base->offset);
 		if (expires < expires_next) {
 			expires_next = expires;
-			hrtimer_update_next_timer(cpu_base, timer);
+			cpu_base->next_timer = timer;
 		}
 	}
 	/*