hrtimers: Reorder clock bases

The ordering of the clock bases is historical due to the
CLOCK_REALTIME and CLOCK_MONOTONIC constants. Now the hrtimer bases
have their own enumeration due to the gap between CLOCK_MONOTONIC and
CLOCK_BOOTTIME. So we can be more clever as most timers end up on the
CLOCK_MONOTONIC base due to the virtue of POSIX declaring that
relative CLOCK_REALTIME timers are not affected by time changes. In
desktop environments this is slowly changing as applications switch to
absolute timers, but I've observed empty CLOCK_REALTIME bases often
enough. There is no performance penalty or overhead when
CLOCK_REALTIME timers are active, but in case they are not we don't
skip over a full cache line.

Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Peter Zijlstra <[email protected]>

include/linux/hrtimer.h

@@ -153,8 +153,8 @@ struct hrtimer_clock_base {
 };
 
 enum  hrtimer_base_type {
-	HRTIMER_BASE_REALTIME,
 	HRTIMER_BASE_MONOTONIC,
+	HRTIMER_BASE_REALTIME,
 	HRTIMER_BASE_BOOTTIME,
 	HRTIMER_MAX_CLOCK_BASES,
 };

kernel/hrtimer.c

@@ -63,18 +63,18 @@ DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) =
 
 	.clock_base =
 	{
-		{
-			.index = HRTIMER_BASE_REALTIME,
-			.clockid = CLOCK_REALTIME,
-			.get_time = &ktime_get_real,
-			.resolution = KTIME_LOW_RES,
-		},
 		{
 			.index = HRTIMER_BASE_MONOTONIC,
 			.clockid = CLOCK_MONOTONIC,
 			.get_time = &ktime_get,
 			.resolution = KTIME_LOW_RES,
 		},
+		{
+			.index = HRTIMER_BASE_REALTIME,
+			.clockid = CLOCK_REALTIME,
+			.get_time = &ktime_get_real,
+			.resolution = KTIME_LOW_RES,
+		},
 		{
 			.index = HRTIMER_BASE_BOOTTIME,
 			.clockid = CLOCK_BOOTTIME,