timekeeping: Update clocksource with stop_machine

update_wall_time calls change_clocksource HZ times per second to check
if a new clock source is available. In close to 100% of all calls
there is no new clock. Replace the tick based check by an update done
with stop_machine.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: John Stultz <johnstul@us.ibm.com>
Cc: Daniel Walker <dwalker@fifo99.com>
LKML-Reference: <20090814134810.711836357@de.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 26 insertions, 15 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 27ae01b596b7..41579e7fcf9d 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -18,6 +18,7 @@
 #include <linux/jiffies.h>
 #include <linux/time.h>
 #include <linux/tick.h>
+#include <linux/stop_machine.h>
 
 /* Structure holding internal timekeeping values. */
 struct timekeeper {
@@ -179,6 +180,7 @@ void timekeeping_leap_insert(int leapsecond)
 }
 
 #ifdef CONFIG_GENERIC_TIME
+
 /**
  * timekeeping_forward_now - update clock to the current time
  *
@@ -351,31 +353,40 @@ EXPORT_SYMBOL(do_settimeofday);
  *
  * Accumulates current time interval and initializes new clocksource
  */
-static void change_clocksource(void)
+static int change_clocksource(void *data)
 {
         struct clocksource *new, *old;
 
-        new = clocksource_get_next();
-
-        if (!new || timekeeper.clock == new)
-                return;
+        new = (struct clocksource *) data;
 
         timekeeping_forward_now();
+        if (!new->enable || new->enable(new) == 0) {
+                old = timekeeper.clock;
+                timekeeper_setup_internals(new);
+                if (old->disable)
+                        old->disable(old);
+        }
+        return 0;
+}
 
-        if (new->enable && !new->enable(new))
+/**
+ * timekeeping_notify - Install a new clock source
+ * @clock:              pointer to the clock source
+ *
+ * This function is called from clocksource.c after a new, better clock
+ * source has been registered. The caller holds the clocksource_mutex.
+ */
+void timekeeping_notify(struct clocksource *clock)
+{
+        if (timekeeper.clock == clock)
                 return;
-
-        old = timekeeper.clock;
-        timekeeper_setup_internals(new);
-
-        if (old->disable)
-                old->disable(old);
-
+        stop_machine(change_clocksource, clock, NULL);
         tick_clock_notify();
 }
+
 #else /* GENERIC_TIME */
+
 static inline void timekeeping_forward_now(void) { }
-static inline void change_clocksource(void) { }
 
 /**
  * ktime_get - get the monotonic time in ktime_t format
@@ -416,6 +427,7 @@ void ktime_get_ts(struct timespec *ts)
                                 ts->tv_nsec + tomono.tv_nsec);
 }
 EXPORT_SYMBOL_GPL(ktime_get_ts);
+
 #endif /* !GENERIC_TIME */
 
 /**
@@ -773,7 +785,6 @@ void update_wall_time(void)
         update_xtime_cache(nsecs);
 
         /* check to see if there is a new clocksource to use */
-        change_clocksource();
         update_vsyscall(&xtime, timekeeper.clock);
 }