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>

3 files changed, 72 insertions, 83 deletions

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index e34015effeb6..9ea40ff26f0e 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -291,4 +291,6 @@ static inline void update_vsyscall_tz(void)
 }
 #endif
 
+extern void timekeeping_notify(struct clocksource *clock);
+
 #endif /* _LINUX_CLOCKSOURCE_H */
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index f18c9a6bdcf4..a1657b5fdeb9 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -109,35 +109,17 @@ EXPORT_SYMBOL(timecounter_cyc2time);
 /*[Clocksource internal variables]---------
  * curr_clocksource:
  *      currently selected clocksource.
- * next_clocksource:
- *      pending next selected clocksource.
  * clocksource_list:
  *      linked list with the registered clocksources
- * clocksource_lock:
- *      protects manipulations to curr_clocksource and next_clocksource
- *      and the clocksource_list
+ * clocksource_mutex:
+ *      protects manipulations to curr_clocksource and the clocksource_list
  * override_name:
  *      Name of the user-specified clocksource.
  */
 static struct clocksource *curr_clocksource;
-static struct clocksource *next_clocksource;
 static LIST_HEAD(clocksource_list);
-static DEFINE_SPINLOCK(clocksource_lock);
+static DEFINE_MUTEX(clocksource_mutex);
 static char override_name[32];
-static int finished_booting;
-
-/* clocksource_done_booting - Called near the end of core bootup
- *
- * Hack to avoid lots of clocksource churn at boot time.
- * We use fs_initcall because we want this to start before
- * device_initcall but after subsys_initcall.
- */
-static int __init clocksource_done_booting(void)
-{
-        finished_booting = 1;
-        return 0;
-}
-fs_initcall(clocksource_done_booting);
 
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
 static LIST_HEAD(watchdog_list);
@@ -356,18 +338,16 @@ static inline void clocksource_resume_watchdog(void) { }
 void clocksource_resume(void)
 {
         struct clocksource *cs;
-        unsigned long flags;
 
-        spin_lock_irqsave(&clocksource_lock, flags);
+        mutex_lock(&clocksource_mutex);
 
-        list_for_each_entry(cs, &clocksource_list, list) {
+        list_for_each_entry(cs, &clocksource_list, list)
                 if (cs->resume)
                         cs->resume();
-        }
 
         clocksource_resume_watchdog();
 
-        spin_unlock_irqrestore(&clocksource_lock, flags);
+        mutex_unlock(&clocksource_mutex);
 }
 
 /**
@@ -383,28 +363,13 @@ void clocksource_touch_watchdog(void)
 }
 
 #ifdef CONFIG_GENERIC_TIME
-/**
- * clocksource_get_next - Returns the selected clocksource
- *
- */
-struct clocksource *clocksource_get_next(void)
-{
-        unsigned long flags;
 
-        spin_lock_irqsave(&clocksource_lock, flags);
-        if (next_clocksource && finished_booting) {
-                curr_clocksource = next_clocksource;
-                next_clocksource = NULL;
-        }
-        spin_unlock_irqrestore(&clocksource_lock, flags);
-
-        return curr_clocksource;
-}
+static int finished_booting;
 
 /**
  * clocksource_select - Select the best clocksource available
  *
- * Private function. Must hold clocksource_lock when called.
+ * Private function. Must hold clocksource_mutex when called.
  *
  * Select the clocksource with the best rating, or the clocksource,
  * which is selected by userspace override.
@@ -413,7 +378,7 @@ static void clocksource_select(void)
 {
         struct clocksource *best, *cs;
 
-        if (list_empty(&clocksource_list))
+        if (!finished_booting || list_empty(&clocksource_list))
                 return;
         /* First clocksource on the list has the best rating. */
         best = list_first_entry(&clocksource_list, struct clocksource, list);
@@ -438,13 +403,31 @@ static void clocksource_select(void)
                         best = cs;
                 break;
         }
-        if (curr_clocksource != best)
-                next_clocksource = best;
+        if (curr_clocksource != best) {
+                printk(KERN_INFO "Switching to clocksource %s\n", best->name);
+                curr_clocksource = best;
+                timekeeping_notify(curr_clocksource);
+        }
 }
 
+/*
+ * clocksource_done_booting - Called near the end of core bootup
+ *
+ * Hack to avoid lots of clocksource churn at boot time.
+ * We use fs_initcall because we want this to start before
+ * device_initcall but after subsys_initcall.
+ */
+static int __init clocksource_done_booting(void)
+{
+        finished_booting = 1;
+        clocksource_select();
+        return 0;
+}
+fs_initcall(clocksource_done_booting);
+
 #else /* CONFIG_GENERIC_TIME */
 
-static void clocksource_select(void) { }
+static inline void clocksource_select(void) { }
 
 #endif
 
@@ -471,13 +454,11 @@ static void clocksource_enqueue(struct clocksource *cs)
  */
 int clocksource_register(struct clocksource *cs)
 {
-        unsigned long flags;
-
-        spin_lock_irqsave(&clocksource_lock, flags);
+        mutex_lock(&clocksource_mutex);
         clocksource_enqueue(cs);
         clocksource_select();
-        spin_unlock_irqrestore(&clocksource_lock, flags);
         clocksource_enqueue_watchdog(cs);
+        mutex_unlock(&clocksource_mutex);
         return 0;
 }
 EXPORT_SYMBOL(clocksource_register);
@@ -487,14 +468,12 @@ EXPORT_SYMBOL(clocksource_register);
  */
 void clocksource_change_rating(struct clocksource *cs, int rating)
 {
-        unsigned long flags;
-
-        spin_lock_irqsave(&clocksource_lock, flags);
+        mutex_lock(&clocksource_mutex);
         list_del(&cs->list);
         cs->rating = rating;
         clocksource_enqueue(cs);
         clocksource_select();
-        spin_unlock_irqrestore(&clocksource_lock, flags);
+        mutex_unlock(&clocksource_mutex);
 }
 EXPORT_SYMBOL(clocksource_change_rating);
 
@@ -503,13 +482,11 @@ EXPORT_SYMBOL(clocksource_change_rating);
  */
 void clocksource_unregister(struct clocksource *cs)
 {
-        unsigned long flags;
-
+        mutex_lock(&clocksource_mutex);
         clocksource_dequeue_watchdog(cs);
-        spin_lock_irqsave(&clocksource_lock, flags);
         list_del(&cs->list);
         clocksource_select();
-        spin_unlock_irqrestore(&clocksource_lock, flags);
+        mutex_unlock(&clocksource_mutex);
 }
 EXPORT_SYMBOL(clocksource_unregister);
 
@@ -527,9 +504,9 @@ sysfs_show_current_clocksources(struct sys_device *dev,
 {
         ssize_t count = 0;
 
-        spin_lock_irq(&clocksource_lock);
+        mutex_lock(&clocksource_mutex);
         count = snprintf(buf, PAGE_SIZE, "%s\n", curr_clocksource->name);
-        spin_unlock_irq(&clocksource_lock);
+        mutex_unlock(&clocksource_mutex);
 
         return count;
 }
@@ -557,14 +534,14 @@ static ssize_t sysfs_override_clocksource(struct sys_device *dev,
         if (buf[count-1] == '\n')
                 count--;
 
-        spin_lock_irq(&clocksource_lock);
+        mutex_lock(&clocksource_mutex);
 
         if (count > 0)
                 memcpy(override_name, buf, count);
         override_name[count] = 0;
         clocksource_select();
 
-        spin_unlock_irq(&clocksource_lock);
+        mutex_unlock(&clocksource_mutex);
 
         return ret;
 }
@@ -584,7 +561,7 @@ sysfs_show_available_clocksources(struct sys_device *dev,
         struct clocksource *src;
         ssize_t count = 0;
 
-        spin_lock_irq(&clocksource_lock);
+        mutex_lock(&clocksource_mutex);
         list_for_each_entry(src, &clocksource_list, list) {
                 /*
                  * Don't show non-HRES clocksource if the tick code is
@@ -596,7 +573,7 @@ sysfs_show_available_clocksources(struct sys_device *dev,
                                   max((ssize_t)PAGE_SIZE - count, (ssize_t)0),
                                   "%s ", src->name);
         }
-        spin_unlock_irq(&clocksource_lock);
+        mutex_unlock(&clocksource_mutex);
 
         count += snprintf(buf + count,
                           max((ssize_t)PAGE_SIZE - count, (ssize_t)0), "\n");
@@ -651,11 +628,10 @@ device_initcall(init_clocksource_sysfs);
  */
 static int __init boot_override_clocksource(char* str)
 {
-        unsigned long flags;
-        spin_lock_irqsave(&clocksource_lock, flags);
+        mutex_lock(&clocksource_mutex);
         if (str)
                 strlcpy(override_name, str, sizeof(override_name));
-        spin_unlock_irqrestore(&clocksource_lock, flags);
+        mutex_unlock(&clocksource_mutex);
         return 1;
 }
 
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);
 }