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Subject[PATCH 12/12] clocksource: atomic signals
Modifies the way clocks are switched to in the timekeeping code. The original
code would constantly monitor the clocksource list checking for newly added
clocksources. I modified this by using atomic types to signal when a new clock
is added. This allows the operation to be used only when it's needed.

The fast path is also reduced to checking a single atomic value.

Signed-Off-By: Daniel Walker <dwalker@mvista.com>

---
include/linux/clocksource.h | 5 ++++
include/linux/timekeeping.h | 10 --------
kernel/time/clocksource.c | 6 +++++
kernel/time/timekeeping.c | 51 +++++++++++++++++++++++++++++++-------------
4 files changed, 49 insertions(+), 23 deletions(-)
Index: linux-2.6.19/include/linux/clocksource.h
===================================================================
--- linux-2.6.19.orig/include/linux/clocksource.h
+++ linux-2.6.19/include/linux/clocksource.h
@@ -26,6 +26,11 @@ typedef u64 cycle_t;
extern struct clocksource clocksource_jiffies;

/*
+ * Atomic signal that is specific to timekeeping.
+ */
+extern atomic_t clock_check;
+
+/*
* Allows inlined calling for notifier routines.
*/
extern struct atomic_notifier_head clocksource_list_notifier;
Index: linux-2.6.19/include/linux/timekeeping.h
===================================================================
--- linux-2.6.19.orig/include/linux/timekeeping.h
+++ linux-2.6.19/include/linux/timekeeping.h
@@ -5,15 +5,7 @@

extern void update_wall_time(void);

-#ifdef CONFIG_GENERIC_TIME
-
-extern struct clocksource *clock;
-
-#else /* CONFIG_GENERIC_TIME */
-static inline int change_clocksource(void)
-{
- return 0;
-}
+#ifndef CONFIG_GENERIC_TIME

static inline void change_clocksource(void) { }
static inline void timekeeping_init_notifier(void) { }
Index: linux-2.6.19/kernel/time/clocksource.c
===================================================================
--- linux-2.6.19.orig/kernel/time/clocksource.c
+++ linux-2.6.19/kernel/time/clocksource.c
@@ -50,6 +50,7 @@ static char override_name[32];
static int finished_booting;

ATOMIC_NOTIFIER_HEAD(clocksource_list_notifier);
+atomic_t clock_check = ATOMIC_INIT(0);

/* clocksource_done_booting - Called near the end of bootup
*
@@ -58,6 +59,8 @@ ATOMIC_NOTIFIER_HEAD(clocksource_list_no
static int __init clocksource_done_booting(void)
{
finished_booting = 1;
+ /* Check for a new clock now */
+ atomic_inc(&clock_check);
return 0;
}

@@ -291,6 +294,9 @@ static ssize_t sysfs_override_clocksourc
/* try to select it: */
next_clocksource = select_clocksource();

+ /* Signal that there is a new clocksource */
+ atomic_inc(&clock_check);
+
spin_unlock_irq(&clocksource_lock);

return ret;
Index: linux-2.6.19/kernel/time/timekeeping.c
===================================================================
--- linux-2.6.19.orig/kernel/time/timekeeping.c
+++ linux-2.6.19/kernel/time/timekeeping.c
@@ -3,6 +3,7 @@
#include <linux/module.h>
#include <linux/timekeeping.h>
#include <linux/time.h>
+#include <linux/timex.h>
#include <linux/hrtimer.h>

/*
@@ -19,7 +20,6 @@ static unsigned long timekeeping_suspend
* Clock used for timekeeping
*/
struct clocksource *clock = &clocksource_jiffies;
-atomic_t clock_recalc_interval = ATOMIC_INIT(0);

/*
* The current time
@@ -150,11 +150,12 @@ int do_settimeofday(struct timespec *tv)
EXPORT_SYMBOL(do_settimeofday);

/**
- * change_clocksource - Swaps clocksources if a new one is available
+ * timkeeping_change_clocksource - Swaps clocksources if a new one is available
*
* Accumulates current time interval and initializes new clocksource
+ * Needs to be called with the xtime_lock held.
*/
-static int change_clocksource(void)
+static int timekeeping_change_clocksource(void)
{
struct clocksource *new;
cycle_t now;
@@ -169,9 +170,15 @@ static int change_clocksource(void)
clock->cycle_last = now;
printk(KERN_INFO "Time: %s clocksource has been installed.\n",
clock->name);
+ hrtimer_clock_notify();
+ clock->error = 0;
+ clock->xtime_nsec = 0;
+ clocksource_calculate_interval(clock, tick_nsec);
return 1;
- } else if (unlikely(atomic_read(&clock_recalc_interval))) {
- atomic_set(&clock_recalc_interval, 0);
+ } else {
+ clock->error = 0;
+ clock->xtime_nsec = 0;
+ clocksource_calculate_interval(clock, tick_nsec);
return 1;
}
return 0;
@@ -198,9 +205,14 @@ int timekeeping_is_continuous(void)
static int
clocksource_callback(struct notifier_block *nb, unsigned long op, void *c)
{
- if (c == clock && op == CLOCKSOURCE_NOTIFY_FREQ &&
- !atomic_read(&clock_recalc_interval))
- atomic_inc(&clock_recalc_interval);
+ if (likely(c != clock))
+ return 0;
+
+ switch (op) {
+ case CLOCKSOURCE_NOTIFY_FREQ:
+ case CLOCKSOURCE_NOTIFY_RATING:
+ atomic_inc(&clock_check);
+ }

return 0;
}
@@ -325,6 +337,13 @@ static int __init timekeeping_init_devic
int error = sysdev_class_register(&timekeeping_sysclass);
if (!error)
error = sysdev_register(&device_timer);
+
+#ifdef CONFIG_GENERIC_TIME
+ /*
+ * Now we signal to switch to a high-res clock.
+ */
+ atomic_inc(&clock_check);
+#endif
return error;
}

@@ -467,13 +486,17 @@ void update_wall_time(void)
xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift;
clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;

- /* check to see if there is a new clocksource to use */
- if (change_clocksource()) {
- clock->error = 0;
- clock->xtime_nsec = 0;
- hrtimer_clock_notify();
- clocksource_calculate_interval(clock, tick_nsec);
+#ifdef CONFIG_GENERIC_TIME
+ /*
+ * check to see if there is a new clocksource to use
+ */
+ if (unlikely(atomic_read(&clock_check))) {
+
+ timekeeping_change_clocksource();
+
+ atomic_set(&clock_check, 0);
}
+#endif /* CONFIG_GENERIC_TIME */
update_vsyscall(&xtime, clock);
}

--
-
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