[RFC][patch 03/12] cleanup clocksource selection

Martin Schwidefsky
2009-07-29T13:42:33Z

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From: Martin Schwidefsky <schwidefsky@de.ibm.com>

Sorting the list of clocksources according to their rating is not
necessary as the clocksource selection is not performance critical.
Simplify clocksource_register and sysfs_override_clocksource,
remove clocksource_enqueue and let clocksource_select do the job.
The check in clocksource_register for double registration of the
same clocksource is removed without replacement.

To find the initial clocksource a new weak function in jiffies.c is
defined that returns the jiffies clocksource. The architecture code
can then override the weak function with a more suitable clocksource,
e.g. the TOD clock on s390.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Daniel Walker <dwalker@fifo99.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
arch/s390/kernel/time.c | 4 +
include/linux/clocksource.h | 2
kernel/time/clocksource.c | 148 ++++++++++++++------------------------------
kernel/time/jiffies.c | 6 +
kernel/time/timekeeping.c | 4 -
5 files changed, 62 insertions(+), 102 deletions(-)

Index: linux-2.6/kernel/time/clocksource.c
===================================================================
--- linux-2.6.orig/kernel/time/clocksource.c
+++ linux-2.6/kernel/time/clocksource.c
@@ -21,7 +21,6 @@
*
* TODO WishList:
* o Allow clocksource drivers to be unregistered
- * o get rid of clocksource_jiffies extern
*/

#include <linux/clocksource.h>
@@ -107,9 +106,6 @@ u64 timecounter_cyc2time(struct timecoun
}
EXPORT_SYMBOL(timecounter_cyc2time);

-/* XXX - Would like a better way for initializing curr_clocksource */
-extern struct clocksource clocksource_jiffies;
-
/*[Clocksource internal variables]---------
* curr_clocksource:
* currently selected clocksource. Initialized to clocksource_jiffies.
@@ -123,9 +119,8 @@ extern struct clocksource clocksource_ji
* override_name:
* Name of the user-specified clocksource.
*/
-static struct clocksource *curr_clocksource = &clocksource_jiffies;
+static struct clocksource *curr_clocksource;
static struct clocksource *next_clocksource;
-static struct clocksource *clocksource_override;
static LIST_HEAD(clocksource_list);
static DEFINE_SPINLOCK(clocksource_lock);
static char override_name[32];
@@ -320,6 +315,7 @@ void clocksource_touch_watchdog(void)
clocksource_resume_watchdog();
}

+#ifdef CONFIG_GENERIC_TIME
/**
* clocksource_get_next - Returns the selected clocksource
*
@@ -339,77 +335,69 @@ struct clocksource *clocksource_get_next
}

/**
- * select_clocksource - Selects the best registered clocksource.
- *
- * Private function. Must hold clocksource_lock when called.
- *
- * Select the clocksource with the best rating, or the clocksource,
- * which is selected by userspace override.
+ * clocksource_select - Select the best clocksource available
*/
-static struct clocksource *select_clocksource(void)
+static int clocksource_select(void)
{
- struct clocksource *next;
-
- if (list_empty(&clocksource_list))
- return NULL;
-
- if (clocksource_override)
- next = clocksource_override;
- else
- next = list_entry(clocksource_list.next, struct clocksource,
- list);
+ struct clocksource *best, *cs;
+ int rc;

- if (next == curr_clocksource)
- return NULL;
-
- return next;
-}
-
-/*
- * Enqueue the clocksource sorted by rating
- */
-static int clocksource_enqueue(struct clocksource *c)
-{
- struct list_head *tmp, *entry = &clocksource_list;
-
- list_for_each(tmp, &clocksource_list) {
- struct clocksource *cs;
-
- cs = list_entry(tmp, struct clocksource, list);
- if (cs == c)
- return -EBUSY;
- /* Keep track of the place, where to insert */
- if (cs->rating >= c->rating)
- entry = tmp;
+ rc = 0;
+ best = NULL;
+ list_for_each_entry(cs, &clocksource_list, list) {
+ /* Check for the override clocksource. */
+ if (strcmp(cs->name, override_name) == 0) {
+ /*
+ * Check to make sure we don't switch to a non-highres
+ * capable clocksource if the tick code is in oneshot
+ * mode (highres or nohz)
+ */
+ if ((cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) ||
+ !tick_oneshot_mode_active()) {
+ /* Override clocksource can be used. */
+ best = cs;
+ break;
+ }
+ /* Override clocksource cannot be used. */
+ printk(KERN_WARNING "Override clocksource %s is not "
+ "HRT compatible. Cannot switch while in "
+ "HRT/NOHZ mode\n", cs->name);
+ override_name[0] = 0;
+ }
+ /* Check rating */
+ if (!best || cs->rating > best->rating)
+ best = cs;
}
- list_add(&c->list, entry);
+ if (curr_clocksource != best)
+ next_clocksource = best;
+ return rc;
+}

- if (strlen(c->name) == strlen(override_name) &&
- !strcmp(c->name, override_name))
- clocksource_override = c;
+#else /* CONFIG_GENERIC_TIME */

+static inline int clocksource_select(void)
+{
return 0;
}

+#endif
+
/**
* clocksource_register - Used to install new clocksources
* @t: clocksource to be registered
*
* Returns -EBUSY if registration fails, zero otherwise.
*/
-int clocksource_register(struct clocksource *c)
+int clocksource_register(struct clocksource *cs)
{
unsigned long flags;
- int ret;

spin_lock_irqsave(&clocksource_lock, flags);
- ret = clocksource_enqueue(c);
- if (!ret)
- next_clocksource = select_clocksource();
+ list_add(&cs->list, &clocksource_list);
+ clocksource_select();
spin_unlock_irqrestore(&clocksource_lock, flags);
- if (!ret)
- clocksource_check_watchdog(c);
- return ret;
+ clocksource_check_watchdog(cs);
+ return 0;
}
EXPORT_SYMBOL(clocksource_register);

@@ -422,10 +410,8 @@ void clocksource_change_rating(struct cl
unsigned long flags;

spin_lock_irqsave(&clocksource_lock, flags);
- list_del(&cs->list);
cs->rating = rating;
- clocksource_enqueue(cs);
- next_clocksource = select_clocksource();
+ clocksource_select();
spin_unlock_irqrestore(&clocksource_lock, flags);
}

@@ -438,9 +424,7 @@ void clocksource_unregister(struct clock

spin_lock_irqsave(&clocksource_lock, flags);
list_del(&cs->list);
- if (clocksource_override == cs)
- clocksource_override = NULL;
- next_clocksource = select_clocksource();
+ clocksource_select();
spin_unlock_irqrestore(&clocksource_lock, flags);
}

@@ -478,10 +462,6 @@ static ssize_t sysfs_override_clocksourc
struct sysdev_attribute *attr,
const char *buf, size_t count)
{
- struct clocksource *ovr = NULL;
- size_t ret = count;
- int len;
-
/* strings from sysfs write are not 0 terminated! */
if (count >= sizeof(override_name))
return -EINVAL;
@@ -495,41 +475,11 @@ static ssize_t sysfs_override_clocksourc
if (count > 0)
memcpy(override_name, buf, count);
override_name[count] = 0;
-
- len = strlen(override_name);
- if (len) {
- struct clocksource *cs;
-
- ovr = clocksource_override;
- /* try to select it: */
- list_for_each_entry(cs, &clocksource_list, list) {
- if (strlen(cs->name) == len &&
- !strcmp(cs->name, override_name))
- ovr = cs;
- }
- }
-
- /*
- * Check to make sure we don't switch to a non-highres capable
- * clocksource if the tick code is in oneshot mode (highres or nohz)
- */
- if (tick_oneshot_mode_active() && ovr &&
- !(ovr->flags & CLOCK_SOURCE_VALID_FOR_HRES)) {
- printk(KERN_WARNING "%s clocksource is not HRT compatible. "
- "Cannot switch while in HRT/NOHZ mode\n", ovr->name);
- ovr = NULL;
- override_name[0] = 0;
- }
-
- /* Reselect, when the override name has changed */
- if (ovr != clocksource_override) {
- clocksource_override = ovr;
- next_clocksource = select_clocksource();
- }
+ clocksource_select();

spin_unlock_irq(&clocksource_lock);

- return ret;
+ return count;
}

/**
Index: linux-2.6/include/linux/clocksource.h
===================================================================
--- linux-2.6.orig/include/linux/clocksource.h
+++ linux-2.6/include/linux/clocksource.h
@@ -14,6 +14,7 @@
#include <linux/list.h>
#include <linux/cache.h>
#include <linux/timer.h>
+#include <linux/init.h>
#include <asm/div64.h>
#include <asm/io.h>

@@ -322,6 +323,7 @@ extern void clocksource_touch_watchdog(v
extern struct clocksource* clocksource_get_next(void);
extern void clocksource_change_rating(struct clocksource *cs, int rating);
extern void clocksource_resume(void);
+extern struct clocksource * __init __weak clocksource_default_clock(void);

#ifdef CONFIG_GENERIC_TIME_VSYSCALL
extern void update_vsyscall(struct timespec *ts, struct clocksource *c);
Index: linux-2.6/kernel/time/timekeeping.c
===================================================================
--- linux-2.6.orig/kernel/time/timekeeping.c
+++ linux-2.6/kernel/time/timekeeping.c
@@ -269,7 +269,7 @@ static void change_clocksource(void)

new = clocksource_get_next();

- if (clock == new)
+ if (!new || clock == new)
return;

clocksource_forward_now();
@@ -436,7 +436,7 @@ void __init timekeeping_init(void)

ntp_init();

- clock = clocksource_get_next();
+ clock = clocksource_default_clock();
if (clock->enable)
clock->enable(clock);
/* save mult_orig on enable */
Index: linux-2.6/kernel/time/jiffies.c
===================================================================
--- linux-2.6.orig/kernel/time/jiffies.c
+++ linux-2.6/kernel/time/jiffies.c
@@ -61,7 +61,6 @@ struct clocksource clocksource_jiffies =
.read = jiffies_read,
.mask = 0xffffffff, /*32bits*/
.mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
- .mult_orig = NSEC_PER_JIFFY << JIFFIES_SHIFT,
.shift = JIFFIES_SHIFT,
};

@@ -71,3 +70,8 @@ static int __init init_jiffies_clocksour
}

core_initcall(init_jiffies_clocksource);
+
+struct clocksource * __init __weak clocksource_default_clock(void)
+{
+ return &clocksource_jiffies;
+}
Index: linux-2.6/arch/s390/kernel/time.c
===================================================================
--- linux-2.6.orig/arch/s390/kernel/time.c
+++ linux-2.6/arch/s390/kernel/time.c
@@ -205,6 +205,10 @@ static struct clocksource clocksource_to
.flags = CLOCK_SOURCE_IS_CONTINUOUS,
};

+struct clocksource * __init clocksource_default_clock(void)
+{
+ return &clocksource_tod;
+}

void update_vsyscall(struct timespec *wall_time, struct clocksource *clock)
{