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Subject[patch 08/22] dynticks: extend next_timer_interrupt() to use a reference jiffie
From: Thomas Gleixner <tglx@linutronix.de>

For CONFIG_NO_HZ we need to calculate the next timer wheel event based to a
given jiffie value. Extend the existing code to allow the extra now argument.
Provide a compability function for the existing implementations to call the
function with now = jiffies. This also solves the racyness of the original
code vs. jiffies changing during the iteration.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
include/linux/timer.h | 10 +++++
kernel/timer.c | 97 ++++++++++++++++++++++++++++++++++++--------------
2 files changed, 81 insertions(+), 26 deletions(-)
Index: linux-2.6.18-mm3/include/linux/timer.h
===================================================================
--- linux-2.6.18-mm3.orig/include/linux/timer.h 2006-10-04 18:13:52.000000000 +0200
+++ linux-2.6.18-mm3/include/linux/timer.h 2006-10-04 18:13:55.000000000 +0200
@@ -61,7 +61,17 @@ extern int del_timer(struct timer_list *
extern int __mod_timer(struct timer_list *timer, unsigned long expires);
extern int mod_timer(struct timer_list *timer, unsigned long expires);

+/*
+ * Return when the next timer-wheel timeout occurs (in absolute jiffies),
+ * locks the timer base:
+ */
extern unsigned long next_timer_interrupt(void);
+/*
+ * Return when the next timer-wheel timeout occurs (in absolute jiffies),
+ * locks the timer base and does the comparison against the given
+ * jiffie.
+ */
+extern unsigned long get_next_timer_interrupt(unsigned long now);

/***
* add_timer - start a timer
Index: linux-2.6.18-mm3/kernel/timer.c
===================================================================
--- linux-2.6.18-mm3.orig/kernel/timer.c 2006-10-04 18:13:54.000000000 +0200
+++ linux-2.6.18-mm3/kernel/timer.c 2006-10-04 18:13:55.000000000 +0200
@@ -468,29 +468,14 @@ static inline void __run_timers(tvec_bas
* is used on S/390 to stop all activity when a cpus is idle.
* This functions needs to be called disabled.
*/
-unsigned long next_timer_interrupt(void)
+unsigned long __next_timer_interrupt(tvec_base_t *base, unsigned long now)
{
- tvec_base_t *base;
struct list_head *list;
- struct timer_list *nte;
+ struct timer_list *nte, *found = NULL;
unsigned long expires;
- unsigned long hr_expires = MAX_JIFFY_OFFSET;
- ktime_t hr_delta;
tvec_t *varray[4];
int i, j;

- hr_delta = hrtimer_get_next_event();
- if (hr_delta.tv64 != KTIME_MAX) {
- struct timespec tsdelta;
- tsdelta = ktime_to_timespec(hr_delta);
- hr_expires = timespec_to_jiffies(&tsdelta);
- if (hr_expires < 3)
- return hr_expires + jiffies;
- }
- hr_expires += jiffies;
-
- base = __get_cpu_var(tvec_bases);
- spin_lock(&base->lock);
expires = base->timer_jiffies + (LONG_MAX >> 1);
list = NULL;

@@ -499,6 +484,7 @@ unsigned long next_timer_interrupt(void)
do {
list_for_each_entry(nte, base->tv1.vec + j, entry) {
expires = nte->expires;
+ found = nte;
if (j < (base->timer_jiffies & TVR_MASK))
list = base->tv2.vec + (INDEX(0));
goto found;
@@ -518,9 +504,12 @@ unsigned long next_timer_interrupt(void)
j = (j + 1) & TVN_MASK;
continue;
}
- list_for_each_entry(nte, varray[i]->vec + j, entry)
- if (time_before(nte->expires, expires))
+ list_for_each_entry(nte, varray[i]->vec + j, entry) {
+ if (time_before(nte->expires, expires)) {
expires = nte->expires;
+ found = nte;
+ }
+ }
if (j < (INDEX(i)) && i < 3)
list = varray[i + 1]->vec + (INDEX(i + 1));
goto found;
@@ -534,10 +523,59 @@ found:
* where we found the timer element.
*/
list_for_each_entry(nte, list, entry) {
- if (time_before(nte->expires, expires))
+ if (time_before(nte->expires, expires)) {
expires = nte->expires;
+ found = nte;
+ }
}
}
+ WARN_ON(!found);
+
+ return expires;
+}
+
+#ifdef CONFIG_NO_HZ
+
+unsigned long get_next_timer_interrupt(unsigned long now)
+{
+ tvec_base_t *base = __get_cpu_var(tvec_bases);
+ unsigned long expires;
+
+ spin_lock(&base->lock);
+ expires = __next_timer_interrupt(base, now);
+ spin_unlock(&base->lock);
+
+ /*
+ * 'Timer wheel time' can lag behind 'jiffies time' due to
+ * delayed processing, so make sure we return a value that
+ * makes sense externally. base->timer_jiffies is unchanged,
+ * so it is safe to access it outside the lock.
+ */
+
+ return expires - (now - base->timer_jiffies);
+}
+
+#else
+
+unsigned long next_timer_interrupt(void)
+{
+ tvec_base_t *base = __get_cpu_var(tvec_bases);
+ unsigned long expires;
+ unsigned long now = jiffies;
+ unsigned long hr_expires = MAX_JIFFY_OFFSET;
+ ktime_t hr_delta = hrtimer_get_next_event();
+
+ if (hr_delta.tv64 != KTIME_MAX) {
+ struct timespec tsdelta;
+ tsdelta = ktime_to_timespec(hr_delta);
+ hr_expires = timespec_to_jiffies(&tsdelta);
+ if (hr_expires < 3)
+ return hr_expires + now;
+ }
+ hr_expires += now;
+
+ spin_lock(&base->lock);
+ expires = __next_timer_interrupt(base, now);
spin_unlock(&base->lock);

/*
@@ -553,16 +591,23 @@ found:
* would falsely evaluate to true. If that is the case, just
* return jiffies so that we can immediately fire the local timer
*/
- if (time_before(expires, jiffies))
- return jiffies;
+ if (time_before(expires, now))
+ expires = now;
+ else if (time_before(hr_expires, expires))
+ expires = hr_expires;

- if (time_before(hr_expires, expires))
- return hr_expires;
-
- return expires;
+ /*
+ * 'Timer wheel time' can lag behind 'jiffies time' due to
+ * delayed processing, so make sure we return a value that
+ * makes sense externally. base->timer_jiffies is unchanged,
+ * so it is safe to access it outside the lock.
+ */
+ return expires - (now - base->timer_jiffies);
}
#endif

+#endif
+
/******************************************************************/

/*
--

-
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