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Date: Wed, 2 Apr 2008 13:39:43 +0200 (CEST)
From: Roman Zippel <zippel@...ux-m68k.org>
To: john stultz <johnstul@...ibm.com>
cc: lkml <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 1/2] Introduce clocksource_forward_now
Hi,
On Tue, 18 Mar 2008, john stultz wrote:
> My solution is to introduce CLOCK_MONOTONIC_RAW. This exposes a
> nanosecond based time value, that increments starting at bootup and has
> no frequency adjustments made to it what so ever.
There is a problem with the time offset since the last update_wall_time()
call, which isn't taken into account when switching clocks (possibly
during suspend/resume too), so that the clock might jump back during a
clock switch.
To avoid making the whole more complex it's better to do small cleanup
first, so this patch introduces clocksource_forward_now() which takes care
of this offset since the last update_wall_time() call and adds it to the
clock, so there is no need anymore to deal with it explicitly.
This is also gets rid of the timekeeping_suspend_nsecs hack, instead of
waiting until resume, the value is accumulated during suspend. In the end
there is only a single user of __get_nsec_offset() left, so I integrated
it back to getnstimeofday().
bye, Roman
Signed-off-by: Roman Zippel <zippel@...ux-m68k.org>
---
kernel/time/timekeeping.c | 67 ++++++++++++++++++++--------------------------
1 file changed, 30 insertions(+), 37 deletions(-)
Index: linux-2.6/kernel/time/timekeeping.c
===================================================================
--- linux-2.6.orig/kernel/time/timekeeping.c
+++ linux-2.6/kernel/time/timekeeping.c
@@ -58,27 +58,23 @@ struct clocksource *clock;
#ifdef CONFIG_GENERIC_TIME
/**
- * __get_nsec_offset - Returns nanoseconds since last call to periodic_hook
+ * clocksource_forward_now - update clock to the current time
*
- * private function, must hold xtime_lock lock when being
- * called. Returns the number of nanoseconds since the
- * last call to update_wall_time() (adjusted by NTP scaling)
+ * Forward the current clock to update its state since the last call to
+ * update_wall_time(). This is useful before significant clock changes,
+ * as it avoids having to deal with this time offset explicitly.
*/
-static inline s64 __get_nsec_offset(void)
+static void clocksource_forward_now(void)
{
cycle_t cycle_now, cycle_delta;
- s64 ns_offset;
+ s64 nsec;
- /* read clocksource: */
cycle_now = clocksource_read(clock);
-
- /* calculate the delta since the last update_wall_time: */
cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
+ clock->cycle_last = cycle_now;
- /* convert to nanoseconds: */
- ns_offset = cyc2ns(clock, cycle_delta);
-
- return ns_offset;
+ nsec = cyc2ns(clock, cycle_delta);
+ timespec_add_ns(&xtime, nsec);
}
/**
@@ -89,6 +85,7 @@ static inline s64 __get_nsec_offset(void
*/
void getnstimeofday(struct timespec *ts)
{
+ cycle_t cycle_now, cycle_delta;
unsigned long seq;
s64 nsecs;
@@ -96,7 +93,15 @@ void getnstimeofday(struct timespec *ts)
seq = read_seqbegin(&xtime_lock);
*ts = xtime;
- nsecs = __get_nsec_offset();
+
+ /* read clocksource: */
+ cycle_now = clocksource_read(clock);
+
+ /* calculate the delta since the last update_wall_time: */
+ cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
+
+ /* convert to nanoseconds: */
+ nsecs = cyc2ns(clock, cycle_delta);
} while (read_seqretry(&xtime_lock, seq));
@@ -130,21 +135,19 @@ EXPORT_SYMBOL(do_gettimeofday);
int do_settimeofday(struct timespec *tv)
{
unsigned long flags;
- time_t wtm_sec, sec = tv->tv_sec;
- long wtm_nsec, nsec = tv->tv_nsec;
if ((unsigned long)tv->tv_nsec >= NSEC_PER_SEC)
return -EINVAL;
write_seqlock_irqsave(&xtime_lock, flags);
- nsec -= __get_nsec_offset();
+ clocksource_forward_now();
- wtm_sec = wall_to_monotonic.tv_sec + (xtime.tv_sec - sec);
- wtm_nsec = wall_to_monotonic.tv_nsec + (xtime.tv_nsec - nsec);
+ wall_to_monotonic.tv_sec += xtime.tv_sec - tv->tv_sec;
+ timespec_add_ns(&wall_to_monotonic, xtime.tv_nsec - tv->tv_nsec);
+
+ xtime = *tv;
- set_normalized_timespec(&xtime, sec, nsec);
- set_normalized_timespec(&wall_to_monotonic, wtm_sec, wtm_nsec);
update_xtime_cache(0);
clock->error = 0;
@@ -170,21 +173,16 @@ EXPORT_SYMBOL(do_settimeofday);
static void change_clocksource(void)
{
struct clocksource *new;
- cycle_t now;
- u64 nsec;
new = clocksource_get_next();
if (clock == new)
return;
- now = clocksource_read(new);
- nsec = __get_nsec_offset();
- timespec_add_ns(&xtime, nsec);
+ clocksource_forward_now();
clock = new;
- clock->cycle_last = now;
-
+ clock->cycle_last = clocksource_read(new);
clock->error = 0;
clock->xtime_nsec = 0;
clocksource_calculate_interval(clock, NTP_INTERVAL_LENGTH);
@@ -199,8 +197,8 @@ static void change_clocksource(void)
*/
}
#else
+static inline void clocksource_forward_now(void) { }
static inline void change_clocksource(void) { }
-static inline s64 __get_nsec_offset(void) { return 0; }
#endif
/**
@@ -264,8 +262,6 @@ void __init timekeeping_init(void)
static int timekeeping_suspended;
/* time in seconds when suspend began */
static unsigned long timekeeping_suspend_time;
-/* xtime offset when we went into suspend */
-static s64 timekeeping_suspend_nsecs;
/**
* timekeeping_resume - Resumes the generic timekeeping subsystem.
@@ -291,8 +287,6 @@ static int timekeeping_resume(struct sys
wall_to_monotonic.tv_sec -= sleep_length;
total_sleep_time += sleep_length;
}
- /* Make sure that we have the correct xtime reference */
- timespec_add_ns(&xtime, timekeeping_suspend_nsecs);
update_xtime_cache(0);
/* re-base the last cycle value */
clock->cycle_last = clocksource_read(clock);
@@ -317,8 +311,7 @@ static int timekeeping_suspend(struct sy
timekeeping_suspend_time = read_persistent_clock();
write_seqlock_irqsave(&xtime_lock, flags);
- /* Get the current xtime offset */
- timekeeping_suspend_nsecs = __get_nsec_offset();
+ clocksource_forward_now();
timekeeping_suspended = 1;
write_sequnlock_irqrestore(&xtime_lock, flags);
@@ -459,10 +452,10 @@ void update_wall_time(void)
*/
while (offset >= clock->cycle_interval) {
/* accumulate one interval */
- clock->xtime_nsec += clock->xtime_interval;
- clock->cycle_last += clock->cycle_interval;
offset -= clock->cycle_interval;
+ clock->cycle_last += clock->cycle_interval;
+ clock->xtime_nsec += clock->xtime_interval;
if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
xtime.tv_sec++;
--
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