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Cache xtime every call to update_wall_time

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This avoids xtime lag seen with dynticks, because while 'xtime' itself
is still not updated often, we keep a 'xtime_cache' variable around that
contains the approximate real-time that _is_ updated each time we do a
'update_wall_time()', and is thus never off by more than one tick.

IOW, this restores the original semantics for 'xtime' users, as long as
you use the proper abstraction functions (ie 'current_kernel_time()' or
'get_seconds()' depending on whether you want a timespec or just the
seconds field).

[ Updated Patch.  As penance for my sins I've also yanked another #ifdef
  that was added to avoid the xtime lag w/ hrtimers.  ]

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
john stultz 2007-07-24 18:38:34 -07:00 committed by Linus Torvalds
parent 2c6b47de17
commit 17c38b7490
3 changed files with 24 additions and 12 deletions
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6
include/linux/time.h
View file

@ -99,11 +99,7 @@ extern int update_persistent_clock(struct timespec now);
extern int no_sync_cmos_clock __read_mostly;
void timekeeping_init(void);
static inline unsigned long get_seconds(void)
{
return xtime.tv_sec;
}
unsigned long get_seconds(void);
struct timespec current_kernel_time(void);
#define CURRENT_TIME (current_kernel_time())

4
kernel/hrtimer.c
View file

@ -141,11 +141,7 @@ static void hrtimer_get_softirq_time(struct hrtimer_cpu_base *base)
do {
seq = read_seqbegin(&xtime_lock);
#ifdef CONFIG_NO_HZ
getnstimeofday(&xts);
#else
xts = current_kernel_time();
#endif
tom = wall_to_monotonic;
} while (read_seqretry(&xtime_lock, seq));

26
kernel/time/timekeeping.c
View file

@ -47,10 +47,22 @@ EXPORT_SYMBOL(xtime_lock);
struct timespec xtime __attribute__ ((aligned (16)));
struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
static unsigned long total_sleep_time; /* seconds */
EXPORT_SYMBOL(xtime);
#ifdef CONFIG_NO_HZ
static struct timespec xtime_cache __attribute__ ((aligned (16)));
static inline void update_xtime_cache(u64 nsec)
{
xtime_cache = xtime;
timespec_add_ns(&xtime_cache, nsec);
}
#else
#define xtime_cache xtime
/* We do *not* want to evaluate the argument for this case */
#define update_xtime_cache(n) do { } while (0)
#endif
static struct clocksource *clock; /* pointer to current clocksource */
@ -478,6 +490,8 @@ void update_wall_time(void)
xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift;
clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
update_xtime_cache(cyc2ns(clock, offset));
/* check to see if there is a new clocksource to use */
change_clocksource();
update_vsyscall(&xtime, clock);
@ -510,6 +524,13 @@ void monotonic_to_bootbased(struct timespec *ts)
ts->tv_sec += total_sleep_time;
}
unsigned long get_seconds(void)
{
return xtime_cache.tv_sec;
}
EXPORT_SYMBOL(get_seconds);
struct timespec current_kernel_time(void)
{
struct timespec now;
@ -518,10 +539,9 @@ struct timespec current_kernel_time(void)
do {
seq = read_seqbegin(&xtime_lock);
now = xtime;
now = xtime_cache;
} while (read_seqretry(&xtime_lock, seq));
return now;
}
EXPORT_SYMBOL(current_kernel_time);