Commit 17c38b74 authored by john stultz's avatar john stultz Committed by Linus Torvalds
Browse files

Cache xtime every call to update_wall_time 



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: default avatarJohn Stultz <johnstul@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2c6b47de

include/linux/time.h

+1 −5
Original line number Diff line number Diff line
@@ -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())

kernel/hrtimer.c

+0 −4
Original line number Diff line number Diff line
@@ -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));

kernel/time/timekeeping.c

+23 −3
Original line number Diff line number Diff line
@@ -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);

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