time: introduce xtime_seconds

Subject: time: introduce xtime_seconds
From: Ingo Molnar <[email protected]>

improve performance of sys_time(). sys_time() returns time in seconds, but it does so by calling do_gettimeofday() and then returning the tv_sec portion of the GTOD time. But the data structure "xtime", which is updated by every timer/scheduler tick, already offers HZ granularity time.

the patch improves the sysbench oltp macrobenchmark by 4-5% on an AMD dual-core system:

v2.6.23:

#threads

   1:     transactions:                        4073   (407.23 per sec.)
   2:     transactions:                        8530   (852.81 per sec.)
   3:     transactions:                        8321   (831.88 per sec.)
   4:     transactions:                        8407   (840.58 per sec.)
   5:     transactions:                        8070   (806.74 per sec.)

v2.6.23 + sys_time-speedup.patch:

   1:     transactions:                        4281   (428.09 per sec.)
   2:     transactions:                        8910   (890.85 per sec.)
   3:     transactions:                        8659   (865.79 per sec.)
   4:     transactions:                        8676   (867.34 per sec.)
   5:     transactions:                        8532   (852.91 per sec.)

and by 4-5% on an Intel dual-core system too:

2.6.23:

  1:     transactions:                        4560   (455.94 per sec.)
  2:     transactions:                        10094  (1009.30 per sec.)
  3:     transactions:                        9755   (975.36 per sec.)
  4:     transactions:                        9859   (985.78 per sec.)
  5:     transactions:                        9701   (969.72 per sec.)

2.6.23 + sys_time-speedup.patch:

  1:     transactions:                        4779   (477.84 per sec.)
  2:     transactions:                        10103  (1010.14 per sec.)
  3:     transactions:                        10141  (1013.93 per sec.)
  4:     transactions:                        10371  (1036.89 per sec.)
  5:     transactions:                        10178  (1017.50 per sec.)

(the more CPUs the system has, the more speedup this patch gives for this particular workload.)

---
 kernel/time.c             |    6 +-----
 kernel/time/timekeeping.c |    7 -------
 2 files changed, 1 insertion(+), 12 deletions(-)

Index: linux/kernel/time.c
===================================================================
@@ -57,11 +57,7 @@ EXPORT_SYMBOL(sys_tz);
  */
 asmlinkage long sys_time(time_t __user * tloc)
 {
-	time_t i;
-	struct timespec tv;
-
-	getnstimeofday(&tv);
-	i = tv.tv_sec;
+	time_t i = get_seconds();
 
 	if (tloc) {
 		if (put_user(i,tloc))

Index: linux/kernel/time/timekeeping.c
===================================================================
@@ -49,19 +49,12 @@ struct timespec wall_to_monotonic __attr
 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 */