time: introduce xtime_seconds

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.)

Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/time.c

···
  */
 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))

kernel/time/timekeeping.c

···
 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 */