sys_time() speedup
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 significantly:
2.6.22-rc6:
#threads
1: transactions: 3733 (373.21 per sec.)
2: transactions: 6676 (667.46 per sec.)
3: transactions: 6957 (695.50 per sec.)
4: transactions: 7055 (705.48 per sec.)
5: transactions: 6596 (659.33 per sec.)
2.6.22-rc6 + sys_time.patch:
1: transactions: 4005 (400.47 per sec.)
2: transactions: 7379 (737.77 per sec.)
3: transactions: 7347 (734.49 per sec.)
4: transactions: 7468 (746.65 per sec.)
5: transactions: 7428 (742.47 per sec.)
Mixed API uses of gettimeofday() and time() are guaranteed to be coherent
via the use of a at-most-once-per-second slowpath that updates xtime.
[akpm@linux-foundation.org: build fixes]
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/time.c b/kernel/time.c
index f04791f..ffe1914 100644
--- a/kernel/time.c
+++ b/kernel/time.c
@@ -57,14 +57,17 @@
*/
asmlinkage long sys_time(time_t __user * tloc)
{
- time_t i;
- struct timeval tv;
+ /*
+ * We read xtime.tv_sec atomically - it's updated
+ * atomically by update_wall_time(), so no need to
+ * even read-lock the xtime seqlock:
+ */
+ time_t i = xtime.tv_sec;
- do_gettimeofday(&tv);
- i = tv.tv_sec;
+ smp_rmb(); /* sys_time() results are coherent */
if (tloc) {
- if (put_user(i,tloc))
+ if (put_user(i, tloc))
i = -EFAULT;
}
return i;
@@ -373,12 +376,25 @@
tv->tv_sec = sec;
tv->tv_usec = usec;
-}
+ /*
+ * Make sure xtime.tv_sec [returned by sys_time()] always
+ * follows the gettimeofday() result precisely. This
+ * condition is extremely unlikely, it can hit at most
+ * once per second:
+ */
+ if (unlikely(xtime.tv_sec != tv->tv_sec)) {
+ unsigned long flags;
+
+ write_seqlock_irqsave(&xtime_lock, flags);
+ update_wall_time();
+ write_sequnlock_irqrestore(&xtime_lock, flags);
+ }
+}
EXPORT_SYMBOL(do_gettimeofday);
+#else /* CONFIG_TIME_INTERPOLATION */
-#else
#ifndef CONFIG_GENERIC_TIME
/*
* Simulate gettimeofday using do_gettimeofday which only allows a timeval
@@ -394,7 +410,7 @@
}
EXPORT_SYMBOL_GPL(getnstimeofday);
#endif
-#endif
+#endif /* CONFIG_TIME_INTERPOLATION */
/* Converts Gregorian date to seconds since 1970-01-01 00:00:00.
* Assumes input in normal date format, i.e. 1980-12-31 23:59:59
