AcoSail/kernel_optimize_test
8
0
Fork 0
forked from luck/tmp_suning_uos_patched
Code Pull Requests Activity

timekeeping: fix rounding problem during clock update

Browse Source 
Due to a rounding problem during a clock update it's possible for readers
to observe the clock jumping back by 1nsec.  The following simplified
example demonstrates the problem:

cycle	xtime
0	0
1000	999999.6
2000	1999999.2
3000	2999998.8
...

1500 =	1499999.4
=	0.0 + 1499999.4
=	999999.6 + 499999.8

When reading the clock only the full nanosecond part is used, while
timekeeping internally keeps nanosecond fractions.  If the clock is now
updated at cycle 1500 here, a nanosecond is missing due to the truncation.

The simple fix is to round up the xtime value during the update, this also
changes the distance to the reference time, but the adjustment will
automatically take care that it stays under control.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Roman Zippel 2008-09-22 14:42:43 -07:00 committed by Thomas Gleixner
parent eb3f938fd6
commit 5cd1c9c5cf
1 changed files with 6 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
kernel/time/timekeeping.c
View File

@ -454,7 +454,7 @@ void update_wall_time(void)
#else
offset = clock->cycle_interval;
#endif
clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
clock->xtime_nsec = (s64)xtime.tv_nsec << clock->shift;
/* normally this loop will run just once, however in the
* case of lost or late ticks, it will accumulate correctly.
@ -479,9 +479,12 @@ void update_wall_time(void)
/* correct the clock when NTP error is too big */
clocksource_adjust(offset);
/* store full nanoseconds into xtime */
xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift;
/* store full nanoseconds into xtime after rounding it up and
* add the remainder to the error difference.
*/
xtime.tv_nsec = ((s64)clock->xtime_nsec >> clock->shift) + 1;
clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
clock->error += clock->xtime_nsec << (NTP_SCALE_SHIFT - clock->shift);
update_xtime_cache(cyc2ns(clock, offset));