With keeping the max and min sched time within one jiffy of the gtod clock
was too tight. Just before a schedule tick the max could easily be hit, as
well as just after a schedule_tick the min could be hit. This caused the
clock to jump around by a jiffy.
This patch widens the minimum to
last gtod + (delta_jiffies ? delta_jiffies - 1 : 0) * TICK_NSECS
and the maximum to
last gtod + (2 + delta_jiffies) * TICK_NSECS
This keeps the minum to gtod or if one jiffy less than delta jiffies
and the maxim 2 jiffies ahead of gtod. This may cause unstable TSCs to be
a bit more sporadic, but it helps keep a clock with a stable TSC working well.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
---
kernel/sched_clock.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
Index: linux-tip.git/kernel/sched_clock.c
===================================================================
--- linux-tip.git.orig/kernel/sched_clock.c 2008-07-06 22:35:35.000000000 -0400
+++ linux-tip.git/kernel/sched_clock.c 2008-07-06 22:33:49.000000000 -0400
@@ -96,14 +96,21 @@ static void __update_sched_clock(struct
s64 delta = now - scd->prev_raw;
WARN_ON_ONCE(!irqs_disabled());
- min_clock = scd->tick_gtod + delta_jiffies * TICK_NSEC;
+
+ min_clock = scd->tick_gtod +
+ (delta_jiffies ? delta_jiffies - 1 : 0) * TICK_NSEC;
if (unlikely(delta < 0)) {
clock++;
goto out;
}
- max_clock = min_clock + TICK_NSEC;
+ /*
+ * The clock must stay within a jiffie of the gtod.
+ * But since we may be at the start of a jiffy or the end of one
+ * we add another jiffy buffer.
+ */
+ max_clock = scd->tick_gtod + (2 + delta_jiffies) * TICK_NSEC;
if (unlikely(clock + delta > max_clock)) {
if (clock < max_clock)
--
--
