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Subject[PATCH] dynticks: fix hrtimer rounding error in next_timer_interrupt
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The rework of next_timer_interrupt() fixed the timer wheel bugs, but
invented a rounding error versus the next hrtimer event. This is caused
by the conversion of the hrtimer internal representation to relative
jiffies.

This causes bug #8100:
http://bugzilla.kernel.org/show_bug.cgi?id=8100

next_timer_interrupt() returns "now" in such a case and causes the code
in tick_nohz_stop_sched_tick() to trigger the timer softirq, which is
bogus as no timer is due for expiry. This results in an endless context
switching between idle and ksoftirqd until a timer is due for expiry.

Modify the hrtimer evaluation so that, it returns now + 1, when the
conversion results in a delta < 1 jiffie.

It's confirmed to resolve bug #8100

Reported-by: Emil Karlson <jkarlson@cc.hut.fi>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/timer.c b/kernel/timer.c
index 797cccb..440048a 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -695,15 +695,28 @@ static unsigned long cmp_next_hrtimer_event(unsigned long now,
{
ktime_t hr_delta = hrtimer_get_next_event();
struct timespec tsdelta;
+ unsigned long delta;

if (hr_delta.tv64 == KTIME_MAX)
return expires;

- if (hr_delta.tv64 <= TICK_NSEC)
- return now;
+ /*
+ * Expired timer available, let it expire in the next tick
+ */
+ if (hr_delta.tv64 <= 0)
+ return now + 1;

tsdelta = ktime_to_timespec(hr_delta);
- now += timespec_to_jiffies(&tsdelta);
+ delta = timespec_to_jiffies(&tsdelta);
+ /*
+ * Take rounding errors in to account and make sure, that it
+ * expires in the next tick. Otherwise we go into an endless
+ * ping pong due to tick_nohz_stop_sched_tick() retriggering
+ * the timer softirq
+ */
+ if (delta < 1)
+ delta = 1;
+ now += delta;
if (time_before(now, expires))
return now;
return expires;

-
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