Commit 9501b6cf authored Mar 25, 2007 by Thomas Gleixner Committed by Linus Torvalds Mar 25, 2007

[PATCH] dynticks: fix hrtimer rounding error in next_timer_interrupt

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 317ec6cd

kernel/timer.c

+16 −3

Original line number	Diff line number	Diff line
		@@ -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;