From 9501b6cf5541f0d576d566a463f1e7d3eaaab4eb Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Sun, 25 Mar 2007 14:31:17 +0200 Subject: [PATCH] [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 Signed-off-by: Thomas Gleixner Signed-off-by: Linus Torvalds --- kernel/timer.c | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-) diff --git a/kernel/timer.c b/kernel/timer.c index 797cccb864..440048acae 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; -- 2.39.5