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authorThomas Gleixner <tglx@linutronix.de>2012-05-25 22:08:59 +0000
committerThomas Gleixner <tglx@linutronix.de>2012-06-06 13:49:02 +0200
commite40468a54882ef7411fb178dbf2e465ec2349af7 (patch)
tree5c7859bb5b325752694b3fd0bccbce7b4997dab2 /kernel/timer.c
parent99d5f3aac674fe081ffddd2dbb8946ccbc14c410 (diff)
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timers: Improve get_next_timer_interrupt()
Gilad reported at http://lkml.kernel.org/r/1336056962-10465-2-git-send-email-gilad@benyossef.com "Current timer code fails to correctly return a value meaning that there is no future timer event, with the result that the timer keeps getting re-armed in HZ one shot mode even when we could turn it off, generating unneeded interrupts. What is happening is that when __next_timer_interrupt() wishes to return a value that signifies "there is no future timer event", it returns (base->timer_jiffies + NEXT_TIMER_MAX_DELTA). However, the code in tick_nohz_stop_sched_tick(), which called __next_timer_interrupt() via get_next_timer_interrupt(), compares the return value to (last_jiffies + NEXT_TIMER_MAX_DELTA) to see if the timer needs to be re-armed. base->timer_jiffies != last_jiffies and so tick_nohz_stop_sched_tick() interperts the return value as indication that there is a distant future event 12 days from now and programs the timer to fire next after KTIME_MAX nsecs instead of avoiding to arm it. This ends up causing a needless interrupt once every KTIME_MAX nsecs." Fix this by using the new active timer accounting. This avoids scans when no active timer is enqueued completely, so we don't have to rely on base->timer_next and base->timer_jiffies anymore. Reported-by: Gilad Ben-Yossef <gilad@benyossef.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Link: http://lkml.kernel.org/r/20120525214819.317535385@linutronix.de
Diffstat (limited to 'kernel/timer.c')
-rw-r--r--kernel/timer.c13
1 files changed, 8 insertions, 5 deletions
diff --git a/kernel/timer.c b/kernel/timer.c
index 7fada698bd1a..a61c09374eba 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1326,18 +1326,21 @@ static unsigned long cmp_next_hrtimer_event(unsigned long now,
unsigned long get_next_timer_interrupt(unsigned long now)
{
struct tvec_base *base = __this_cpu_read(tvec_bases);
- unsigned long expires;
+ unsigned long expires = now + NEXT_TIMER_MAX_DELTA;
/*
* Pretend that there is no timer pending if the cpu is offline.
* Possible pending timers will be migrated later to an active cpu.
*/
if (cpu_is_offline(smp_processor_id()))
- return now + NEXT_TIMER_MAX_DELTA;
+ return expires;
+
spin_lock(&base->lock);
- if (time_before_eq(base->next_timer, base->timer_jiffies))
- base->next_timer = __next_timer_interrupt(base);
- expires = base->next_timer;
+ if (base->active_timers) {
+ if (time_before_eq(base->next_timer, base->timer_jiffies))
+ base->next_timer = __next_timer_interrupt(base);
+ expires = base->next_timer;
+ }
spin_unlock(&base->lock);
if (time_before_eq(expires, now))