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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
This commit is contained in:
Thomas Gleixner 2012-05-25 22:08:59 +00:00
parent 99d5f3aac6
commit e40468a548
1 changed files with 8 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
kernel/timer.c
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@ -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))