Commit-ID: e40468a54882ef7411fb178dbf2e465ec2349af7
Gitweb: http://git.kernel.org/tip/e40468a54882ef7411fb178dbf2e465ec2349af7
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Fri, 25 May 2012 22:08:59 +0000
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Wed, 6 Jun 2012 13:49:02 +0200
timers: Improve get_next_timer_interrupt()
Gilad reported at
http://lkml.kernel.org/r/1336056962-10465-2-git-send-email-gilad@xxxxxxxxxxxxx
"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@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20120525214819.317535385@xxxxxxxxxxxxx
---
kernel/timer.c | 13 ++++++++-----
1 files changed, 8 insertions(+), 5 deletions(-)
diff --git a/kernel/timer.c b/kernel/timer.c
index 7fada69..a61c093 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))
--
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