diff options
authorThomas Gleixner <tglx@linutronix.de>2011-05-16 11:07:48 +0200
committerThomas Gleixner <tglx@linutronix.de>2011-05-16 23:35:41 +0200
commit07f4beb0b5bbfaf36a64aa00d59e670ec578a95a (patch)
parent557d97d57446f55d2c4a66593794ea31ffd0a74d (diff)
tick: Clear broadcast active bit when switching to oneshot
The first cpu which switches from periodic to oneshot mode switches also the broadcast device into oneshot mode. The broadcast device serves as a backup for per cpu timers which stop in deeper C-states. To avoid starvation of the cpus which might be in idle and depend on broadcast mode it marks the other cpus as broadcast active and sets the brodcast expiry value of those cpus to the next tick. The oneshot mode broadcast bit for the other cpus is sticky and gets only cleared when those cpus exit idle. If a cpu was not idle while the bit got set in consequence the bit prevents that the broadcast device is armed on behalf of that cpu when it enters idle for the first time after it switched to oneshot mode. In most cases that goes unnoticed as one of the other cpus has usually a timer pending which keeps the broadcast device armed with a short timeout. Now if the only cpu which has a short timer active has the bit set then the broadcast device will not be armed on behalf of that cpu and will fire way after the expected timer expiry. In the case of Christians bug report it took ~145 seconds which is about half of the wrap around time of HPET (the limit for that device) due to the fact that all other cpus had no timers armed which expired before the 145 seconds timeframe. The solution is simply to clear the broadcast active bit unconditionally when a cpu switches to oneshot mode after the first cpu switched the broadcast device over. It's not idle at that point otherwise it would not be executing that code. [ I fundamentally hate that broadcast crap. Why the heck thought some folks that when going into deep idle it's a brilliant concept to switch off the last device which brings the cpu back from that state? ] Thanks to Christian for providing all the valuable debug information! Reported-and-tested-by: Christian Hoffmann <email@christianhoffmann.info> Cc: John Stultz <johnstul@us.ibm.com> Link: http://lkml.kernel.org/r/%3Calpine.LFD.2.02.1105161105170.3078%40ionos%3E Cc: stable@kernel.org Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
1 files changed, 11 insertions, 1 deletions
diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index da800ffa810c..723c7637e55a 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -522,10 +522,11 @@ static void tick_broadcast_init_next_event(struct cpumask *mask,
void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
+ int cpu = smp_processor_id();
/* Set it up only once ! */
if (bc->event_handler != tick_handle_oneshot_broadcast) {
int was_periodic = bc->mode == CLOCK_EVT_MODE_PERIODIC;
- int cpu = smp_processor_id();
bc->event_handler = tick_handle_oneshot_broadcast;
clockevents_set_mode(bc, CLOCK_EVT_MODE_ONESHOT);
@@ -551,6 +552,15 @@ void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
tick_broadcast_set_event(tick_next_period, 1);
} else
bc->next_event.tv64 = KTIME_MAX;
+ } else {
+ /*
+ * The first cpu which switches to oneshot mode sets
+ * the bit for all other cpus which are in the general
+ * (periodic) broadcast mask. So the bit is set and
+ * would prevent the first broadcast enter after this
+ * to program the bc device.
+ */
+ tick_broadcast_clear_oneshot(cpu);