hrtimers: allow the hot-unplugging of all cpus

Impact: fix CPU hotplug hang on Power6 testbox

On architectures that support offlining all cpus (at least powerpc/pseries),
hot-unpluging the tick_do_timer_cpu can result in a system hang.

This comes from the fact that if the cpu going down happens to be the
cpu doing the tick, then as the tick_do_timer_cpu handover happens after the
cpu is dead (via the CPU_DEAD notification), we're left without ticks,
jiffies are frozen and any task relying on timers (msleep, ...) is stuck.
That's particularly the case for the cpu looping in __cpu_die() waiting
for the dying cpu to be dead.

This patch addresses this by having the tick_do_timer_cpu handover happen
earlier during the CPU_DYING notification. For this, a new clockevent
notification type is introduced (CLOCK_EVT_NOTIFY_CPU_DYING) which is triggered
in hrtimer_cpu_notify().

Signed-off-by: Sebastien Dugue <sebastien.dugue@bull.net>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index cea1536..3a1dbba 100644 (file)
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -36,6 +36,7 @@ enum clock_event_nofitiers {
        CLOCK_EVT_NOTIFY_BROADCAST_EXIT,
        CLOCK_EVT_NOTIFY_SUSPEND,
        CLOCK_EVT_NOTIFY_RESUME,
+       CLOCK_EVT_NOTIFY_CPU_DYING,
        CLOCK_EVT_NOTIFY_CPU_DEAD,
 };
 

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 8fea312..647a40e 100644 (file)
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1608,6 +1608,10 @@ static int __cpuinit hrtimer_cpu_notify(struct notifier_block *self,
                break;
 
 #ifdef CONFIG_HOTPLUG_CPU
+       case CPU_DYING:
+       case CPU_DYING_FROZEN:
+               clockevents_notify(CLOCK_EVT_NOTIFY_CPU_DYING, &scpu);
+               break;
        case CPU_DEAD:
        case CPU_DEAD_FROZEN:
        {

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 63e05d4..21a5ca8 100644 (file)
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -273,6 +273,21 @@ out_bc:
        return ret;
 }
 
+/*
+ * Transfer the do_timer job away from a dying cpu.
+ *
+ * Called with interrupts disabled.
+ */
+static void tick_handover_do_timer(int *cpup)
+{
+       if (*cpup == tick_do_timer_cpu) {
+               int cpu = cpumask_first(cpu_online_mask);
+
+               tick_do_timer_cpu = (cpu < nr_cpu_ids) ? cpu :
+                       TICK_DO_TIMER_NONE;
+       }
+}
+
 /*
  * Shutdown an event device on a given cpu:
  *
@@ -297,13 +312,6 @@ static void tick_shutdown(unsigned int *cpup)
                clockevents_exchange_device(dev, NULL);
                td->evtdev = NULL;
        }
-       /* Transfer the do_timer job away from this cpu */
-       if (*cpup == tick_do_timer_cpu) {
-               int cpu = cpumask_first(cpu_online_mask);
-
-               tick_do_timer_cpu = (cpu < nr_cpu_ids) ? cpu :
-                       TICK_DO_TIMER_NONE;
-       }
        spin_unlock_irqrestore(&tick_device_lock, flags);
 }
 
@@ -357,6 +365,10 @@ static int tick_notify(struct notifier_block *nb, unsigned long reason,
                tick_broadcast_oneshot_control(reason);
                break;
 
+       case CLOCK_EVT_NOTIFY_CPU_DYING:
+               tick_handover_do_timer(dev);
+               break;
+
        case CLOCK_EVT_NOTIFY_CPU_DEAD:
                tick_shutdown_broadcast_oneshot(dev);
                tick_shutdown_broadcast(dev);