[RFC][PATCH] Fix cpu hotplug hang

[Sebastien Dugue <sebastien.dugue@bull.net>]

  Hi Thomas, Ingo,

  here is a patch that fixes a CPU hotplug hang I get on a Power6 box. It may
not be the only possible fix but it appears to be the cleanest I can think of
at the moment.

  Comments welcomed.

  Thanks,

  Sebastien.


>From b3bf273f7a91a686db25112278fc554b47aa30c6 Mon Sep 17 00:00:00 2001
From: Sebastien Dugue <sebastien.dugue@bull.net>
Date: Mon, 1 Dec 2008 12:22:06 +0100
Subject: [PATCH] Fix cpu hotplug hang

  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: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
---
 include/linux/clockchips.h |    1 +
 kernel/hrtimer.c           |    4 ++++
 kernel/time/tick-common.c  |   26 +++++++++++++++++++-------
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index ed3a5d4..c6de413 100644
--- 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 47e6334..b870bd0 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1794,6 +1794,10 @@ 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, &cpu);
+		break;
 	case CPU_DEAD:
 	case CPU_DEAD_FROZEN:
 		clockevents_notify(CLOCK_EVT_NOTIFY_CPU_DEAD, &cpu);
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index df12434..152871c 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -274,6 +274,21 @@ out_bc:
 }
 
 /*
+ * Transfer the do_timer job away from a dying cpu.
+ *
+ * Called with interrupts disabled.
+ */
+static void tick_handover_do_timer(unsigned int *cpup)
+{
+	if (*cpup == tick_do_timer_cpu) {
+		int cpu = first_cpu(cpu_online_map);
+
+		tick_do_timer_cpu = (cpu != NR_CPUS) ? cpu :
+			TICK_DO_TIMER_NONE;
+	}
+}
+
+/*
  * Shutdown an event device on a given cpu:
  *
  * This is called on a life CPU, when a CPU is dead. So we cannot
@@ -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 = first_cpu(cpu_online_map);
-
-		tick_do_timer_cpu = (cpu != NR_CPUS) ? 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);
-- 
1.6.0.1.308.gede4c