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Subject[RFC][PATCH] Fix cpu hotplug hang

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


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