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Commit 5762ba18 authored by Sebastien Dugue's avatar Sebastien Dugue Committed by Ingo Molnar
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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: default avatarSebastien Dugue <sebastien.dugue@bull.net>
Cc: <stable@kernel.org>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 0ce74d92

include/linux/clockchips.h

+1 −0
Original line number Diff line number Diff line
@@ -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,

};

kernel/hrtimer.c

+4 −0
Original line number Diff line number Diff line
@@ -1609,6 +1609,10 @@ 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:

	{

kernel/time/tick-common.c

+19 −7
Original line number Diff line number Diff line
@@ -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 = 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:

 *
@@ -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);