[PATCH] NOHZ, check to see if tick device is initialized in IRQ handling path

[Prarit Bhargava]

2nd try at this ... going with a more global cc.

I think the linux.git "system hang" isn't really a hang.  For some reason the
panic text wasn't displayed on the console.  I've seen this behaviour a few
times now ... maybe there's a bug in the panic output path?

It seems that the power interrupt is an error with the CPU exceeded the
OSes current requested frequency on the package.  If I disable on demand
cpu frequency, the problem goes away.

Anyhoo, here's a patch...

----8<----

When adding a CPU there is a small window in which interrupts are enabled and
the clock tick device has not been initialized.  If an interrupt occurs in
this window, irq_exit() will be called which calls tick_nohz_irq_exit() which
in turn calls __tick_nohz_idle_enter().

__tick_nohz_idle() enter assumes that the tick has been initialized.  In the
above case, however, it has not and this leads to what appears to be a system
hang on latest linux.git or a the following panic on RHEL6:

Pid: 0, comm: swapper Not tainted 2.6.32-358.el6.x86_64 #1
RIP: 0010:[<ffffffff810a89e5>]  [<ffffffff810a89e5>] tick_nohz_stop_sched_tick+0x2a5/0x3e0
RSP: 0018:ffff88089c503f38  EFLAGS: 00010046
RAX: ffffffff81c07520 RBX: ffff88089c5116a0 RCX: 000002f04bb18cd8
RDX: 0000000000000000 RSI: 000000000000a1b5 RDI: 000002f04bb0eb23
RBP: ffff88089c503f88 R08: ffff88089c50e060 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000017
R13: 000002f04bb17dd5 R14: 0000000000000000 R15: 0000000000000092
FS:  0000000000000000(0000) GS:ffff88089c500000(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 0000000000000078 CR3: 0000000001a85000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process swapper (pid: 0, threadinfo ffff8810745c0000, task ffff8808740f2080)
Stack:
 00000000000116a0 0000000000000087 ffff88089c503f78 0000000000000046
<d> ffff88089c503f98 0000000000000000 0000000000000000 0000000000000000
<d> 0000000000000000 0000000000000000 ffff88089c503f98 ffffffff81076d86
Call Trace:
 <IRQ>
 [<ffffffff81076d86>] irq_exit+0x76/0x90
 [<ffffffff81028dd6>] smp_thermal_interrupt+0x26/0x40
 [<ffffffff8100bcf3>] thermal_interrupt+0x13/0x20
 <EOI>
 [<ffffffff81506997>] ? start_secondary+0x127/0x2ef
 [<ffffffff81506990>] ? start_secondary+0x120/0x2ef

The code currently assumes that the tick device is initialized when
irq_enter() and irq_exit() are called.  This is not correct and a check must
be performed prior to entering the tick code through these code paths to
ensure that the tick device is initialized and running.

I've only seen this occur on a few systems.  I've tested with and without the
patch and as far as I can tell this patch resolves the problem on
linux.git top of tree.

Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
---
 kernel/time/tick-sched.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a19a399..5027187 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -567,6 +567,12 @@ EXPORT_SYMBOL_GPL(tick_nohz_idle_enter);
 void tick_nohz_irq_exit(void)
 {
 	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
+	struct clock_event_device *dev =
+				     __get_cpu_var(tick_cpu_device).evtdev;
+
+	/* Has the tick been initialized yet? */
+	if (unlikely(!dev || dev->mode == CLOCK_EVT_MODE_UNUSED))
+		return;
 
 	if (!ts->inidle)
 		return;
@@ -809,6 +815,12 @@ static inline void tick_check_nohz(int cpu) { }
  */
 void tick_check_idle(int cpu)
 {
+	struct clock_event_device *dev = per_cpu(tick_cpu_device, cpu).evtdev;
+
+	/* Has the tick been initialized yet? */
+	if (unlikely(!dev || dev->mode == CLOCK_EVT_MODE_UNUSED))
+		return;
+
 	tick_check_oneshot_broadcast(cpu);
 	tick_check_nohz(cpu);
 }
-- 
1.7.9.3

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