Booting an RT kernel as kvm guest with "highres=off" results in this:
Switching to clocksource kvm-clock
------------[ cut here ]------------
WARNING: at kernel/lockdep.c:2593 trace_hardirqs_on_caller+0x1dc/0x210()
DEBUG_LOCKS_WARN_ON(current->hardirq_context)
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.10.40-rt40_preempt-rt #1
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
0000000000000009 ffff88007fc03db8 ffffffff8171fb1c ffff88007fc03df0
ffffffff8103fb31 0000000000000000 ffffffff8109c3b4 ffff88007fc0e6c0
ffff88007647b900 ffff88007647b900 ffff88007fc03e50 ffffffff8103fbbc
Call Trace:
<IRQ> [<ffffffff8171fb1c>] dump_stack+0x19/0x1b
[<ffffffff8103fb31>] warn_slowpath_common+0x61/0xa0
[<ffffffff8109c3b4>] ? tick_check_oneshot_change+0xf4/0x120
[<ffffffff8103fbbc>] warn_slowpath_fmt+0x4c/0x50
[<ffffffff81092b43>] ? ktime_get+0x43/0xc0
[<ffffffff8109f8cc>] trace_hardirqs_on_caller+0x1dc/0x210
[<ffffffff8109f90d>] trace_hardirqs_on+0xd/0x10
[<ffffffff8109c3b4>] tick_check_oneshot_change+0xf4/0x120 <---
[<ffffffff8106d383>] hrtimer_run_queues+0x63/0x300
[<ffffffff81052128>] run_local_timers+0x18/0x70
[<ffffffff810521b7>] update_process_times+0x37/0x70
[<ffffffff81099c1c>] tick_periodic+0x2c/0xd0
[<ffffffff81099d50>] tick_handle_periodic+0x20/0x70
[<ffffffff81727a26>] smp_apic_timer_interrupt+0x66/0x9b
[<ffffffff81726c32>] apic_timer_interrupt+0x72/0x80
<EOI> [<ffffffff81381169>] ? memset+0x9/0xb0
[<ffffffff81164acb>] ? address_space_init_once+0x1b/0x120
[<ffffffff81164c34>] inode_init_once+0x64/0x80
<...snip...>
------------[ cut here ]------------
If you don't have lock debugging on, you'll instead get this:
kernel BUG at kernel/posix-cpu-timers.c:1469!
which is just a sanity check to ensure irqs are disabled in
run_posix_cpu_timers(). The debug lock splat is more informative
since it points right at tick_check_oneshot_change as the culprit
who re-enabled interrupts [which in turn has called the
tick_nohz_switch_to_nohz before returning].
This isn't a problem on mainline, since there, the call path is
clearly limited to softirq context as follows:
run_timer_softirq
--> hrtimer_run_pending
--> tick_check_oneshot_change
--> tick_nohz_switch_to_nohz
On rt, there is also only one call path, and since we know irqs
are off for it, we can delete the local_irq ops vs making them
the save/restore variants. Insert a BUG_ON, since as above
we'll bug anyway later on if we screw this up. And fix up
the comments to match the way -rt does things.
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
[found on 3.10-rt -- have not checked others yet, but I bet they
have the same thing, since who disables highres normally anyway?]
kernel/time/tick-sched.c | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index aa1e4b2411d5..836cf05831db 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -974,20 +974,21 @@ static void tick_nohz_handler(struct clock_event_device *dev)
/**
* tick_nohz_switch_to_nohz - switch to nohz mode
+ * Called with interrupts disabled.
*/
static void tick_nohz_switch_to_nohz(void)
{
struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
ktime_t next;
+ BUG_ON(!irqs_disabled());
+
if (!tick_nohz_enabled)
return;
- local_irq_disable();
- if (tick_switch_to_oneshot(tick_nohz_handler)) {
- local_irq_enable();
+ if (tick_switch_to_oneshot(tick_nohz_handler))
return;
- }
+
ts->nohz_mode = NOHZ_MODE_LOWRES;
@@ -1005,7 +1006,6 @@ static void tick_nohz_switch_to_nohz(void)
break;
next = ktime_add(next, tick_period);
}
- local_irq_enable();
}
/*
@@ -1191,15 +1191,17 @@ void tick_oneshot_notify(void)
/**
* Check, if a change happened, which makes oneshot possible.
*
- * Called cyclic from the hrtimer softirq (driven by the timer
- * softirq) allow_nohz signals, that we can switch into low-res nohz
- * mode, because high resolution timers are disabled (either compile
+ * Called cyclic from the hardirq (each jiffie from hrtimer_run_queues)
+ * allow_nohz signals, that we can switch into low-res nohz mode,
+ * because high resolution timers are disabled (either compile
* or runtime).
*/
int tick_check_oneshot_change(int allow_nohz)
{
struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
+ BUG_ON(!irqs_disabled());
+
if (!test_and_clear_bit(0, &ts->check_clocks))
return 0;
--
1.9.0
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