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SubjectRe: Timer refuses to expire
Hi Paul,

On Wed, Dec 06, 2017 at 02:04:21PM -0800, Paul E. McKenney wrote:
> On Tue, Dec 05, 2017 at 03:37:44PM -0800, Paul E. McKenney wrote:
> > On Mon, Dec 04, 2017 at 09:42:08AM -0800, Paul E. McKenney wrote:
> > > On Fri, Dec 01, 2017 at 10:25:29AM -0800, Paul E. McKenney wrote:
> > > > Hello, Anna-Maria,
> > > >
> > > > It turned out that one set of problems was due to NO_HZ_FULL issues,
> > > > which are addressed with a pair of patches. However, there is still one
> > > > case of a timer refusing to expire in rcutorture's TREE01 test scenario.
> > > > The takes from two to four hours to trigger, and the most recent one
> > > > gives the following diagnostic (see patch at the very end of this email):
> > > >
> > > > [13166.127458] schedule_timeout: Waylayed timer base->clk: 0x100c40004 jiffies: 0x100c4524e base->next_expiry: 0x100c40004 timer->flags: 0x1000003 timer->expires 0x100c40003 idx: 4 idx_now: ea base->pending_map 000000000000001000000000001080000000000000040000000002000000000000000000000000000000000000040000000000000000000000000000000000000000000000000000
> > > >
> > > > The message appears any time a timer for less than five jiffies takes
> > > > more than eight seconds to expire. In all cases, this expiry did not
> > > > happen naturally, but rather via an unsolicited wakeup from the RCU CPU
> > > > stall code. If I am interpreting this message correctly, base->clk
> > > > has advanced past this timer, and the timer is correctly flagged in
> > > > base->pending_map. This seems like part of the problem because the
> > > > timer's timeout was only three jiffies. However, base->clk has not
> > > > advanced for more than 20 seconds, which seems like another problem.
> > > >
> > > > What additional diagnostics would be helpful? I can capture data
> > > > at the beginning of the schedule_timeout in the timer_list structure,
> > > > and of course can print more information at the time of the wakeup.
> > >
> > > And on the off-chance that the following messages from this weekend's
> > > runs are at all helpful. One interesting point is that starting at
> > > time 4731.872311, there are repeated enqueues to CPU 5's timer wheel,
> > > but CPU 5's ->clk does not advance. Things seem to advance at
> > > time 4891.721190.
> >
> > Another layer on the onion... For at least some of the failures, there
> > is a stalled CPU-hotplug operation. This of course can mean that the
> > timers are stuck on the partially offlined CPU. So I dumped the stack
> > of the task that is taking the CPU offline. Please see below for console
> > output and patch.
> >
> > I am considering doing an unsolicited wakeup of the task doing the
> > hotplug operation, but I am not convinced that the entirely of the CPU
> > hotplug code is willing to put up with that sort of thing.
>
> What I did instead was to dump out the state of the task that
> __cpuhp_kick_ap() waits on, please see the patch at the very end of this
> email. This triggered as shown below, and you guessed it, that task is
> waiting on a grace period. Which I am guessing won't happen until the
> outgoing CPU reaches CPUHP_TIMERS_DEAD state and calls timers_dead_cpu().
> Which will prevent RCU's grace-period kthread from ever awakening, which
> will prevent the task that __cpuhp_kick_ap() waits on from ever awakening,
> which will prevent the outgoing CPU from reaching CPUHP_TIMERS_DEAD state.
>
> Deadlock.
>

There is one thing I'm confused here. Sure, this is a deadlock, but the
timer should still work in such a deadlock, right? I mean, the timer of
schedule_timeout() should be able to wake up rcu_gp_kthread() even in
this case? And yes, the gp kthread will continue to wait due to the
deadlock, but the deadlock can not explain the "Waylayed timer", right?

Regards,
Boqun

> Does that make sense, or am I missing a trick here?
>
> I tried invoking timers_dead_cpu() from sched_cpu_deactivate(), but
> that triggered the BUG_ON(cpu_online(cpu)). I removed this BUG_ON(),
> and appear to have deadlocked on the timer ->lock.
>
> Any thoughts on what else to try?
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> [ 2939.381210] schedule_timeout: Waylayed timer base->clk: 0x1002709c3 jiffies: 0x100284614 base->next_expiry: 0x1002709c3 timer->flags: 0x40000007 timer->expires 0x10027f3a2 idx: 100 idx_now: 105 base->pending_map 000000000000000800000000000000000002000000000000020080400002000868000000000000010000000001000000000000000000000000000000000000000000000000000000
> [ 2939.381219] Torture onoff task state:
> [ 2939.381221] torture_onoff D14584 842 2 0x80000000
> [ 2939.381239] Call Trace:
> [ 2939.381245] ? __schedule+0x33c/0x6f0
> [ 2939.381248] ? preempt_count_add+0x51/0x90
> [ 2939.381250] schedule+0x37/0x90
> [ 2939.381270] schedule_timeout+0x20d/0x4c0
> [ 2939.381276] wait_for_completion+0x95/0x100
> [ 2939.381282] ? wake_up_q+0x60/0x60
> [ 2939.381299] __cpuhp_kick_ap+0x49/0x60
> [ 2939.381313] cpuhp_kick_ap+0x29/0x70
> [ 2939.381316] cpuhp_kick_ap_work+0x34/0x130
> [ 2939.381322] _cpu_down+0x17e/0x1f0
> [ 2939.381326] do_cpu_down+0x30/0x50
> [ 2939.381354] torture_offline+0x76/0x140
> [ 2939.381368] torture_onoff+0xef/0x1c0
> [ 2939.381371] kthread+0xf0/0x130
> [ 2939.381374] ? torture_kthread_stopping+0x70/0x70
> [ 2939.381376] ? kthread_destroy_worker+0x40/0x40
> [ 2939.381378] ret_from_fork+0x1f/0x30
> [ 2939.381381] __cpuhp_kick_ap task state:
> [ 2939.381383] cpuhp/7 D14504 46 2 0x80000000
> [ 2939.381399] Call Trace:
> [ 2939.381403] ? __schedule+0x33c/0x6f0
> [ 2939.381406] schedule+0x37/0x90
> [ 2939.381408] schedule_timeout+0x20d/0x4c0
> [ 2939.381414] ? enqueue_task_fair+0x556/0x12b0
> [ 2939.381417] wait_for_completion+0x95/0x100
> [ 2939.381419] ? wake_up_q+0x60/0x60
> [ 2939.381431] __wait_rcu_gp+0x10d/0x140
> [ 2939.381441] ? sched_cpu_activate+0xc0/0xc0
> [ 2939.381443] sched_cpu_deactivate+0x36/0xa0
> [ 2939.381453] ? kfree_call_rcu+0x20/0x20
> [ 2939.381454] ? __call_rcu+0x530/0x530
> [ 2939.381457] ? __rcu_read_unlock+0x50/0x50
> [ 2939.381459] ? __rcu_read_unlock+0x50/0x50
> [ 2939.381462] cpuhp_invoke_callback+0xa8/0x610
> [ 2939.381466] cpuhp_thread_fun+0xc5/0x150
> [ 2939.381469] smpboot_thread_fn+0x15c/0x220
> [ 2939.381476] kthread+0xf0/0x130
> [ 2939.381478] ? sort_range+0x20/0x20
> [ 2939.381480] ? kthread_destroy_worker+0x40/0x40
> [ 2939.381482] ret_from_fork+0x1f/0x30
>
> ------------------------------------------------------------------------
>
> commit aa2b802cb432a725589884d2ba47833de21e0871
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date: Wed Dec 6 09:46:30 2017 -0800
>
> EXP: timer: Dump __cpuhp_kick_ap task state
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 04892a82f6ac..dc9098946b3a 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -381,6 +381,8 @@ cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
> st->bringup = !st->bringup;
> }
>
> +void schedule_timeout_set_task3dump(struct task_struct *t);
> +
> /* Regular hotplug invocation of the AP hotplug thread */
> static void __cpuhp_kick_ap(struct cpuhp_cpu_state *st)
> {
> @@ -395,7 +397,9 @@ static void __cpuhp_kick_ap(struct cpuhp_cpu_state *st)
> smp_mb();
> st->should_run = true;
> wake_up_process(st->thread);
> + schedule_timeout_set_task3dump(st->thread);
> wait_for_ap_thread(st, st->bringup);
> + schedule_timeout_set_task3dump(NULL);
> }
>
> static int cpuhp_kick_ap(struct cpuhp_cpu_state *st, enum cpuhp_state target)
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 0a67df4cdeb6..532b43d05219 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1725,6 +1725,7 @@ static void process_timeout(struct timer_list *t)
> }
>
> static struct task_struct *schedule_timeout_task2dump;
> +static struct task_struct *schedule_timeout_task3dump;
> static DEFINE_MUTEX(schedule_timeout_task2dump_mutex);
> void schedule_timeout_set_task2dump(struct task_struct *t)
> {
> @@ -1732,7 +1733,12 @@ void schedule_timeout_set_task2dump(struct task_struct *t)
> WRITE_ONCE(schedule_timeout_task2dump, t);
> mutex_unlock(&schedule_timeout_task2dump_mutex);
> }
> -EXPORT_SYMBOL_GPL(schedule_timeout_set_task2dump);
> +void schedule_timeout_set_task3dump(struct task_struct *t)
> +{
> + mutex_lock(&schedule_timeout_task2dump_mutex);
> + WRITE_ONCE(schedule_timeout_task3dump, t);
> + mutex_unlock(&schedule_timeout_task2dump_mutex);
> +}
>
> /**
> * schedule_timeout - sleep until timeout
> @@ -1824,6 +1830,10 @@ signed long __sched schedule_timeout(signed long timeout)
> pr_info("Torture onoff task state:\n");
> sched_show_task(schedule_timeout_task2dump);
> }
> + if (schedule_timeout_task3dump) {
> + pr_info("__cpuhp_kick_ap task state:\n");
> + sched_show_task(schedule_timeout_task3dump);
> + }
> mutex_unlock(&schedule_timeout_task2dump_mutex);
> }
> }
>
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