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 <paul...@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 <paul...@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); > } > } >
signature.asc
Description: PGP signature