Date:   Tue, 17 Nov 2020 11:52:36 +0100
From:   Marco Elver <elver@...gle.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kasan-dev <kasan-dev@...glegroups.com>, rcu@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] kfence: Avoid stalling work queue task without
 allocations

On Fri, Nov 13, 2020 at 09:57AM -0800, Paul E. McKenney wrote:
> On Thu, Nov 12, 2020 at 09:54:06AM -0800, Paul E. McKenney wrote:
> > On Thu, Nov 12, 2020 at 05:14:39PM +0100, Marco Elver wrote:
> 
> [ . . . ]
> 
> > > | [  334.160218] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 15s!
> > 
> > It might be instructive to cause this code to provoke a backtrace.
> > I suggest adding something like "trigger_single_cpu_backtrace(cpu)"
> > in kernel/workqueue.c's function named wq_watchdog_timer_fn()
> > somewhere within its "if" statement that is preceded with the "did we
> > stall?" comment.  Or just search for "BUG: workqueue lockup - pool"
> > within kernel/workqueue.c.
> 
> And I did get a small but unexpected gift of time, so here is an
> (untested) patch.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 437935e..f3d4ff7 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -5792,6 +5792,7 @@ static void wq_watchdog_timer_fn(struct timer_list *unused)
>  			pr_cont_pool_info(pool);
>  			pr_cont(" stuck for %us!\n",
>  				jiffies_to_msecs(jiffies - pool_ts) / 1000);
> +			trigger_single_cpu_backtrace(cpu);
>  		}
>  	}
>  

That didn't quite work, although the system this is running on only has
1 CPU, so dump_stack() should give us the information we want (see
below). Not sure if that helps though.

Thanks,
-- Marco

------ >8 ------

| Testing all events: OK
| Running tests again, along with the function tracer
| Running tests on all trace events:
| Testing all events: 
| hrtimer: interrupt took 10322624 ns
| BUG: workqueue lockup - pool cpus=0 flags=0x4 nice=0 stuck for 19s!
| CPU: 0 PID: 26 Comm: kcompactd0 Not tainted 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #2
| Hardware name: linux,dummy-virt (DT)
| Call trace:
|  dump_backtrace+0x0/0x240 arch/arm64/kernel/stacktrace.c:100
|  show_stack+0x34/0x88 arch/arm64/kernel/stacktrace.c:196
|  __dump_stack lib/dump_stack.c:77 [inline]
|  dump_stack+0x140/0x1bc lib/dump_stack.c:118
|  wq_watchdog_timer_fn+0x338/0x340 kernel/workqueue.c:5796
|  call_timer_fn+0xe8/0x480 kernel/time/timer.c:1410
|  expire_timers kernel/time/timer.c:1455 [inline]
|  __run_timers.part.0+0x2e8/0x470 kernel/time/timer.c:1747
|  __run_timers kernel/time/timer.c:1728 [inline]
|  run_timer_softirq+0x90/0xa8 kernel/time/timer.c:1762
|  __do_softirq+0x188/0x6b4 kernel/softirq.c:298
|  do_softirq_own_stack include/linux/interrupt.h:568 [inline]
|  invoke_softirq kernel/softirq.c:393 [inline]
|  __irq_exit_rcu kernel/softirq.c:423 [inline]
|  irq_exit+0x1cc/0x1e0 kernel/softirq.c:447
|  __handle_domain_irq+0xb4/0x130 kernel/irq/irqdesc.c:690
|  handle_domain_irq include/linux/irqdesc.h:170 [inline]
|  gic_handle_irq+0x70/0x108 drivers/irqchip/irq-gic.c:370
|  el1_irq+0xc0/0x180 arch/arm64/kernel/entry.S:651
|  arch_local_irq_restore arch/arm64/include/asm/irqflags.h:124 [inline]
|  rcu_irq_exit_irqson+0x40/0x78 kernel/rcu/tree.c:832
|  trace_preempt_enable_rcuidle include/trace/events/preemptirq.h:55 [inline]
|  trace_preempt_on+0x144/0x1a0 kernel/trace/trace_preemptirq.c:123
|  preempt_latency_stop kernel/sched/core.c:4197 [inline]
|  preempt_latency_stop kernel/sched/core.c:4194 [inline]
|  preempt_schedule_common+0xf8/0x1a8 kernel/sched/core.c:4682
|  preempt_schedule+0x38/0x40 kernel/sched/core.c:4706
|  __raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:161 [inline]
|  _raw_spin_unlock_irqrestore+0x90/0xa0 kernel/locking/spinlock.c:191
|  spin_unlock_irqrestore include/linux/spinlock.h:409 [inline]
|  finish_wait+0x78/0xa0 kernel/sched/wait.c:382
|  kcompactd+0x3a0/0x4b8 mm/compaction.c:2817
|  kthread+0x13c/0x188 kernel/kthread.c:292
|  ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
| Showing busy workqueues and worker pools:
| workqueue events: flags=0x0
|   pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
|     pending: vmstat_shepherd
| workqueue events_power_efficient: flags=0x82
|   pwq 2: cpus=0 flags=0x4 nice=0 active=3/256 refcnt=5
|     in-flight: 7:check_lifetime
|     pending: do_cache_clean, neigh_periodic_work
| pool 2: cpus=0 flags=0x4 nice=0 hung=19s workers=2 idle: 61
| rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
| 	(detected by 0, t=3752 jiffies, g=2501, q=1)
| rcu: All QSes seen, last rcu_preempt kthread activity 2981 (4295017301-4295014320), jiffies_till_next_fqs=1, root ->qsmask 0x0
| rcu: rcu_preempt kthread starved for 2981 jiffies! g2501 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
| rcu: 	Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
| rcu: RCU grace-period kthread stack dump:
| task:rcu_preempt     state:R  running task     stack:    0 pid:   10 ppid:     2 flags:0x00000428
| Call trace:
|  __switch_to+0x10c/0x200 arch/arm64/kernel/process.c:578
|  context_switch kernel/sched/core.c:3773 [inline]
|  __schedule+0x2d8/0x980 kernel/sched/core.c:4522
|  preempt_schedule_notrace+0x70/0x1c0 kernel/sched/core.c:4754
|  __ftrace_ops_list_func kernel/trace/ftrace.c:6956 [inline]
|  ftrace_ops_list_func+0x108/0x230 kernel/trace/ftrace.c:6977
|  ftrace_graph_call+0x0/0x4
|  preempt_count_add+0x8/0x1a0 arch/arm64/include/asm/atomic.h:65
|  schedule+0x44/0x100 kernel/sched/core.c:4599
|  schedule_timeout+0x240/0x538 kernel/time/timer.c:1871
|  rcu_gp_fqs_loop kernel/rcu/tree.c:1942 [inline]
|  rcu_gp_kthread+0x618/0x1bd8 kernel/rcu/tree.c:2115
|  kthread+0x13c/0x188 kernel/kthread.c:292
|  ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
| 
| ================================
| WARNING: inconsistent lock state
| 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #2 Not tainted
| --------------------------------
| inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
| event_benchmark/105 [HC0[0]:SC0[0]:HE0:SE1] takes:
| ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: print_other_cpu_stall kernel/rcu/tree_stall.h:493 [inline]
| ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: check_cpu_stall kernel/rcu/tree_stall.h:652 [inline]
| ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_pending kernel/rcu/tree.c:3752 [inline]
| ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x428/0xd40 kernel/rcu/tree.c:2581
| {IN-HARDIRQ-W} state was registered at:
|   mark_lock kernel/locking/lockdep.c:4374 [inline]
|   mark_usage kernel/locking/lockdep.c:4302 [inline]
|   __lock_acquire+0xab8/0x1a50 kernel/locking/lockdep.c:4785
|   lock_acquire kernel/locking/lockdep.c:5436 [inline]
|   lock_acquire+0x268/0x508 kernel/locking/lockdep.c:5401
|   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
|   _raw_spin_lock_irqsave+0x78/0x14c kernel/locking/spinlock.c:159
|   print_other_cpu_stall kernel/rcu/tree_stall.h:493 [inline]
|   check_cpu_stall kernel/rcu/tree_stall.h:652 [inline]
|   rcu_pending kernel/rcu/tree.c:3752 [inline]
|   rcu_sched_clock_irq+0x428/0xd40 kernel/rcu/tree.c:2581
|   update_process_times+0x6c/0xb8 kernel/time/timer.c:1709
|   tick_sched_handle.isra.0+0x58/0x88 kernel/time/tick-sched.c:176
|   tick_sched_timer+0x68/0xe0 kernel/time/tick-sched.c:1328
|   __run_hrtimer kernel/time/hrtimer.c:1519 [inline]
|   __hrtimer_run_queues+0x288/0x730 kernel/time/hrtimer.c:1583
|   hrtimer_interrupt+0x114/0x288 kernel/time/hrtimer.c:1645
|   timer_handler drivers/clocksource/arm_arch_timer.c:647 [inline]
|   arch_timer_handler_virt+0x50/0x70 drivers/clocksource/arm_arch_timer.c:658
|   handle_percpu_devid_irq+0x104/0x4c0 kernel/irq/chip.c:930
|   generic_handle_irq_desc include/linux/irqdesc.h:152 [inline]
|   generic_handle_irq+0x54/0x78 kernel/irq/irqdesc.c:650
|   __handle_domain_irq+0xac/0x130 kernel/irq/irqdesc.c:687
|   handle_domain_irq include/linux/irqdesc.h:170 [inline]
|   gic_handle_irq+0x70/0x108 drivers/irqchip/irq-gic.c:370
|   el1_irq+0xc0/0x180 arch/arm64/kernel/entry.S:651
|   arch_local_irq_enable arch/arm64/include/asm/irqflags.h:37 [inline]
|   preempt_schedule_irq+0x44/0xa0 kernel/sched/core.c:4782
|   arm64_preempt_schedule_irq+0xd0/0x118 arch/arm64/kernel/process.c:726
|   el1_irq+0xd8/0x180 arch/arm64/kernel/entry.S:664
|   arch_local_irq_enable arch/arm64/include/asm/irqflags.h:37 [inline]
|   trace_do_benchmark kernel/trace/trace_benchmark.c:56 [inline]
|   benchmark_event_kthread+0x144/0x4b0 kernel/trace/trace_benchmark.c:154
|   kthread+0x13c/0x188 kernel/kthread.c:292
|   ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
| irq event stamp: 12978
| hardirqs last  enabled at (12977): [<ffffbb6e0a62ea50>] preempt_schedule_irq+0x40/0xa0 kernel/sched/core.c:4782
| hardirqs last disabled at (12978): [<ffffbb6e092123c0>] el1_irq+0x80/0x180 arch/arm64/kernel/entry.S:648
| softirqs last  enabled at (8540): [<ffffbb6e09210b80>] __do_softirq+0x630/0x6b4 kernel/softirq.c:325
| softirqs last disabled at (8531): [<ffffbb6e092c6c54>] do_softirq_own_stack include/linux/interrupt.h:568 [inline]
| softirqs last disabled at (8531): [<ffffbb6e092c6c54>] invoke_softirq kernel/softirq.c:393 [inline]
| softirqs last disabled at (8531): [<ffffbb6e092c6c54>] __irq_exit_rcu kernel/softirq.c:423 [inline]
| softirqs last disabled at (8531): [<ffffbb6e092c6c54>] irq_exit+0x1cc/0x1e0 kernel/softirq.c:447
| 
| other info that might help us debug this:
|  Possible unsafe locking scenario:
| 
|        CPU0
|        ----
|   lock(rcu_node_0);
|   <Interrupt>
|     lock(rcu_node_0);
| 
|  *** DEADLOCK ***
| 
| 1 lock held by event_benchmark/105:
|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: print_other_cpu_stall kernel/rcu/tree_stall.h:493 [inline]
|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: check_cpu_stall kernel/rcu/tree_stall.h:652 [inline]
|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_pending kernel/rcu/tree.c:3752 [inline]
|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x428/0xd40 kernel/rcu/tree.c:2581
| 
| stack backtrace:
| CPU: 0 PID: 105 Comm: event_benchmark Not tainted 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #2
| Hardware name: linux,dummy-virt (DT)
| Call trace:
|  dump_backtrace+0x0/0x240 arch/arm64/kernel/stacktrace.c:100
|  show_stack+0x34/0x88 arch/arm64/kernel/stacktrace.c:196
|  __dump_stack lib/dump_stack.c:77 [inline]
|  dump_stack+0x140/0x1bc lib/dump_stack.c:118
|  print_usage_bug kernel/locking/lockdep.c:3739 [inline]
|  print_usage_bug+0x2a0/0x2f0 kernel/locking/lockdep.c:3706
|  valid_state kernel/locking/lockdep.c:3750 [inline]
|  mark_lock_irq kernel/locking/lockdep.c:3953 [inline]
|  mark_lock.part.0+0x438/0x4e8 kernel/locking/lockdep.c:4410
|  mark_lock kernel/locking/lockdep.c:4008 [inline]
|  mark_held_locks+0x54/0x90 kernel/locking/lockdep.c:4011
|  __trace_hardirqs_on_caller kernel/locking/lockdep.c:4029 [inline]
|  lockdep_hardirqs_on_prepare+0xe0/0x290 kernel/locking/lockdep.c:4097
|  trace_hardirqs_on+0x90/0x370 kernel/trace/trace_preemptirq.c:49
|  el1_irq+0xdc/0x180 arch/arm64/kernel/entry.S:685
|  arch_local_irq_enable arch/arm64/include/asm/irqflags.h:37 [inline]
|  preempt_schedule_irq+0x44/0xa0 kernel/sched/core.c:4782
|  arm64_preempt_schedule_irq+0xd0/0x118 arch/arm64/kernel/process.c:726
|  el1_irq+0xd8/0x180 arch/arm64/kernel/entry.S:664
|  arch_local_irq_enable arch/arm64/include/asm/irqflags.h:37 [inline]
|  trace_do_benchmark kernel/trace/trace_benchmark.c:56 [inline]
|  benchmark_event_kthread+0x144/0x4b0 kernel/trace/trace_benchmark.c:154
|  kthread+0x13c/0x188 kernel/kthread.c:292
|  ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
| BUG: scheduling while atomic: event_benchmark/105/0x00000002
| INFO: lockdep is turned off.
| Modules linked in:
| Preemption disabled at:
| [<ffffbb6e0a62ea4c>] preempt_schedule_irq+0x3c/0xa0 kernel/sched/core.c:4781
| CPU: 0 PID: 105 Comm: event_benchmark Not tainted 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #2
| Hardware name: linux,dummy-virt (DT)
| Call trace:
|  dump_backtrace+0x0/0x240 arch/arm64/kernel/stacktrace.c:100
|  show_stack+0x34/0x88 arch/arm64/kernel/stacktrace.c:196
|  __dump_stack lib/dump_stack.c:77 [inline]
|  dump_stack+0x140/0x1bc lib/dump_stack.c:118
|  __schedule_bug+0xcc/0xe0 kernel/sched/core.c:4262
|  schedule_debug kernel/sched/core.c:4289 [inline]
|  __schedule+0x878/0x980 kernel/sched/core.c:4417
|  preempt_schedule_irq+0x4c/0xa0 kernel/sched/core.c:4783
|  arm64_preempt_schedule_irq+0xd0/0x118 arch/arm64/kernel/process.c:726
|  el1_irq+0xd8/0x180 arch/arm64/kernel/entry.S:664
|  arch_local_irq_enable arch/arm64/include/asm/irqflags.h:37 [inline]
|  trace_do_benchmark kernel/trace/trace_benchmark.c:56 [inline]
|  benchmark_event_kthread+0x144/0x4b0 kernel/trace/trace_benchmark.c:154
|  kthread+0x13c/0x188 kernel/kthread.c:292
|  ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961

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