Re: [PATCH] kfence: Avoid stalling work queue task without allocations

Marco Elver
2020-11-18T22:56:53Z

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On Tue, Nov 17, 2020 at 10:29AM -0800, Paul E. McKenney wrote:
[...]
> But it would be good to get the kcompactd() people to look at this (not
> immediately seeing who they are in MAINTAINERS). Perhaps preemption is
> disabled somehow and I am failing to see it.
>
> Failing that, maybe someone knows of a way to check for overly long
> timeout handlers.

I think I figured out one piece of the puzzle. Bisection keeps pointing
me at some -rcu merge commit, which kept throwing me off. Nor did it
help that reproduction is a bit flaky. However, I think there are 2
independent problems, but the manifestation of 1 problem triggers the
2nd problem:

1. problem: slowed forward progress (workqueue lockup / RCU stall reports)

2. problem: DEADLOCK which causes complete system lockup

| ...
| 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
| ...

Problem 2 can with reasonable confidence (5 trials) be fixed by reverting:

rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled

At which point the system always boots to user space -- albeit with a
bunch of warnings still (attached). The supposed "good" version doesn't
end up with all those warnings deterministically, so I couldn't say if
the warnings are expected due to recent changes or not (Arm64 QEMU
emulation, 1 CPU, and lots of debugging tools on).

Does any of that make sense?

Thanks,
-- Marco

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Testing all events: OK
Running tests again, along with the function tracer
Running tests on all trace events:
Testing all events:
hrtimer: interrupt took 10156432 ns
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
(detected by 0, t=3752 jiffies, g=2785, q=6)
rcu: All QSes seen, last rcu_preempt kthread activity 3752 (4295396561-4295392809), jiffies_till_next_fqs=1, root ->qsmask 0x0
rcu: rcu_preempt kthread starved for 3752 jiffies! g2785 f0x0 RCU_GP_ONOFF(3) ->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_common+0x4c/0x1a8 kernel/sched/core.c:4681
preempt_schedule+0x38/0x40 kernel/sched/core.c:4706
__raw_spin_unlock include/linux/spinlock_api_smp.h:152 [inline]
_raw_spin_unlock+0x94/0xa8 kernel/locking/spinlock.c:183
rcu_gp_init kernel/rcu/tree.c:1820 [inline]
rcu_gp_kthread+0x34c/0x1bd8 kernel/rcu/tree.c:2105
kthread+0x13c/0x188 kernel/kthread.c:292
ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
(detected by 0, t=3752 jiffies, g=2817, q=6)
rcu: All QSes seen, last rcu_preempt kthread activity 856 (4295412565-4295411709), jiffies_till_next_fqs=1, root ->qsmask 0x0
rcu: rcu_preempt kthread starved for 856 jiffies! g2817 f0x2 RCU_GP_CLEANUP(7) ->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_common+0x4c/0x1a8 kernel/sched/core.c:4681
preempt_schedule+0x38/0x40 kernel/sched/core.c:4706
__raw_spin_unlock_irq include/linux/spinlock_api_smp.h:169 [inline]
_raw_spin_unlock_irq+0x84/0x98 kernel/locking/spinlock.c:199
rcu_gp_cleanup kernel/rcu/tree.c:2015 [inline]
rcu_gp_kthread+0x1038/0x1bd8 kernel/rcu/tree.c:2119
kthread+0x13c/0x188 kernel/kthread.c:292
ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
BUG: workqueue lockup - pool cpus=0 flags=0x4 nice=0 stuck for 16s!
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=2/256 refcnt=4
pending: neigh_periodic_work, do_cache_clean
workqueue rcu_gp: flags=0x8
pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
in-flight: 15:srcu_invoke_callbacks
pool 0: cpus=0 node=0 flags=0x0 nice=0 hung=0s workers=3 idle: 111 5
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
(detected by 0, t=3752 jiffies, g=2829, q=6)
rcu: All QSes seen, last rcu_preempt kthread activity 1522 (4295422970-4295421448), jiffies_till_next_fqs=1, root ->qsmask 0x0
rcu: rcu_preempt kthread starved for 1522 jiffies! g2829 f0x2 RCU_GP_CLEANUP(7) ->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:
dump_backtrace+0x0/0x240 arch/arm64/kernel/stacktrace.c:100
show_stack+0x34/0x88 arch/arm64/kernel/stacktrace.c:196
sched_show_task kernel/sched/core.c:6445 [inline]
sched_show_task+0x1fc/0x228 kernel/sched/core.c:6420
rcu_check_gp_kthread_starvation+0xc8/0xe4 kernel/rcu/tree_stall.h:452
print_other_cpu_stall kernel/rcu/tree_stall.h:520 [inline]
check_cpu_stall kernel/rcu/tree_stall.h:640 [inline]
rcu_pending kernel/rcu/tree.c:3752 [inline]
rcu_sched_clock_irq+0xb34/0xc48 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]
__raw_spin_unlock_irq include/linux/spinlock_api_smp.h:168 [inline]
_raw_spin_unlock_irq+0x50/0x98 kernel/locking/spinlock.c:199
finish_lock_switch kernel/sched/core.c:3513 [inline]
finish_task_switch+0xa8/0x290 kernel/sched/core.c:3613
context_switch kernel/sched/core.c:3776 [inline]
__schedule+0x2dc/0x980 kernel/sched/core.c:4522
preempt_schedule_common+0x4c/0x1a8 kernel/sched/core.c:4681
preempt_schedule+0x38/0x40 kernel/sched/core.c:4706
__raw_spin_unlock_irq include/linux/spinlock_api_smp.h:169 [inline]
_raw_spin_unlock_irq+0x84/0x98 kernel/locking/spinlock.c:199
rcu_gp_cleanup kernel/rcu/tree.c:2046 [inline]
rcu_gp_kthread+0x1144/0x1bd8 kernel/rcu/tree.c:2119
kthread+0x13c/0x188 kernel/kthread.c:292
ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
(detected by 0, t=3752 jiffies, g=2845, q=6)
rcu: All QSes seen, last rcu_preempt kthread activity 2796 (4295435367-4295432571), jiffies_till_next_fqs=1, root ->qsmask 0x0
rcu: rcu_preempt kthread starved for 2796 jiffies! g2845 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:0x0000042a
Call trace:
dump_backtrace+0x0/0x240 arch/arm64/kernel/stacktrace.c:100
show_stack+0x34/0x88 arch/arm64/kernel/stacktrace.c:196
sched_show_task kernel/sched/core.c:6445 [inline]
sched_show_task+0x1fc/0x228 kernel/sched/core.c:6420
rcu_check_gp_kthread_starvation+0xc8/0xe4 kernel/rcu/tree_stall.h:452
print_other_cpu_stall kernel/rcu/tree_stall.h:520 [inline]
check_cpu_stall kernel/rcu/tree_stall.h:640 [inline]
rcu_pending kernel/rcu/tree.c:3752 [inline]
rcu_sched_clock_irq+0xb34/0xc48 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_restore arch/arm64/include/asm/irqflags.h:124 [inline]
rcu_irq_enter_irqson+0x40/0x78 kernel/rcu/tree.c:1078
trace_preempt_disable_rcuidle include/trace/events/preemptirq.h:51 [inline]
trace_preempt_off+0x108/0x1f8 kernel/trace/trace_preemptirq.c:130
preempt_latency_start kernel/sched/core.c:4164 [inline]
preempt_latency_start kernel/sched/core.c:4157 [inline]
preempt_schedule_notrace+0x170/0x1c0 kernel/sched/core.c:4747
__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
OK