Date:   Wed, 11 Nov 2020 13:05:43 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Marco Elver <elver@...gle.com>
Cc:     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,
        paulmck@...nel.org, peterz@...radead.org
Subject: Re: [PATCH] kfence: Avoid stalling work queue task without
 allocations

On Wed, 11 Nov 2020 14:38:13 +0100
Marco Elver <elver@...gle.com> wrote:

> [+Cc folks who can maybe help figure out what's going on, since I get
>   warnings even without KFENCE on next-20201110.]
> 
> On Wed, Nov 11, 2020 at 09:29AM +0100, Marco Elver wrote:
> > On Wed, 11 Nov 2020 at 00:23, Anders Roxell <anders.roxell@...aro.org> wrote:
> > [...]
> > > I gave them a spin on next-20201105 [1] and on next-20201110 [2].
> > >
> > > I eventually got to a prompt on next-20201105.
> > > However, I got to this kernel panic on the next-20201110:
> > >
> > > [...]
> > > [ 1514.089966][    T1] Testing event system initcall: OK
> > > [ 1514.806232][    T1] Running tests on all trace events:
> > > [ 1514.857835][    T1] Testing all events:
> > > [ 1525.503262][    C0] hrtimer: interrupt took 10902600 ns
> > > [ 1623.861452][    C0] BUG: workqueue lockup - pool cpus=0 node=0
> > > flags=0x0 nice=0 stuck for 65s!
> > > [...]

OK, so this blows up when you enable all events?

Note, it could just be adding overhead (which is exasperated with other
debug options enabled), which could open up a race window.
 

> > > [ 7823.104349][   T28]       Tainted: G        W
> > > 5.10.0-rc3-next-20201110-00008-g8dc06700529d #3
> > > [ 7833.206491][   T28] "echo 0 >
> > > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > [ 7840.750700][   T28] task:kworker/0:1     state:D stack:26640 pid:
> > > 1872 ppid:     2 flags:0x00000428
> > > [ 7875.642531][   T28] Workqueue: events toggle_allocation_gate
> > > [ 7889.178334][   T28] Call trace:
> > > [ 7897.066649][   T28]  __switch_to+0x1cc/0x1e0
> > > [ 7905.326856][   T28]  0xffff00000f7077b0
> > > [ 7928.354644][   T28] INFO: lockdep is turned off.
> > > [ 7934.022572][   T28] Kernel panic - not syncing: hung_task: blocked tasks
> > > [ 7934.032039][   T28] CPU: 0 PID: 28 Comm: khungtaskd Tainted: G
> > >   W         5.10.0-rc3-next-20201110-00008-g8dc06700529d #3
> > > [ 7934.045586][   T28] Hardware name: linux,dummy-virt (DT)
> > > [ 7934.053677][   T28] Call trace:
> > > [ 7934.060276][   T28]  dump_backtrace+0x0/0x420
> > > [ 7934.067635][   T28]  show_stack+0x38/0xa0
> > > [ 7934.091277][   T28]  dump_stack+0x1d4/0x278
> > > [ 7934.098878][   T28]  panic+0x304/0x5d8
> > > [ 7934.114923][   T28]  check_hung_uninterruptible_tasks+0x5e4/0x640
> > > [ 7934.123823][   T28]  watchdog+0x138/0x160
> > > [ 7934.131561][   T28]  kthread+0x23c/0x260
> > > [ 7934.138590][   T28]  ret_from_fork+0x10/0x18
> > > [ 7934.146631][   T28] Kernel Offset: disabled
> > > [ 7934.153749][   T28] CPU features: 0x0240002,20002004
> > > [ 7934.161476][   T28] Memory Limit: none
> > > [ 7934.171272][   T28] ---[ end Kernel panic - not syncing: hung_task:
> > > blocked tasks ]---
> > >
> > > Cheers,
> > > Anders
> > > [1] https://people.linaro.org/~anders.roxell/output-next-20201105-test.log
> > > [2] https://people.linaro.org/~anders.roxell/output-next-20201110-test.log
> > 
> > Thanks for testing. The fact that it passes on next-20201105 but not
> > on 20201110 is strange. If you boot with KFENCE disabled (boot param
> > kfence.sample_interval=0), does it boot?
> [...]
> 
> Right, so I think this is no longer KFENCE's fault. This looks like
> something scheduler/RCU/ftrace related?! I notice that there have been
> scheduler changes between next-20201105 and next-20201110.

I'm not sure any of that would cause this.

> 
> I get this with KFENCE disabled:
> 
> | Running tests on all trace events:
> | Testing all events: 
> | BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 32s!
> | 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=0x5 nice=0 active=2/256 refcnt=4
> |     in-flight: 107:neigh_periodic_work
> |     pending: do_cache_clean
> | pool 2: cpus=0 flags=0x5 nice=0 hung=3s workers=2 manager: 7
> | rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> | 	(detected by 0, t=6502 jiffies, g=2885, q=4)
> | rcu: All QSes seen, last rcu_preempt kthread activity 5174 (4295523265-4295518091), jiffies_till_next_fqs=1, root ->qsmask 0x0
> | rcu: rcu_preempt kthread starved for 5174 jiffies! g2885 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+0x100/0x1e0
> |  __schedule+0x2d0/0x890
> |  preempt_schedule_notrace+0x70/0x1c0
> |  ftrace_ops_no_ops+0x174/0x250
> |  ftrace_graph_call+0x0/0xc

Note, just because ftrace is called here, the blocked task was preempted
when the ftrace code called preempt_enable_notrace().


> |  preempt_count_add+0x1c/0x180
> |  schedule+0x44/0x108
> |  schedule_timeout+0x394/0x530
> |  rcu_gp_kthread+0x76c/0x19a8
> |  kthread+0x174/0x188
> |  ret_from_fork+0x10/0x18
> | 
> | ================================
> | WARNING: inconsistent lock state
> | 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #18 Not tainted
> | --------------------------------
> | inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
> | kcompactd0/26 [HC0[0]:SC0[0]:HE0:SE1] takes:
> | ffffae32e6bd4358 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x4a0/0xd18
> | {IN-HARDIRQ-W} state was registered at:

I did some digging here and it looks like the rcu_node lock could be taken
without interrupts enabled when it does a stall print. That probably should
be fixed, but it's a symptom of the underlining bug and not the cause.

-- Steve


> |   __lock_acquire+0x7bc/0x15b8
> |   lock_acquire+0x244/0x498
> |   _raw_spin_lock_irqsave+0x78/0x144
> |   rcu_sched_clock_irq+0x4a0/0xd18
> |   update_process_times+0x68/0x98
> |   tick_sched_handle.isra.16+0x54/0x80
> |   tick_sched_timer+0x64/0xd8
> |   __hrtimer_run_queues+0x2a4/0x750
> | [...]
> | irq event stamp: 270278
> | hardirqs last  enabled at (270277): [<ffffae32e5a0bff8>] _raw_spin_unlock_irq+0x48/0x90
> | hardirqs last disabled at (270278): [<ffffae32e46122bc>] el1_irq+0x7c/0x180
> | softirqs last  enabled at (268786): [<ffffae32e4610b58>] __do_softirq+0x650/0x6a4
> | softirqs last disabled at (268783): [<ffffae32e46c0b80>] irq_exit+0x1a8/0x1b0
> | 
> | 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 kcompactd0/26:
> |  #0: ffffae32e6bd4358 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x4a0/0xd18
> | [...]
> 
> Full log and config attached. Also, I can provoke this quicker with the
> attached diff.
> 
> Thanks,
> -- Marco

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