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

Paul E. McKenney
2020-11-12T17:54:13Z

On Thu, Nov 12, 2020 at 05:14:39PM +0100, Marco Elver wrote:
> On Thu, Nov 12, 2020 at 01:49PM +0100, Marco Elver wrote:
> > On Thu, 12 Nov 2020 at 01:11, Paul E. McKenney <paulmck@kernel.org> wrote:
> [...]
> > > > This assert didn't fire yet, I just get more of the below. I'll keep
> > > > rerunning, but am not too hopeful...
> > >
> > > Is bisection a possibility?
> >
> > I've been running a bisection for past ~12h, and am making slow
> > progress. It might be another 12h, but I think it'll get there.
>
> Bisection gave me this:
>
> | git bisect start
> | # bad: [c07b306d7fa5680777e2132662d2e6c19fb53579] kfence: Avoid stalling work queue task without allocations
> | git bisect bad c07b306d7fa5680777e2132662d2e6c19fb53579
> | # good: [3cea11cd5e3b00d91caf0b4730194039b45c5891] Linux 5.10-rc2
> | git bisect good 27598e7e73260ed0b2917eb02d4a515ebb578313
> | # good: [3e5acbea719e66ef3be64fe74c99cc905ca697dc] Merge remote-tracking branch 'wireless-drivers-next/master' into master
> | git bisect good 3e5acbea719e66ef3be64fe74c99cc905ca697dc
> | # good: [491a5a9a2fea28353d99621b8abb83b6928b4e36] Merge remote-tracking branch 'sound-asoc/for-next' into master
> | git bisect good 491a5a9a2fea28353d99621b8abb83b6928b4e36
> | # bad: [502f8643d6e21c7e370a0b75131130cc51609055] Merge remote-tracking branch 'phy-next/next' into master
> | git bisect bad 502f8643d6e21c7e370a0b75131130cc51609055
> | # good: [6693cb1fa5ea7b91ec00f9404776a095713face5] Merge remote-tracking branch 'tip/auto-latest' into master
> | git bisect good 6693cb1fa5ea7b91ec00f9404776a095713face5
> | # bad: [b790e3afead9357195b6d1e1b6cd9b3521503ad2] Merge branch 'tglx-pc.2020.10.30a' into HEAD
> | git bisect bad b790e3afead9357195b6d1e1b6cd9b3521503ad2
> | # bad: [765b512bb3d639bfad7dd43c288ee085236c7267] Merge branches 'cpuinfo.2020.11.06a', 'doc.2020.11.06a', 'fixes.2020.11.02a', 'lockdep.2020.11.02a', 'tasks.2020.11.06a' and 'torture.2020.11.06a' into HEAD
> | git bisect bad 765b512bb3d639bfad7dd43c288ee085236c7267
> | # good: [01f9e708d9eae6335ae9ff25ab09893c20727a55] tools/rcutorture: Fix BUG parsing of console.log

So torture.2020.11.06a is OK.

> | git bisect good 01f9e708d9eae6335ae9ff25ab09893c20727a55
> | # good: [1be6ab91e2db157faedb7f16ab0636a80745a073] srcu: Take early exit on memory-allocation failure

As is fixes.2020.11.02a.

> | git bisect good 1be6ab91e2db157faedb7f16ab0636a80745a073
> | # good: [65e9eb1ccfe56b41a0d8bfec651ea014968413cb] rcu: Prevent RCU_LOCKDEP_WARN() from swallowing the condition

And lockdep.2020.11.02a.

> | git bisect good 65e9eb1ccfe56b41a0d8bfec651ea014968413cb
> | # good: [c386e29d43728778ddd642fa73cc582bee684171] docs/rcu: Update the call_rcu() API

And doc.2020.11.06a.

> | git bisect good c386e29d43728778ddd642fa73cc582bee684171
> | # good: [27c0f1448389baf7f309b69e62d4b531c9395e88] rcutorture: Make grace-period kthread report match RCU flavor being tested

And the first three commits of tasks.2020.11.06a.

> | git bisect good 27c0f1448389baf7f309b69e62d4b531c9395e88
> | # good: [3fcd6a230fa7d03bffcb831a81b40435c146c12b] x86/cpu: Avoid cpuinfo-induced IPIing of idle CPUs

And cpuinfo.2020.11.06a.

> | git bisect good 3fcd6a230fa7d03bffcb831a81b40435c146c12b
> | # good: [75dc2da5ecd65bdcbfc4d59b9d9b7342c61fe374] rcu-tasks: Make the units of ->init_fract be jiffies

And the remaining commit of tasks.2020.11.06a.

> | git bisect good 75dc2da5ecd65bdcbfc4d59b9d9b7342c61fe374
> | # first bad commit: [765b512bb3d639bfad7dd43c288ee085236c7267] Merge branches 'cpuinfo.2020.11.06a', 'doc.2020.11.06a', 'fixes.2020.11.02a', 'lockdep.2020.11.02a', 'tasks.2020.11.06a' and 'torture.2020.11.06a' into HEAD
>
> This doesn't look very satisfying, given it's the merge commit. :-/

So each individual branch is just fine, but the merge of them is not. Fun.

These have been passing quite a bit of rcutorture over here, including
preemptible kernels running !SMP, but admittedly on x86 rather than ARMv8.

One approach would be to binary-search the combinations of merges.
Except that there are six of them, so there are 64 combinations, of
which you have tested only 8 thus far (none, one each, and all).

But are you sure that the bisection points labeled "good" really are good?
For example, what is the distribution of first failure times in the
points labeled "bad" vs. the runtime used to make a "good" determination?
Alternatively, just try a longer run on each of the commits feeding into
the merge point.

> > > Failing that, please see the updated patch below. This adds a few more
> > > calls to lockdep_assert_irqs_disabled(), but perhaps more helpfully dumps
> > > the current stack of the CPU that the RCU grace-period kthread wants to
> > > run on in the case where this kthread has been starved of CPU.
> >
> > Thanks, I will apply that after the bisection runs.
>
> Here's a new log with it applied:

Even more strangeness! ;-)

> | [ 118.480959] Key type dns_resolver registered
> | [ 118.487752] registered taskstats version 1
> | [ 118.489798] Running tests on all trace events:
> | [ 118.490164] Testing all events: OK
> | [ 173.304186] Running tests again, along with the function tracer
> | [ 173.320155] Running tests on all trace events:
> | [ 173.331638] Testing all events:
> | [ 173.485044] hrtimer: interrupt took 14340976 ns

Fourteen milliseconds, so annoying from a real-time perspective, but
unlikely to be the cause of this.

Was the system responsive at this point, between three and ten minutes
after boot? Similar question for the other gaps in the dmesg log.
The reason for the question is that workqueue's reported stall times
don't span these intervals.

> | [ 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.

> | [ 334.259490] Showing busy workqueues and worker pools:
> | [ 334.265398] workqueue events: flags=0x0
> | [ 334.289070] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 334.300659] pending: vmstat_shepherd
> | [ 453.541827] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 10s!
> | [ 453.655731] BUG: workqueue lockup - pool cpus=0 flags=0x4 nice=0 stuck for 10s!
> | [ 453.759839] Showing busy workqueues and worker pools:
> | [ 453.784294] workqueue events: flags=0x0
> | [ 453.812207] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 453.822108] pending: vmstat_shepherd
> | [ 453.839855] workqueue events_power_efficient: flags=0x82
> | [ 453.865152] pwq 2: cpus=0 flags=0x4 nice=0 active=2/256 refcnt=4
> | [ 453.874553] pending: neigh_periodic_work, do_cache_clean
> | [ 481.424362] BUG: workqueue lockup - pool cpus=0 flags=0x4 nice=0 stuck for 10s!
> | [ 481.508136] Showing busy workqueues and worker pools:
> | [ 481.524265] workqueue events: flags=0x0
> | [ 481.550480] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 481.560690] pending: vmstat_shepherd
> | [ 481.571255] workqueue events_power_efficient: flags=0x82
> | [ 481.592515] pwq 2: cpus=0 flags=0x4 nice=0 active=1/256 refcnt=3
> | [ 481.601153] pending: neigh_periodic_work
> | [ 532.108407] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 10s!
> | [ 532.203476] Showing busy workqueues and worker pools:
> | [ 532.215930] workqueue events: flags=0x0
> | [ 532.244203] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 532.254428] pending: vmstat_shepherd
> | [ 739.567892] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 19s!
> | [ 739.656419] Showing busy workqueues and worker pools:
> | [ 739.699514] workqueue events: flags=0x0
> | [ 739.705111] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 739.715393] pending: vmstat_shepherd
> | [ 739.733403] workqueue events_power_efficient: flags=0x82
> | [ 739.739433] pwq 2: cpus=0 flags=0x4 nice=0 active=2/256 refcnt=4
> | [ 739.748156] pending: check_lifetime, neigh_periodic_work
> | [ 811.578165] BUG: workqueue lockup - pool cpus=0 flags=0x5 nice=0 stuck for 14s!
> | [ 811.602913] Showing busy workqueues and worker pools:
> | [ 811.620424] workqueue events: flags=0x0
> | [ 811.652479] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 811.662686] pending: vmstat_shepherd
> | [ 811.683811] workqueue events_power_efficient: flags=0x82
> | [ 811.716123] pwq 2: cpus=0 flags=0x5 nice=0 active=1/256 refcnt=3
> | [ 811.724857] pending: neigh_periodic_work
> | [ 811.749989] pool 2: cpus=0 flags=0x5 nice=0 hung=14s workers=2 manager: 61 idle: 7
> | [ 822.456290] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 11s!
> | [ 822.600359] BUG: workqueue lockup - pool cpus=0 flags=0x5 nice=0 stuck for 25s!
> | [ 822.675814] Showing busy workqueues and worker pools:
> | [ 822.720098] workqueue events: flags=0x0
> | [ 822.747304] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 822.757174] pending: vmstat_shepherd
> | [ 822.768047] workqueue events_power_efficient: flags=0x82
> | [ 822.799954] pwq 2: cpus=0 flags=0x5 nice=0 active=1/256 refcnt=3
> | [ 822.808488] pending: neigh_periodic_work
> | [ 822.831900] pool 2: cpus=0 flags=0x5 nice=0 hung=25s workers=2 manager: 61 idle: 7
> | [ 834.116239] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 22s!
> | [ 834.246557] BUG: workqueue lockup - pool cpus=0 flags=0x5 nice=0 stuck for 37s!
> | [ 834.271069] Showing busy workqueues and worker pools:
> | [ 834.276687] workqueue events: flags=0x0
> | [ 834.296267] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> | [ 834.306148] pending: vmstat_shepherd
> | [ 834.324273] workqueue events_power_efficient: flags=0x82
> | [ 834.344433] pwq 2: cpus=0 flags=0x5 nice=0 active=2/256 refcnt=4
> | [ 834.352891] pending: neigh_periodic_work, do_cache_clean
> | [ 834.384530] pool 2: cpus=0 flags=0x5 nice=0 hung=37s workers=2 manager: 61 idle: 7
> | [ 840.906940] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> | [ 840.912685] (detected by 0, t=3752 jiffies, g=2709, q=1)

CPU 0 detected the stall.

> | [ 840.914587] rcu: All QSes seen, last rcu_preempt kthread activity 620 (4295099794-4295099174), jiffies_till_next_fqs=1, root ->qsmask 0x0

As before, the grace period is not stalled, but instead the grace-period
kthread is failing to detect the end of an already-ended grace period.

> | [ 840.925016] rcu: rcu_preempt kthread starved for 620 jiffies! g2709 f0x2 RCU_GP_CLEANUP(7) ->state=0x0 ->cpu=0

And CPU 0 is where the RCU grace-period kthread was last seen running.

> | [ 840.930687] rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> | [ 840.936056] rcu: RCU grace-period kthread stack dump:
> | [ 840.940433] task:rcu_preempt state:R running task stack: 0 pid: 10 ppid: 2 flags:0x00000428
> | [ 840.949160] Call trace:
> | [ 840.952822] dump_backtrace+0x0/0x278
> | [ 840.956816] show_stack+0x30/0x80
> | [ 840.960643] sched_show_task+0x1a8/0x240
> | [ 840.964684] rcu_check_gp_kthread_starvation+0x170/0x358
> | [ 840.969113] rcu_sched_clock_irq+0x744/0xd18
> | [ 840.973232] update_process_times+0x68/0x98
> | [ 840.977308] tick_sched_handle.isra.16+0x54/0x80
> | [ 840.981504] tick_sched_timer+0x64/0xd8
> | [ 840.985500] __hrtimer_run_queues+0x2a4/0x750
> | [ 840.989628] hrtimer_interrupt+0xf4/0x2a0
> | [ 840.993669] arch_timer_handler_virt+0x44/0x70
> | [ 840.997841] handle_percpu_devid_irq+0xfc/0x4d0
> | [ 841.002043] generic_handle_irq+0x50/0x70
> | [ 841.006098] __handle_domain_irq+0x9c/0x120
> | [ 841.010188] gic_handle_irq+0xcc/0x108
> | [ 841.014132] el1_irq+0xbc/0x180
> | [ 841.017935] arch_local_irq_restore+0x4/0x8
> | [ 841.021993] trace_preempt_on+0xf4/0x190
> | [ 841.026016] preempt_schedule_common+0x12c/0x1b0
> | [ 841.030193] preempt_schedule.part.88+0x20/0x28
> | [ 841.034373] preempt_schedule+0x20/0x28
> | [ 841.038369] _raw_spin_unlock_irq+0x80/0x90
> | [ 841.042498] rcu_gp_kthread+0xe5c/0x19a8
> | [ 841.046504] kthread+0x174/0x188
> | [ 841.050320] ret_from_fork+0x10/0x18
> | [ 841.054312] rcu: Stack dump where RCU grace-period kthread last ran:
> | [ 841.058980] Task dump for CPU 0:
> | [ 841.062736] task:rcu_preempt state:R running task stack: 0 pid: 10 ppid: 2 flags:0x00000428

And RCU's grace-period kthread really is running on CPU 0 right now.
It is just not making any forward progress.

> | [ 841.071073] Call trace:
> | [ 841.074662] dump_backtrace+0x0/0x278
> | [ 841.078596] show_stack+0x30/0x80
> | [ 841.082386] sched_show_task+0x1a8/0x240
> | [ 841.086367] dump_cpu_task+0x48/0x58
> | [ 841.090311] rcu_check_gp_kthread_starvation+0x214/0x358
> | [ 841.094736] rcu_sched_clock_irq+0x744/0xd18
> | [ 841.098852] update_process_times+0x68/0x98
> | [ 841.102949] tick_sched_handle.isra.16+0x54/0x80
> | [ 841.107119] tick_sched_timer+0x64/0xd8
> | [ 841.111127] __hrtimer_run_queues+0x2a4/0x750
> | [ 841.115264] hrtimer_interrupt+0xf4/0x2a0
> | [ 841.119319] arch_timer_handler_virt+0x44/0x70
> | [ 841.123525] handle_percpu_devid_irq+0xfc/0x4d0
> | [ 841.127690] generic_handle_irq+0x50/0x70
> | [ 841.131702] __handle_domain_irq+0x9c/0x120
> | [ 841.135779] gic_handle_irq+0xcc/0x108
> | [ 841.139743] el1_irq+0xbc/0x180

The code above this point was detecting and printing the RCU CPU stall
warning. The code below this point was doing what?

Any chance of getting file names and line numbers for the rest of this
stack?

> | [ 841.143527] arch_local_irq_restore+0x4/0x8

So we are just now restoring interrupts, hence our getting the
interrupt at this point..

> | [ 841.147612] trace_preempt_on+0xf4/0x190

From within the trace code, which is apparently recording the fact
that preemption is being enabled.

> | [ 841.151656] preempt_schedule_common+0x12c/0x1b0
> | [ 841.155869] preempt_schedule.part.88+0x20/0x28
> | [ 841.160036] preempt_schedule+0x20/0x28

I was not aware that releasing a raw spinlock could result in a direct
call to preempt_schedule().

> | [ 841.164051] _raw_spin_unlock_irq+0x80/0x90
> | [ 841.168139] rcu_gp_kthread+0xe5c/0x19a8

So the RCU grace-period kthread has spent many seconds attempting to
release a lock? Am I reading this correctly? Mark Rutland, am I missing
something here?

> | [ 841.172134] kthread+0x174/0x188
> | [ 841.175953] ret_from_fork+0x10/0x18
> | [ 841.191371]
> | [ 841.193648] ================================
> | [ 841.196605] WARNING: inconsistent lock state
> | [ 841.199764] 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #23 Not tainted
> | [ 841.203564] --------------------------------

Has lockdep recorded the fact that the lock is actually released?
It had better, given that interrupts are now enabled.

> | [ 841.206550] inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
> | [ 841.210074] rcu_preempt/10 [HC0[0]:SC0[0]:HE0:SE1] takes:
> | [ 841.213453] ffffd787e91d4358 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x4a0/0xd18
> | [ 841.221240] {IN-HARDIRQ-W} state was registered at:
> | [ 841.224538] __lock_acquire+0x7bc/0x15b8
> | [ 841.227541] lock_acquire+0x244/0x498
> | [ 841.230442] _raw_spin_lock_irqsave+0x78/0x144
> | [ 841.233555] rcu_sched_clock_irq+0x4a0/0xd18
> | [ 841.236621] update_process_times+0x68/0x98
> | [ 841.239645] tick_sched_handle.isra.16+0x54/0x80
> | [ 841.242801] tick_sched_timer+0x64/0xd8
> | [ 841.245745] __hrtimer_run_queues+0x2a4/0x750
> | [ 841.248842] hrtimer_interrupt+0xf4/0x2a0
> | [ 841.251846] arch_timer_handler_virt+0x44/0x70
> | [ 841.254976] handle_percpu_devid_irq+0xfc/0x4d0
> | [ 841.258131] generic_handle_irq+0x50/0x70
> | [ 841.261146] __handle_domain_irq+0x9c/0x120
> | [ 841.264169] gic_handle_irq+0xcc/0x108
> | [ 841.267096] el1_irq+0xbc/0x180
> | [ 841.269844] arch_local_irq_restore+0x4/0x8
> | [ 841.272881] trace_preempt_on+0xf4/0x190
> | [ 841.275847] preempt_schedule_common+0x12c/0x1b0
> | [ 841.279017] preempt_schedule.part.88+0x20/0x28
> | [ 841.282149] preempt_schedule+0x20/0x28
> | [ 841.285112] _raw_spin_unlock_irq+0x80/0x90
> | [ 841.288154] rcu_gp_kthread+0xe5c/0x19a8
> | [ 841.291175] kthread+0x174/0x188
> | [ 841.293952] ret_from_fork+0x10/0x18
> | [ 841.296780] irq event stamp: 39750
> | [ 841.299604] hardirqs last enabled at (39749): [<ffffd787e6d85738>] rcu_irq_enter_irqson+0x48/0x68
> | [ 841.303961] hardirqs last disabled at (39750): [<ffffd787e6c122bc>] el1_irq+0x7c/0x180
> | [ 841.308042] softirqs last enabled at (36704): [<ffffd787e6c10b58>] __do_softirq+0x650/0x6a4
> | [ 841.312250] softirqs last disabled at (36683): [<ffffd787e6cc0b80>] irq_exit+0x1a8/0x1b0
> | [ 841.316257]
> | [ 841.316257] other info that might help us debug this:
> | [ 841.319834] Possible unsafe locking scenario:
> | [ 841.319834]
> | [ 841.323217] CPU0
> | [ 841.325656] ----
> | [ 841.328097] lock(rcu_node_0);
> | [ 841.332433] <Interrupt>
> | [ 841.334966] lock(rcu_node_0);
> | [ 841.339379]
> | [ 841.339379] *** DEADLOCK ***
> | [ 841.339379]
> | [ 841.342829] 1 lock held by rcu_preempt/10:
> | [ 841.345794] #0: ffffd787e91d4358 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x4a0/0xd18
> | [ 841.354415]
> | [ 841.354415] stack backtrace:
> | [ 841.357664] CPU: 0 PID: 10 Comm: rcu_preempt Not tainted 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #23
> | [ 841.362249] Hardware name: linux,dummy-virt (DT)
> | [ 841.365352] Call trace:
> | [ 841.367862] dump_backtrace+0x0/0x278
> | [ 841.370745] show_stack+0x30/0x80
> | [ 841.373517] dump_stack+0x138/0x1b0
> | [ 841.376339] print_usage_bug+0x2d8/0x2f8
> | [ 841.379288] mark_lock.part.46+0x370/0x480
> | [ 841.382304] mark_held_locks+0x58/0x90
> | [ 841.385228] lockdep_hardirqs_on_prepare+0xdc/0x298
> | [ 841.388452] trace_hardirqs_on+0x90/0x388
> | [ 841.391434] el1_irq+0xd8/0x180
> | [ 841.394178] arch_local_irq_restore+0x4/0x8
> | [ 841.397186] trace_preempt_on+0xf4/0x190
> | [ 841.400127] preempt_schedule_common+0x12c/0x1b0
> | [ 841.403246] preempt_schedule.part.88+0x20/0x28
> | [ 841.406347] preempt_schedule+0x20/0x28
> | [ 841.409278] _raw_spin_unlock_irq+0x80/0x90
> | [ 841.412290] rcu_gp_kthread+0xe5c/0x19a8
> | [ 841.415237] kthread+0x174/0x188
> | [ 841.418011] ret_from_fork+0x10/0x18
> | [ 841.423450] BUG: scheduling while atomic: rcu_preempt/10/0x00000002
> | [ 841.431367] INFO: lockdep is turned off.
> | [ 841.439132] Modules linked in:
> | [ 841.450608] Preemption disabled at:
> | [ 841.452261] [<ffffd787e7fffec0>] preempt_schedule.part.88+0x20/0x28
> | [ 841.467324] CPU: 0 PID: 10 Comm: rcu_preempt Not tainted 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #23
> | [ 841.471926] Hardware name: linux,dummy-virt (DT)
> | [ 841.475030] Call trace:
> | [ 841.477581] dump_backtrace+0x0/0x278
> | [ 841.480451] show_stack+0x30/0x80
> | [ 841.483220] dump_stack+0x138/0x1b0
> | [ 841.486057] __schedule_bug+0x8c/0xe8
> | [ 841.488949] __schedule+0x7e8/0x890
> | [ 841.491801] preempt_schedule_common+0x44/0x1b0
> | [ 841.494927] preempt_schedule.part.88+0x20/0x28
> | [ 841.498048] preempt_schedule+0x20/0x28
> | [ 841.500963] _raw_spin_unlock_irq+0x80/0x90
> | [ 841.503988] rcu_gp_kthread+0xe5c/0x19a8
> | [ 841.506965] kthread+0x174/0x188
> | [ 841.509732] ret_from_fork+0x10/0x18
>
> Thanks,
> -- Marco