Re: [PATCH] sched/core: fix affine_move_task failure case

From: Valentin Schneider
Date: Wed Mar 20 2024 - 14:33:16 EST

On 20/03/24 19:17, Valentin Schneider wrote:
> On 20/03/24 15:03, Daniel Vacek wrote:
>> Hi Valentin,
>>
>> On Mon, Mar 18, 2024 at 6:34 PM Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
>>> On 18/03/24 12:17, Daniel Vacek wrote:
>>> > Bill Peters reported CPU hangs while offlining/onlining CPUs on s390.
>>> >
>>> > Analyzing the vmcore data shows `stop_one_cpu_nowait()` in `affine_move_task()`
>>> > can fail when racing with off-/on-lining resulting in a deadlock waiting for
>>> > the pending migration stop work completion which is never done.
>>> >
>>> > Fix this by correctly handling such a condition.
>>> >
>>>
>>> IIUC the problem is that the dest_cpu and its stopper thread can be taken
>>> down by take_cpu_down(), and affine_move_task() currently isn't aware of
>>> that. I thought we had tested this vs hotplug, but oh well...
>>
>> I'm sorry, I should have provided more context in the first place. The machine
>> is an LPAR with 2 CPUs and CPU 0 was onlining (hotplugging?) CPU 1. The traces
>> show this scenario:
>>
>> CPU 0 | CPU 1
>> |
>> cpuplugd task 1429 |
>> holds the `cpu_hotplug_lock` |
>> for writing in _cpu_up+0x16a |
>> blocked on `cpuhp_state:1.done_up` |
>> completion in __cpuhp_kick_ap+0x76 |
>> |
>> | cpuhp/1 task 17
>> |supposed to complete bringup of the CPU
>> | (`cpuhp_state:1.done_up`) in cpuhp_thread_fun+0x108
>> |blocked on `wq_pool_attach_mutex`
>> | in workqueue_online_cpu+0x9e
>> |
>> xfs-conv/dm-0 task 745 |
>> holds the `wq_pool_attach_mutex` |
>> in worker_attach_to_pool+0x66 \
>> blocked on `task->migration_pending->done`|
>> completion in affine_move_task+0x10a/
>>
>> ~~~
>> crash> b 1429
>> PID: 1429 TASK: 99398000 CPU: 0 COMMAND: "cpuplugd"
>> #0 [997df970] __schedule+0x34c at 3089c424
>> #1 [997df9e0] schedule+0x7e at 3089cafe
>> #2 [997dfa20] schedule_timeout+0x26e at 308a1d8e
>> [inlined] do_wait_for_common
>> [inlined] __wait_for_common
>> #3 [997dfad8] wait_for_common+0x14a at 3089d902
>> [ret call] wait_for_completion+0x1a at 3089d96a
>>
>> [inlined] wait_for_ap_thread <<< blocked on `cpuhp_state:1.done_up` completion
>> [ret call] __cpuhp_kick_ap+0x76 at 300c610e
>> #4 [997dfb58] cpuhp_kick_ap+0xc4 at 300c61dc
>> [inlined] bringup_wait_for_ap
>> [ret call] bringup_cpu+0xea at 300c6402
>> #5 [997dfba8] cpuhp_invoke_callback+0xcc at 300c4f14
>> #6 [997dfc40] _cpu_up+0x16a at 300c798a <<< holds the `cpu_hotplug_lock` for writing
>> #7 [997dfc98] do_cpu_up+0xc6 at 300c7b66
>> #8 [997dfcd8] cpu_subsys_online+0x58 at 305a0a00
>> #9 [997dfd28] device_online+0x9e at 30598e7e
>> #10 [997dfd68] online_store+0x88 at 30598f28
>> #11 [997dfda8] kernfs_fop_write+0xdc at 3040276c
>> #12 [997dfdf8] vfs_write+0xa8 at 30354760
>> #13 [997dfe58] ksys_write+0x62 at 30354a32
>>
>> crash> cpuhp_cpu_state.state cpuhp_state:a | paste - -
>> [0]: 1aef424e0 state = CPUHP_ONLINE, # (195)
>> [1]: 1aef654e0 state = CPUHP_AP_WORKQUEUE_ONLINE, # (159)
>>
>> crash> cpuhp_cpu_state.bringup,thread,done_up.done cpuhp_state:1 -d | paste - - - -
>> [1]: 1aef654e0 bringup = true, thread = 0x81134400, done_up.done = 0, <<<
>>
>> crash> b 17
>> PID: 17 TASK: 81134400 CPU: 1 COMMAND: "cpuhp/1"
>> #0 [81143b68] __schedule+0x34c at 3089c424
>> #1 [81143bd8] schedule+0x7e at 3089cafe
>> #2 [81143c18] schedule_preempt_disabled+0x2a at 3089cfba
>> #3 [81143c30] __mutex_lock+0x320 at 3089df60
>>
>> #4 [81143cb0] workqueue_online_cpu+0x9e at 300e847e <<< blocked on `wq_pool_attach_mutex`
>> #5 [81143d20] cpuhp_invoke_callback+0xcc at 300c4f14
>> #6 [81143db8] cpuhp_thread_fun+0x108 at 300c6848 <<< supposed to complete the bring-up of the CPU (`cpuhp_state:1.done_up`)
>>
>> crash> b 745
>> PID: 745 TASK: 82359100 CPU: 0 COMMAND: "xfs-conv/dm-0"
>> #0 [8b4bfa20] __schedule+0x34c at 3089c424
>> #1 [8b4bfa90] schedule+0x7e at 3089cafe
>> #2 [8b4bfad0] schedule_timeout+0x26e at 308a1d8e
>> [inlined] do_wait_for_common
>> [inlined] __wait_for_common
>> #3 [8b4bfb88] wait_for_common+0x14a at 3089d902
>> [ret call] wait_for_completion+0x1a at 3089d96a
>>
>> #4 [8b4bfc08] affine_move_task+0x10a at 300fb51a <<< blocked on `task->migration_pending->done` completion
>> #5 [8b4bfd08] __set_cpus_allowed_ptr+0x12e at 300fb926
>> [ret call] set_cpus_allowed_ptr+0xa at 300fba32
>> #6 [8b4bfd78] worker_attach_to_pool+0x66 at 300e1dae <<< holds the `wq_pool_attach_mutex`
>> #7 [8b4bfdc8] rescuer_thread+0x12c at 300e5bac
>>
>> crash> rx 8b4bfea0
>> 8b4bfea0: [863373c0:kmalloc-192]
>>
>> crash> worker.task,rescue_wq 863373c0
>> task = 0x82359100,
>> rescue_wq = 0x8aa44400,
>>
>> crash> list -s pool_workqueue.pool pool_workqueue.mayday_node -hO workqueue_struct.maydays 0x8aa44400 | paste - -
>> 1fffff7f751900 pool = 0x1aef56a00,
>>
>> crash> worker_pool.attrs 0x1aef56a00
>> attrs = 0x80088180,
>>
>> crash> workqueue_attrs.cpumask[0].bits 0x80088180
>> cpumask[0].bits = {0x1, 0x0, ...
>>
>> crash> cpumask.bits __cpu_active_mask
>> bits = {0x1, 0x0, ...
>>
>> crash> cpumask.bits __cpu_online_mask
>> bits = {0x3, 0x0, ...
>>
>> crash> task_struct.migration_pending,flags 0x82359100
>> migration_pending = 0x8b4bfce8,
>> flags = 0x4208060,
>> ^ PF_KTHREAD
>>
>> crash> pd distribute_cpu_mask_prev:0
>> per_cpu(distribute_cpu_mask_prev, 0) = 0
>>
>> crash> set_affinity_pending.refs.refs.counter,arg,stop_pending,done.done 0x8b4bfce8 -d
>> refs.refs.counter = 1
>> arg = {
>> task = 0x82359100,
>> dest_cpu = 0,
>> pending = 0x8b4bfce8
>> }
>> stop_pending = 1,
>> done.done = 0,
>> ~~~
>>
>> In other words the `set_cpus_allowed_ptr()` is called from a worker thread which
>> tries to migrate. The worker pool is only allowed on CPU 0 and that was supposed
>> to be the destination as per the stack structure. In this case I thought it's OK
>> to leave the task on the old CPU
>
> AFAICT if a call to set_cpus_allowed_ptr() ends up in affine_move_task()
> and down to the stopper call, that means the task isn't allowed on its
> current CPU and needs to be moved.
>
>> and the Bill's testing scenario was successful
>> with the proposed patch. IIUC, it's exercising the hotplug due to load-balancing.
>>
>> This was on RHEL 8.8.z kernel. I see upstream changed a bit so I'm not sure it's
>> still reproducible. Also, I'm not sure why this only happens on s390 and not on
>> x86. I imagine the CPU hotplug slightly differs? Anyways this seems to be timing
>> sensitive and the timing will differ greatly for sure.
>>
>
> Thanks for the extra context!
>
> Double checking what I wrote before, I forgot RCU considers preempt-off
> sections as read-side critical sections. __set_cpus_allowed_ptr() already
> has preemption disabled all the way from reading the cpu_active_mask to the
> stop_one_cpu_nowait() call via task_rq_lock() + preempt_disable().
>
> IOW we have:
>
> __set_cpus_allowed_ptr()
> task_rq_lock() <-- PREEMPT OFF
> __set_cpus_allowed_ptr_locked()
> cpu_valid_mask = cpu_active_mask;
> dest_cpu = cpumask_any_and_distribute(cpu_valid_mask, ctx->new_mask);
> affine_move_task()
> preempt_disable();
> task_rq_unlock();
> stop_one_cpu_nowait(); <-- preemption still OFF
>
> And, considering:
>
> sched_cpu_deactivate()
> set_cpu_active(cpu, false);
> synchronize_rcu();
>
> Then, if __set_cpus_allowed_ptr() observes a CPU as being in the
> cpu_active_mask and uses that one as a destination CPU, said CPU cannot
> reach CPUHP_TEARDOWN_CPU:take_cpu_down() and park the stopper thread
> because its hotplug machinery will wait on the synchronize_rcu() in
> CPUHP_AP_ACTIVE:sched_cpu_deactivate().
>
> So "in theory", this shouldn't happen upstream.

Eh nevermind, in your stacktrace the relevant task is a rescuer thread
which is a kthread, so the cpu_valid_mask in use there is
cpu_online_mask, not cpu_valid_mask... Back to reading code :-)