Re: BUG: HANG_DETECT waiting for migration_cpu_stop() complete

[Mukesh Ojha <quic_mojha@quicinc.com>]

This is fixed by this.

https://lore.kernel.org/lkml/YvrWaml3F+x9Dk+T@slm.duckdns.org/

-Mukesh

On 9/5/2022 8:17 AM, Jing-Ting Wu wrote:
> Hi,
> 
> We meet the HANG_DETECT happened in T SW version with kernel-5.15.
> Many tasks have been blocked for a long time.
> 
> 
> Root cause:
> migration_cpu_stop() is not complete due to is_migration_disabled(p) is
> true, complete is false and complete_all() never get executed.
> It let other task wait the rwsem.
> 
> Detail:
> system_server waiting for cgroup_threadgroup_rwsem.
> OomAdjuster is holding the cgroup_threadgroup_rwsem and waiting for
> cpuset_rwsem.
> cpuset_hotplug_workfn is holding the cpuset_rwsem and waiting for
> affine_move_task() complete.
> affine_move_task() waiting for migration_cpu_stop() complete.
> 
> The backtrace of system_server:
> __switch_to
> __schedule
> schedule
> percpu_rwsem_wait
> __percpu_down_read
> cgroup_css_set_fork => wait for cgroup_threadgroup_rwsem
> cgroup_can_fork
> copy_process
> kernel_clone
> 
> The backtrace of OomAdjuster:
> __switch_to
> __schedule
> schedule
> percpu_rwsem_wait
> percpu_down_write
> cpuset_can_attach => wait for cpuset_rwsem
> cgroup_migrate_execute
> cgroup_attach_task
> __cgroup1_procs_write => hold cgroup_threadgroup_rwsem
> cgroup1_procs_write
> cgroup_file_write
> kernfs_fop_write_iter
> vfs_write
> ksys_write
> 
> The backtrace of cpuset_hotplug_workfn:
> __switch_to
> __schedule
> schedule
> schedule_timeout
> wait_for_common
> affine_move_task => wait for complete
> __set_cpus_allowed_ptr_locked
> update_tasks_cpumask
> cpuset_hotplug_update_tasks => hold cpuset_rwsem
> cpuset_hotplug_workfn
> process_one_work
> worker_thread
> kthread
> 
> 
> In affine_move_task() will call migration_cpu_stop() and wait for it
> complete.
> In normal case, if migration_cpu_stop() complete it will inform
> everyone that he is done.
> But there is an exception case that will not notify.
> If is_migration_disabled(p) is true and complete will always is false,
> then complete_all() never get executed.
> 
> static int migration_cpu_stop(void *data)
> {
> ...
>      bool complete = false;
> ...
> 
>      if (task_rq(p) == rq) {
>          if (is_migration_disabled(p))
>                goto out; => is_migration_disabled(p) = true,
>                             so complete = false.
>              ...
>          }
> ...
> 
> out:
> ...
>      if (complete) => complete = false,
>                       so complete_all() never get executed.
>          complete_all(&pending->done);
> 
>          return 0;
> }
> 
> 
> Review the code, we found that there are many places can change
> is_migration_disabled() value.
> (such as: __rt_spin_lock(), rt_read_lock(), rt_write_lock(), ...)
> 
> Do you have any suggestion for this issue?
> Thank you.
> 
> 
> 
> 
> Best regards,
> Jing-Ting Wu
> 
> 

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