Re: [PATCH] sched/deadline: fix inactive_task_timer splat with CONFIG_PREEMPT_RT

[Valentin Schneider]

On 04/01/23 15:17, Wander Lairson Costa wrote:
> inactive_task_timer() executes in interrupt (atomic) context. It calls
> put_task_struct(), which indirectly acquires sleeping locks under
> PREEMPT_RT.
>
> Below is an example of a splat that happened in a test environment:
>
>  CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W ---------
>  Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012
>  Call Trace:
>  dump_stack_lvl+0x57/0x7d
>  mark_lock_irq.cold+0x33/0xba
>  ? stack_trace_save+0x4b/0x70
>  ? save_trace+0x55/0x150
>  mark_lock+0x1e7/0x400
>  mark_usage+0x11d/0x140
>  __lock_acquire+0x30d/0x930
>  lock_acquire.part.0+0x9c/0x210
>  ? refill_obj_stock+0x3d/0x3a0
>  ? rcu_read_lock_sched_held+0x3f/0x70
>  ? trace_lock_acquire+0x38/0x140
>  ? lock_acquire+0x30/0x80
>  ? refill_obj_stock+0x3d/0x3a0
>  rt_spin_lock+0x27/0xe0
>  ? refill_obj_stock+0x3d/0x3a0
>  refill_obj_stock+0x3d/0x3a0
>  ? inactive_task_timer+0x1ad/0x340
>  kmem_cache_free+0x357/0x560
>  inactive_task_timer+0x1ad/0x340
>  ? switched_from_dl+0x2d0/0x2d0
>  __run_hrtimer+0x8a/0x1a0
>  __hrtimer_run_queues+0x91/0x130
>  hrtimer_interrupt+0x10f/0x220
>  __sysvec_apic_timer_interrupt+0x7b/0xd0
>  sysvec_apic_timer_interrupt+0x4f/0xd0
>  ? asm_sysvec_apic_timer_interrupt+0xa/0x20
>  asm_sysvec_apic_timer_interrupt+0x12/0x20
>  RIP: 0033:0x7fff196bf6f5
>
> Instead of calling put_task_struct() directly, we defer it using
> call_rcu(). A more natural approach would use a workqueue, but since
> in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
> the code would become more complex because we would need to put the
> work_struct instance in the task_struct and initialize it when we
> allocate a new task_struct.
>

Sorry to come back on this; Juri reminded me offline that put_task_struct()
is invoked in other non-sleepable contexts, not just inactive_task_timer().

e.g.

  rto_push_irq_work_func() // hard irq work so hardirq context
  `\
    push_rt_task()
    `\
       put_task_struct()

Or

  cpu_stopper_thread() // stopper callbacks must not sleep
  `\
    push_cpu_stop()
    `\
      put_task_struct()

... But then again I'm not aware of any splats happening in these paths. Is
there something special about inactive_task_timer(), or could it be the
issue is there for those other paths but we just haven't had them reported
yet?