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

[Wander Lairson Costa]

On Mon, Jan 9, 2023 at 10:40 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Wed, Jan 04, 2023 at 03:17:01PM -0300, 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.
> >
> > Signed-off-by: Wander Lairson Costa <wander@xxxxxxxxxx>
> > Cc: Paul McKenney <paulmck@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > ---
> >  kernel/sched/build_policy.c |  1 +
> >  kernel/sched/deadline.c     | 24 +++++++++++++++++++++++-
> >  2 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/build_policy.c b/kernel/sched/build_policy.c
> > index d9dc9ab3773f..f159304ee792 100644
> > --- a/kernel/sched/build_policy.c
> > +++ b/kernel/sched/build_policy.c
> > @@ -28,6 +28,7 @@
> >  #include <linux/suspend.h>
> >  #include <linux/tsacct_kern.h>
> >  #include <linux/vtime.h>
> > +#include <linux/rcupdate.h>
> >
> >  #include <uapi/linux/sched/types.h>
> >
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 9ae8f41e3372..ab9301d4cc24 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1405,6 +1405,13 @@ static void update_curr_dl(struct rq *rq)
> >       }
> >  }
> >
> > +static void delayed_put_task_struct(struct rcu_head *rhp)
> > +{
> > +     struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > +
> > +     __put_task_struct(task);
>
> Please note that BH is disabled here.  Don't you therefore
> need to schedule a workqueue handler?  Perhaps directly from
> inactive_task_timer(), or maybe from this point.  If the latter, one
> way to skip the extra step is to use queue_rcu_work().
>

My initial work was using a workqueue [1,2]. However, I realized I
could reach a much simpler code with call_rcu().
I am afraid my ignorance doesn't allow me to get your point. Does
disabling softirq imply atomic context?


[1] https://gitlab.com/walac/kernel-ark/-/commit/ec8addbe38d5c318f1789b4c0fa480a9d2afdb65
[2] https://gitlab.com/walac/kernel-ark/-/commit/0bde233235ffed233a7466a36a4866bc48064f54


>                                                         Thanx, Paul
>
> > +}
> > +
> >  static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
> >  {
> >       struct sched_dl_entity *dl_se = container_of(timer,
> > @@ -1442,7 +1449,22 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
> >       dl_se->dl_non_contending = 0;
> >  unlock:
> >       task_rq_unlock(rq, p, &rf);
> > -     put_task_struct(p);
> > +
> > +     if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +             /*
> > +              * Decrement the refcount explicitly to avoid unnecessarily
> > +              * calling call_rcu.
> > +              */
> > +             if (refcount_dec_and_test(&p->usage))
> > +                     /*
> > +                      * under PREEMPT_RT, we can't call put_task_struct
> > +                      * in atomic context because it will indirectly
> > +                      * acquire sleeping locks.
> > +                      */
> > +                     call_rcu(&p->rcu, delayed_put_task_struct);
> > +     } else {
> > +             put_task_struct(p);
> > +     }
> >
> >       return HRTIMER_NORESTART;
> >  }
> > --
> > 2.39.0
> >
>