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

[Paul E. McKenney]

On Tue, Jan 10, 2023 at 05:52:03PM -0300, Wander Lairson Costa wrote:
> 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?

Given that this problem occurred in PREEMPT_RT, I am assuming that the
appropriate definition of "atomic context" is "cannot call schedule()".
And you are in fact not permitted to call schedule() from a bh-disabled
region.

This also means that you cannot acquire a non-raw spinlock in a
bh-disabled region of code in a PREEMPT_RT kernel, because doing
so can invoke schedule.

Of course, using a workqueue does incur needless overhead in
non-PREEMPT_RT kernels.  So one alternative approach is to use
the workqueue only in PREEMPT_RT kernels and to just invoke
__put_task_struct() directly (without call_rcu() along the way)
otherwise.

Does that help, or am I missing your point?

						Thanx, Paul

> [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
> > >
> >
>