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

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Jan 18, 2023 at 03:57:38PM +0000, Valentin Schneider wrote:
> On 10/01/23 14:27, Paul E. McKenney wrote:
> > 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@kernel.org> 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@redhat.com>
> >> > > Cc: Paul McKenney <paulmck@kernel.org>
> >> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> >> > > ---
> >> > >  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.
> 
> But per the PREEMPT_RT lock "replacement", non-raw spinlocks end up
> invoking schedule_rtlock(), which should be safe vs BH disabled
> (local_lock() + rcu_read_lock()):
> 
>   6991436c2b5d ("sched/core: Provide a scheduling point for RT locks")
> 
> Unless I'm missing something else?

No, you miss nothing.  Apologies for my confusion!

(I could have sworn that someone else corrected me on this earlier,
but I don't see it right off hand.)

							Thanx, Paul