linux-kernel - Re: [PATCH] sched/deadline: fix inactive_task_timer splat with CONFIG_PREEMPT

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Message-ID: <20230110013333.GH4028633@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 9 Jan 2023 17:33:33 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Wander Lairson Costa <wander@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        "open list:SCHEDULER" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/deadline: fix inactive_task_timer splat with
 CONFIG_PREEMPT_RT

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@...hat.com>
> Cc: Paul McKenney <paulmck@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.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().

							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
>