Re: [RFC] rcu: Warn that rcu ktheads cannot be spawned

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Jun 24, 2019 at 05:27:32PM +0900, Byungchul Park wrote:
> Hello rcu folks,
> 
> I thought it'd better to announce it if those spawnings fail because of
> !rcu_scheduler_fully_active.
> 
> Of course, with the current code, it never happens though.
> 
> Thoughts?

It seems in the right spirit, but with your patch a warning always fires.
rcu_prepare_cpu() is called multiple times, once from rcu_init() and then
from hotplug paths.

Warning splat stack looks like:

[    0.398767] Call Trace:                                                                                                        
[    0.398775]  rcu_init+0x6aa/0x724                                             
[    0.398779]  start_kernel+0x220/0x4a2                                    
[    0.398780]  ? copy_bootdata+0x12/0xac                                                                                                                                   
[    0.398782]  secondary_startup_64+0xa4/0xb0    


> 
> Thanks,
> Byungchul
> 
> ---8<---
> From 58a33a85c70f82c406319b4752af95cf6ceb73a3 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@lge.com>
> Date: Mon, 24 Jun 2019 17:08:26 +0900
> Subject: [RFC] rcu: Warn that rcu ktheads cannot be spawned
> 
> In case that rcu ktheads cannot be spawned due to
> !rcu_scheduler_fully_active, it'd be better to anounce it.
> 
> While at it, because the return value of rcu_spawn_one_boost_kthread()
> is not used any longer, changed the return type from int to void.
> 
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
>  kernel/rcu/tree_plugin.h | 31 +++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 1102765..7d74193 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1131,7 +1131,7 @@ static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
>   * already exist.  We only create this kthread for preemptible RCU.
>   * Returns zero if all is well, a negated errno otherwise.
>   */
> -static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
> +static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
>  {
>  	int rnp_index = rnp - rcu_get_root();
>  	unsigned long flags;
> @@ -1139,25 +1139,24 @@ static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
>  	struct task_struct *t;
>  
>  	if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
> -		return 0;
> +		return;
>  
> -	if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)
> -		return 0;
> +	if (rcu_rnp_online_cpus(rnp) == 0)
> +		return;
>  
>  	rcu_state.boost = 1;
>  	if (rnp->boost_kthread_task != NULL)
> -		return 0;
> +		return;
>  	t = kthread_create(rcu_boost_kthread, (void *)rnp,
>  			   "rcub/%d", rnp_index);
>  	if (IS_ERR(t))
> -		return PTR_ERR(t);
> +		return;
>  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
>  	rnp->boost_kthread_task = t;
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  	sp.sched_priority = kthread_prio;
>  	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
>  	wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
> -	return 0;
>  }
>  
>  static void rcu_cpu_kthread_setup(unsigned int cpu)
> @@ -1264,8 +1263,12 @@ static void __init rcu_spawn_boost_kthreads(void)
>  		per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
>  	if (WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__))
>  		return;
> +
> +	if (WARN_ON(!rcu_scheduler_fully_active))
> +		return;
> +
>  	rcu_for_each_leaf_node(rnp)
> -		(void)rcu_spawn_one_boost_kthread(rnp);
> +		rcu_spawn_one_boost_kthread(rnp);
>  }
>  
>  static void rcu_prepare_kthreads(int cpu)
> @@ -1273,9 +1276,11 @@ static void rcu_prepare_kthreads(int cpu)
>  	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>  	struct rcu_node *rnp = rdp->mynode;
>  
> +	if (WARN_ON(!rcu_scheduler_fully_active))
> +		return;
> +
>  	/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
> -	if (rcu_scheduler_fully_active)
> -		(void)rcu_spawn_one_boost_kthread(rnp);
> +	rcu_spawn_one_boost_kthread(rnp);
>  }
>  
>  #else /* #ifdef CONFIG_RCU_BOOST */
> @@ -2198,8 +2203,10 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
>   */
>  static void rcu_spawn_cpu_nocb_kthread(int cpu)
>  {
> -	if (rcu_scheduler_fully_active)
> -		rcu_spawn_one_nocb_kthread(cpu);
> +	if (WARN_ON(!rcu_scheduler_fully_active))
> +		return;
> +
> +	rcu_spawn_one_nocb_kthread(cpu);
>  }
>  
>  /*
> -- 
> 1.9.1
>