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Date:   Thu, 27 Jun 2019 13:57:03 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Byungchul Park <byungchul.park@lge.com>, josh@joshtriplett.org,
        rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@lge.com
Subject: Re: [PATCH v2] rcu: Change return type of
 rcu_spawn_one_boost_kthread()
Reply-To: paulmck@linux.ibm.com
References: <1561619266-8850-1-git-send-email-byungchul.park@lge.com>
 <20190627134240.GB215968@google.com>
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On Thu, Jun 27, 2019 at 09:42:40AM -0400, Joel Fernandes wrote:
> On Thu, Jun 27, 2019 at 04:07:46PM +0900, Byungchul Park wrote:
> > Hello,
> > 
> > I tested if the WARN_ON_ONCE() is fired with my box and it was ok.
> 
> Looks pretty safe to me and nice clean up!
> 
> Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Agreed, but I still cannot find where this applies.  I did try rcu/next,
which is currently here:

commit b989ff070574ad8b8621d866de0a8e9a65d42c80 (rcu/rcu/next, rcu/next)
Merge: 4289ee7d5a83 11ca7a9d541d
Author: Paul E. McKenney <paulmck@linux.ibm.com>
Date:   Mon Jun 24 09:12:39 2019 -0700

    Merge LKMM and RCU commits

Help?

							Thanx, Paul

>  - Joel
> 
> > 
> > Thanks,
> > Byungchul
> > 
> > Changes from v1
> > -. WARN_ON_ONCE() on failing to create rcu_boost_kthread.
> > -. Changed title and commit message a bit.
> > 
> > ---8<---
> > From 7100fcf82202f063f70f45def208ea5198412f5a Mon Sep 17 00:00:00 2001
> > From: Byungchul Park <byungchul.park@lge.com>
> > Date: Thu, 27 Jun 2019 15:58:10 +0900
> > Subject: [PATCH v2] rcu: Change return type of rcu_spawn_one_boost_kthread()
> > 
> > The return value of rcu_spawn_one_boost_kthread() is not used any
> > longer. Change return type of that function from int to void.
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> > ---
> >  kernel/rcu/tree_plugin.h | 17 ++++++++---------
> >  1 file changed, 8 insertions(+), 9 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 1102765..3c8444e 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;
> > +		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);
> > +	if (WARN_ON_ONCE(IS_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)
> > @@ -1265,7 +1264,7 @@ static void __init rcu_spawn_boost_kthreads(void)
> >  	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;
> >  	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)
> > @@ -1275,7 +1274,7 @@ static void rcu_prepare_kthreads(int cpu)
> >  
> >  	/* 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 */
> > -- 
> > 1.9.1
> >