linux-kernel - Re: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree

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Date:   Thu, 21 Mar 2019 06:26:03 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, tglx@...utronix.de,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3] rcu: Allow to eliminate softirq processing from
 rcutree

On Thu, Mar 21, 2019 at 09:27:37AM +0100, Sebastian Andrzej Siewior wrote:
> On 2019-03-20 16:46:01 [-0700], Paul E. McKenney wrote:
> > Thank you!  I reverted v2 and applied this one with the same sort of
> > update.  Testing is going well thus far aside from my failing to add
> > the required "=0" after the rcutree.use_softirq.  I will probably not
> > be the only one who will run afoul of this, so I updated the commit log
> > and the documentation accordingly, as shown below.
> 
> perfect, thank you.
> 
> > 							Thanx, Paul
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index f46b4af96ab9..b807204ffd83 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -629,7 +609,10 @@ static void rcu_read_unlock_special(struct task_struct *t)
> >  		/* Need to defer quiescent state until everything is enabled. */
> >  		if (irqs_were_disabled) {
> >  			/* Enabling irqs does not reschedule, so... */
> > -			raise_softirq_irqoff(RCU_SOFTIRQ);
> > +			if (!use_softirq)
> 
> that exclamation mark needs to go :/

That might explain some of the failures in TREE01, TREE02, TREE03, and
TREE09.  TREE01 got a NULL pointer dereference, but in __do_softirq().
So I suspect that this was related.  Ditto for TREE02, TREE03, and TREE09.
These also all have CONFIG_PREEMPT=y, and are the only ones run by default
that are set up this way.  (Well, so do SRCU-P, TASKS01, and TASKS03, but
they are torturing other forms of RCU.)

Anyway, I applied your fix above and will rerun.  The failures happened
within a few seconds in all cases, so a short run should cover this.

Once I get good rcutorture runs, I will ask you to run a heavy-duty run.
Once that passes, I will look at your changes in more detail.

							Thanx, Paul

> > +				raise_softirq_irqoff(RCU_SOFTIRQ);
> > +			else
> > +				invoke_rcu_core();
> >  		} else {
> >  			/* Enabling BH or preempt does reschedule, so... */
> >  			set_tsk_need_resched(current);
> 
> Sebastian
>