linux-kernel - Re: [PATCH v2] rcu: Allow to eliminate softirq processing from rcutree

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Message-Id: <20190320152146.GH4102@linux.ibm.com>
Date:   Wed, 20 Mar 2019 08:21:46 -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 v2] rcu: Allow to eliminate softirq processing from
 rcutree

On Wed, Mar 20, 2019 at 12:32:19PM +0100, Sebastian Andrzej Siewior wrote:
> On 2019-03-19 12:44:19 [+0100], To Paul E. McKenney wrote:
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 0f31b79eb6761..0a719f726e149 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> …
> > +/*
> > + * Spawn per-CPU RCU core processing kthreads.
> > + */
> > +static int __init rcu_spawn_core_kthreads(void)
> > +{
> > +	int cpu;
> > +
> > +	for_each_possible_cpu(cpu)
> > +		per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
> > +	if (!IS_ENABLED(CONFIG_RCU_BOOST) && !rcu_softirq_enabled)
> 
> and this needs to become
> -       if (!IS_ENABLED(CONFIG_RCU_BOOST) && !rcu_softirq_enabled)
> +       if (!IS_ENABLED(CONFIG_RCU_BOOST) && rcu_softirq_enabled)
> 
> With this change and hunk that I just sent to Joel I get thee three RCU
> modes with and without BOOST booted.
> 
> Unless there is something (and Paul agrees that the Joel hunk is
> correct) I would post a v3 with those changes included.

Well, I did get a bunch of grace-period hangs of various sorts when I
ran rcutorture like this:

	tools/testing/selftests/rcutorture/bin/kvm.sh --memory 1G --bootargs rcunosoftirq

I also got a silent hard hang on TREE03.  And an rcutorture forward-progress
failure, which goes along with the grace-period hangs.

So something does need adjustment.  I will try again with this change.

							Thanx, Paul

> > +		return 0;
> > +	WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__);
> > +	return 0;
> > +}
> > +early_initcall(rcu_spawn_core_kthreads);
> 
> Sebastian
>