linux-kernel - Re: [RFC 1/2] rcu: Do prepare and cleanup idle depending on in

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Date:   Wed, 20 Jun 2018 07:58:40 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Byungchul Park <byungchul.park@....com>, jiangshanlai@...il.com,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        joel@...lfernandes.org
Subject: Re: [RFC 1/2] rcu: Do prepare and cleanup idle depending on in_nmi()

On Wed, Jun 20, 2018 at 09:33:09AM -0400, Steven Rostedt wrote:
> On Wed, 20 Jun 2018 17:47:19 +0900
> Byungchul Park <byungchul.park@....com> wrote:
> 
> > Get rid of dependency on ->dynticks_nmi_nesting.
> 
> This is not a trivial change. Can you please explain the rational and
> background in the change log. Add enough context here to know why this
> change is needed.

What Steve said!!!

							Thanx, Paul

> Thanks!
> 
> -- Steve
> 
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > ---
> >  kernel/rcu/tree.c | 22 ++++++++++------------
> >  1 file changed, 10 insertions(+), 12 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index deb2508..59ae94e 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -797,6 +797,11 @@ void rcu_nmi_exit(void)
> >  		return;
> >  	}
> >  
> > +	if (!in_nmi()) {
> > +		rcu_prepare_for_idle();
> > +		rcu_dynticks_task_enter();
> > +	}
> > +
> >  	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> >  	trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nmi_nesting, 0, rdtp->dynticks);
> >  	WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
> > @@ -824,14 +829,8 @@ void rcu_nmi_exit(void)
> >   */
> >  void rcu_irq_exit(void)
> >  {
> > -	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > -
> >  	lockdep_assert_irqs_disabled();
> > -	if (rdtp->dynticks_nmi_nesting == 1)
> > -		rcu_prepare_for_idle();
> >  	rcu_nmi_exit();
> > -	if (rdtp->dynticks_nmi_nesting == 0)
> > -		rcu_dynticks_task_enter();
> >  }
> >  
> >  /*
> > @@ -944,6 +943,11 @@ void rcu_nmi_enter(void)
> >  	if (rcu_dynticks_curr_cpu_in_eqs()) {
> >  		rcu_dynticks_eqs_exit();
> >  		incby = 1;
> > +
> > +		if (!in_nmi()) {
> > +			rcu_dynticks_task_exit();
> > +			rcu_cleanup_after_idle();
> > +		}
> >  	}
> >  	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
> >  			  rdtp->dynticks_nmi_nesting,
> > @@ -977,14 +981,8 @@ void rcu_nmi_enter(void)
> >   */
> >  void rcu_irq_enter(void)
> >  {
> > -	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > -
> >  	lockdep_assert_irqs_disabled();
> > -	if (rdtp->dynticks_nmi_nesting == 0)
> > -		rcu_dynticks_task_exit();
> >  	rcu_nmi_enter();
> > -	if (rdtp->dynticks_nmi_nesting == 1)
> > -		rcu_cleanup_after_idle();
> >  }
> >  
> >  /*
>