linux-kernel - Re: [PATCH tip/core/rcu 01/19] rcu: Refactor rcu_{nmi,irq}

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Date:   Fri, 31 Aug 2018 11:25:32 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org, kernel-team@....com
Subject: Re: [PATCH tip/core/rcu 01/19] rcu: Refactor
 rcu_{nmi,irq}_{enter,exit}()

On Thu, Aug 30, 2018 at 02:10:32PM -0400, Steven Rostedt wrote:
> On Wed, 29 Aug 2018 15:20:29 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > This commit also changes order of execution from this:
> > 
> > 	rcu_dynticks_task_exit();
> > 	rcu_dynticks_eqs_exit();
> > 	trace_rcu_dyntick();
> > 	rcu_cleanup_after_idle();
> > 
> > To this:
> > 
> > 	rcu_dynticks_task_exit();
> > 	rcu_dynticks_eqs_exit();
> > 	rcu_cleanup_after_idle();
> > 	trace_rcu_dyntick();
> > 
> > In other words, the calls to trace_rcu_dyntick() and trace_rcu_dyntick()
> 
> How is trace_rcu_dyntick() and trace_rcu_dyntick reversed ? ;-)
> 
> > are reversed.  This has no functional effect because the real
> > concern is whether a given call is before or after the call to
> > rcu_dynticks_eqs_exit(), and this patch does not change that.  Before the
> > call to rcu_dynticks_eqs_exit(), RCU is not yet watching the current
> > CPU and after that call RCU is watching.
> > 
> > A similar switch in calling order happens on the idle-entry path, with
> > similar lack of effect for the same reasons.
> > 
> > Suggested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcu/tree.c | 61 +++++++++++++++++++++++++++++++----------------
> >  1 file changed, 41 insertions(+), 20 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 0b760c1369f7..0adf77923e8b 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -771,17 +771,18 @@ void rcu_user_enter(void)
> >  #endif /* CONFIG_NO_HZ_FULL */
> >  
> >  /**
> > - * rcu_nmi_exit - inform RCU of exit from NMI context
> > + * rcu_nmi_exit_common - inform RCU of exit from NMI context
> > + * @irq: Is this call from rcu_irq_exit?
> >   *
> >   * If we are returning from the outermost NMI handler that interrupted an
> >   * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting
> >   * to let the RCU grace-period handling know that the CPU is back to
> >   * being RCU-idle.
> >   *
> > - * If you add or remove a call to rcu_nmi_exit(), be sure to test
> > + * If you add or remove a call to rcu_nmi_exit_common(), be sure to test
> >   * with CONFIG_RCU_EQS_DEBUG=y.
> 
> As this is a static function, this description doesn't make sense. You
> need to move the description down to the new rcu_nmi_exit() below.

Right.. I should've done that.

Thanks you Steve.

Byungchul

> Other than that...
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> 
> -- Steve
> 
> 
> >   */
> > -void rcu_nmi_exit(void)
> > +static __always_inline void rcu_nmi_exit_common(bool irq)
> >  {
> >  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> >  
> > @@ -807,7 +808,22 @@ void rcu_nmi_exit(void)
> >  	/* 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. */
> > +
> > +	if (irq)
> > +		rcu_prepare_for_idle();
> > +
> >  	rcu_dynticks_eqs_enter();
> > +
> > +	if (irq)
> > +		rcu_dynticks_task_enter();
> > +}
> > +
> > +/**
> > + * rcu_nmi_exit - inform RCU of exit from NMI context
> > + */
> > +void rcu_nmi_exit(void)
> > +{
> > +	rcu_nmi_exit_common(false);
> >  }
> >  
> >  /**
> > @@ -831,14 +847,8 @@ 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();
> > +	rcu_nmi_exit_common(true);
> >  }
> >  
> >  /*
> > @@ -921,7 +931,8 @@ void rcu_user_exit(void)
> >  #endif /* CONFIG_NO_HZ_FULL */
> >  
> >  /**
> > - * rcu_nmi_enter - inform RCU of entry to NMI context
> > + * rcu_nmi_enter_common - inform RCU of entry to NMI context
> > + * @irq: Is this call from rcu_irq_enter?
> >   *
> >   * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and
> >   * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know
> > @@ -929,10 +940,10 @@ void rcu_user_exit(void)
> >   * long as the nesting level does not overflow an int.  (You will probably
> >   * run out of stack space first.)
> >   *
> > - * If you add or remove a call to rcu_nmi_enter(), be sure to test
> > + * If you add or remove a call to rcu_nmi_enter_common(), be sure to test
> >   * with CONFIG_RCU_EQS_DEBUG=y.
> >   */
> > -void rcu_nmi_enter(void)
> > +static __always_inline void rcu_nmi_enter_common(bool irq)
> >  {
> >  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> >  	long incby = 2;
> > @@ -949,7 +960,15 @@ void rcu_nmi_enter(void)
> >  	 * period (observation due to Andy Lutomirski).
> >  	 */
> >  	if (rcu_dynticks_curr_cpu_in_eqs()) {
> > +
> > +		if (irq)
> > +			rcu_dynticks_task_exit();
> > +
> >  		rcu_dynticks_eqs_exit();
> > +
> > +		if (irq)
> > +			rcu_cleanup_after_idle();
> > +
> >  		incby = 1;
> >  	}
> >  	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
> > @@ -960,6 +979,14 @@ void rcu_nmi_enter(void)
> >  	barrier();
> >  }
> >  
> > +/**
> > + * rcu_nmi_enter - inform RCU of entry to NMI context
> > + */
> > +void rcu_nmi_enter(void)
> > +{
> > +	rcu_nmi_enter_common(false);
> > +}
> > +
> >  /**
> >   * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
> >   *
> > @@ -984,14 +1011,8 @@ 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();
> > +	rcu_nmi_enter_common(true);
> >  }
> >  
> >  /*