Re: [PATCH] rcu: Refactor rcu_{nmi,irq}_{enter,exit}()

[Byungchul Park]

On Sat, Jun 23, 2018 at 10:49:54AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 22, 2018 at 03:23:51PM +0900, Byungchul Park wrote:
> > On Fri, Jun 22, 2018 at 03:12:06PM +0900, Byungchul Park wrote:
> > > When passing through irq or NMI contexts, the current code uses
> > > ->dynticks_nmi_nesting to detect if it's in the ourmost at the moment.
> > > 
> > > Here, the thing is that all the related functions, rcu_irq_enter(),
> > > rcu_nmi_enter(), rcu_irq_exit() and rcu_nmi_exit() are carrying out
> > > the check within each under the following call relationship so there
> > > must be redundant conditional branches:
> > > 
> > >    rcu_irq_enter()
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> > >       rcu_nmi_enter()
> > >          /* A conditional branch with ->dynticks_nmi_nesting */
> >                                                ^
> >                                   Precisely, ->dynticks
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> > > 
> > >    rcu_irq_exit()
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> > >       rcu_nmi_exit()
> > >          /* A conditional branch with ->dynticks_nmi_nesting */
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> > > 
> > >    rcu_nmi_enter()
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> >                                                ^
> >                                   Precisely, ->dynticks
> > > 
> > >    rcu_nmi_exit()
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> > > 
> > > In order to make the code more readable and remove redundant
> > > conditional branches, we need to refactor rcu_{nmi,irq}_{enter,exit}()
> > > so they use a common function for each like:
> > > 
> > >    rcu_irq_enter() inlining rcu_nmi_enter_common(irq)
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> >                                                ^
> >                                   Precisely, ->dynticks
> > > 
> > >    rcu_irq_exit() inlining rcu_nmi_exit_common(irq)
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> > > 
> > >    rcu_nmi_enter() inlining rcu_nmi_enter_common(nmi)
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> >                                                ^
> >                                   Precisely, ->dynticks
> > > 
> > >    rcu_nmi_exit() inlining rcu_nmi_exit_common(nmi)
> > >       /* A conditional branch with ->dynticks_nmi_nesting */
> 
> I queued this for testing and further review, updating the commit log
> as follows.  Please let me know if I messed up something.

Much better. Thanks a lot.

> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 5e5ea52645b197fb7ae2f59f7927079b91e91aa0
> Author: Byungchul Park <byungchul.park@xxxxxxx>
> Date:   Fri Jun 22 15:12:06 2018 +0900
> 
>     rcu: Refactor rcu_{nmi,irq}_{enter,exit}()
>     
>     When entering or exiting irq or NMI handlers, the current code uses
>     ->dynticks_nmi_nesting to detect if it is in the outermost handler,
>     that is, the one interrupting or returning to an RCU-idle context (the
>     idle loop or nohz_full usermode execution).  When entering the outermost
>     handler via an interrupt (as opposed to NMI), it is necessary to invoke
>     rcu_dynticks_task_exit() just before the CPU is marked non-idle from an
>     RCU perspective and to invoke rcu_cleanup_after_idle() just after the
>     CPU is marked non-idle.  Similarly, when exiting the outermost handler
>     via an interrupt, it is necessary to invoke rcu_prepare_for_idle() just
>     before marking the CPU idle and to invoke rcu_dynticks_task_enter()
>     just after marking the CPU idle.
>     
>     The decision to execute these four functions is currently taken in
>     rcu_irq_enter() and rcu_irq_exit() as follows:
>     
>        rcu_irq_enter()
>           /* A conditional branch with ->dynticks_nmi_nesting */
>           rcu_nmi_enter()
>              /* A conditional branch with ->dynticks */
>           /* A conditional branch with ->dynticks_nmi_nesting */
>     
>        rcu_irq_exit()
>           /* A conditional branch with ->dynticks_nmi_nesting */
>           rcu_nmi_exit()
>              /* A conditional branch with ->dynticks_nmi_nesting */
>           /* A conditional branch with ->dynticks_nmi_nesting */
>     
>        rcu_nmi_enter()
>           /* A conditional branch with ->dynticks */
>     
>        rcu_nmi_exit()
>           /* A conditional branch with ->dynticks_nmi_nesting */
>     
>     This works, but the conditional branches in rcu_irq_enter() and
>     rcu_irq_exit() are redundant with those in rcu_nmi_enter() and
>     rcu_nmi_exit(), respectively.  Redundant branches are not something
>     we want in the to/from-idle fastpaths, so this commit refactors
>     rcu_{nmi,irq}_{enter,exit}() so they use a common inlined function passed
>     a constant argument as follows:
>     
>        rcu_irq_enter() inlining rcu_nmi_enter_common(irq=true)
>           /* A conditional branch with ->dynticks */
>     
>        rcu_irq_exit() inlining rcu_nmi_exit_common(irq=true)
>           /* A conditional branch with ->dynticks_nmi_nesting */
>     
>        rcu_nmi_enter() inlining rcu_nmi_enter_common(irq=false)
>           /* A conditional branch with ->dynticks */
>     
>        rcu_nmi_exit() inlining rcu_nmi_exit_common(irq=false)
>           /* A conditional branch with ->dynticks_nmi_nesting */
>     
>     The combination of the constant function argument and the inlining allows
>     the compiler to discard the conditionals that previously controlled
>     execution of rcu_dynticks_task_exit(), rcu_cleanup_after_idle(),
>     rcu_prepare_for_idle(), and rcu_dynticks_task_enter().  This reduces both
>     the to-idle and from-idle path lengths by two conditional branches each,
>     and improves readability as well.
>     
>     Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>     Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8788ddbc0d13..4ed74f10c852 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -773,17 +773,17 @@ 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
>   *
>   * 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.
>   */
> -void rcu_nmi_exit(void)
> +static __always_inline void rcu_nmi_exit_common(bool irq)
>  {
>  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
>  
> @@ -809,7 +809,22 @@ 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);
>  }
>  
>  /**
> @@ -833,14 +848,8 @@ 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);
>  }
>  
>  /*
> @@ -923,7 +932,7 @@ 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
>   *
>   * 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
> @@ -931,10 +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;
> @@ -951,7 +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("++="),
> @@ -962,6 +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
>   *
> @@ -986,14 +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);
>  }
>  
>  /*