Re: [PATCH tip/core/rcu 1/3] rcu: Further shrink Tiny RCU by making empty functions static inlines

[josh@joshtriplett.org]

On Tue, May 12, 2015 at 03:49:11PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> The Tiny RCU counterparts to rcu_idle_enter(), rcu_idle_exit(),
> rcu_irq_enter(), and rcu_irq_exit() are empty functions, but each
> has EXPORT_SYMBOL_GPL(), which, in kernels built with module support,
> needlessly consumes some memory.  This commit therefore moves these
> functions to static inlines in rcutiny.h, removing the need for
> exports.
> 
> This won't affect the size of the tiniest kernels, which are likely
> built without module support, but might help semi-tiny kernels that
> might include module support.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Even on kernels that don't include module support, this would shave a
few bytes.  Callers can't know a function in another .c file is actually
empty (without LTO, anyway), so callers have to actually *call* these
empty functions, and the functions themselves would need to be emitted.
Turning them into static inlines tells the callers that they can avoid
emitting any code.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  include/linux/rcupdate.h |  4 ----
>  include/linux/rcutiny.h  | 16 ++++++++++++++++
>  include/linux/rcutree.h  |  5 +++++
>  kernel/rcu/tiny.c        | 33 ---------------------------------
>  4 files changed, 21 insertions(+), 37 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 87bb0eee665b..1b3d7bcb3a6c 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -292,10 +292,6 @@ void rcu_sched_qs(void);
>  void rcu_bh_qs(void);
>  void rcu_check_callbacks(int user);
>  struct notifier_block;
> -void rcu_idle_enter(void);
> -void rcu_idle_exit(void);
> -void rcu_irq_enter(void);
> -void rcu_irq_exit(void);
>  int rcu_cpu_notify(struct notifier_block *self,
>  		   unsigned long action, void *hcpu);
>  
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> index 937edaeb150d..3df6c1ec4e25 100644
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -159,6 +159,22 @@ static inline void rcu_cpu_stall_reset(void)
>  {
>  }
>  
> +static inline void rcu_idle_enter(void)
> +{
> +}
> +
> +static inline void rcu_idle_exit(void)
> +{
> +}
> +
> +static inline void rcu_irq_enter(void)
> +{
> +}
> +
> +static inline void rcu_irq_exit(void)
> +{
> +}
> +
>  static inline void exit_rcu(void)
>  {
>  }
> diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> index d2e583a6aaca..f22d83f49e56 100644
> --- a/include/linux/rcutree.h
> +++ b/include/linux/rcutree.h
> @@ -93,6 +93,11 @@ void rcu_force_quiescent_state(void);
>  void rcu_bh_force_quiescent_state(void);
>  void rcu_sched_force_quiescent_state(void);
>  
> +void rcu_idle_enter(void);
> +void rcu_idle_exit(void);
> +void rcu_irq_enter(void);
> +void rcu_irq_exit(void);
> +
>  void exit_rcu(void);
>  
>  void rcu_scheduler_starting(void);
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index 069742d61c68..a501b4ab9b1c 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -49,39 +49,6 @@ static void __call_rcu(struct rcu_head *head,
>  
>  #include "tiny_plugin.h"
>  
> -/*
> - * Enter idle, which is an extended quiescent state if we have fully
> - * entered that mode.
> - */
> -void rcu_idle_enter(void)
> -{
> -}
> -EXPORT_SYMBOL_GPL(rcu_idle_enter);
> -
> -/*
> - * Exit an interrupt handler towards idle.
> - */
> -void rcu_irq_exit(void)
> -{
> -}
> -EXPORT_SYMBOL_GPL(rcu_irq_exit);
> -
> -/*
> - * Exit idle, so that we are no longer in an extended quiescent state.
> - */
> -void rcu_idle_exit(void)
> -{
> -}
> -EXPORT_SYMBOL_GPL(rcu_idle_exit);
> -
> -/*
> - * Enter an interrupt handler, moving away from idle.
> - */
> -void rcu_irq_enter(void)
> -{
> -}
> -EXPORT_SYMBOL_GPL(rcu_irq_enter);
> -
>  #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE)
>  
>  /*
> -- 
> 1.8.1.5
>