Re: [PATCH tip/core/rcu 3/3] rcu: Optimize RCU_FAST_NO_HZ for RCU_NOCB_CPU_ALL

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Feb 17, 2014 at 01:40:27PM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> If CONFIG_RCU_NOCB_CPU_ALL=y, then no CPU will ever have RCU callbacks
> because these callbacks will instead be handled by the rcuo kthreads.
> However, the current version of RCU_FAST_NO_HZ nevertheless checks for RCU
> callbacks.  This commit therefore creates static inline implementations
> of rcu_prepare_for_idle() and rcu_cleanup_after_idle() that are no-ops
> when CONFIG_RCU_NOCB_CPU_ALL=y.

"creates static inline implementations" isn't quite accurate; they're
static, and in the header, and the compiler is likely to inline the
no-op versions, but they're not actually marked as inline.  The commit
message should just say that it turns the implementations into no-ops.

> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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

> ---
>  kernel/rcu/tree_plugin.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 820b06aefbee..41afc3fbfb6c 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1658,7 +1658,7 @@ extern int tick_nohz_active;
>   * only if it has been awhile since the last time we did so.  Afterwards,
>   * if there are any callbacks ready for immediate invocation, return true.
>   */
> -static bool rcu_try_advance_all_cbs(void)
> +static bool __maybe_unused rcu_try_advance_all_cbs(void)
>  {
>  	bool cbs_ready = false;
>  	struct rcu_data *rdp;
> @@ -1743,6 +1743,7 @@ int rcu_needs_cpu(int cpu, unsigned long *dj)
>   */
>  static void rcu_prepare_for_idle(int cpu)
>  {
> +#ifndef CONFIG_RCU_NOCB_CPU_ALL
>  	struct rcu_data *rdp;
>  	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
>  	struct rcu_node *rnp;
> @@ -1794,6 +1795,7 @@ static void rcu_prepare_for_idle(int cpu)
>  		rcu_accelerate_cbs(rsp, rnp, rdp);
>  		raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
>  	}
> +#endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
>  }
>  
>  /*
> @@ -1803,11 +1805,12 @@ static void rcu_prepare_for_idle(int cpu)
>   */
>  static void rcu_cleanup_after_idle(int cpu)
>  {
> -
> +#ifndef CONFIG_RCU_NOCB_CPU_ALL
>  	if (rcu_is_nocb_cpu(cpu))
>  		return;
>  	if (rcu_try_advance_all_cbs())
>  		invoke_rcu_core();
> +#endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
>  }
>  
>  /*
> -- 
> 1.8.1.5
>